PATENT DOCUMENT

Publication Number: US-8976976-B2
Application Number: US-20388008-A
Country: US
Kind Code: B2

Title: Accessory adapter with user input interface

Abstract:
Electronic devices and accessories such as headsets for electronic devices are provided. A microphone may be included in an accessory to capture sound for an associated electronic device. Buttons and other user interfaces may be included in the accessories. An accessory may have an audio plug that connects to a mating audio jack in an electronic device, thereby establishing a wired communications link between the accessory and the electronic device. The electronic device may include power supply circuitry for applying bias voltages to the accessory. The bias voltages may bias a microphone and may adjust settings in the accessory such as settings related to operating modes. User input information may be conveyed between the accessory and the electronic device using ultrasonic tone transmission. The electronic device may also gather input from the accessory using a voltage detector coupled to lines in the communications path.

Claims:
What is claimed is: 
     
       1. An adapter configured to be coupled between a headset and an electronic device, comprising:
 an audio jack that receives an audio plug associated with the headset, wherein the audio jack includes at least tip, ring, and sleeve contacts; 
 a user interface that receives user input from a user; 
 circuitry that transmits the user input to the electronic device; and 
 circuitry that transmits identity information of the adapter to the electronic device. 
 
     
     
       2. The adapter defined in  claim 1  further comprising a cable with an audio plug that is connected to the electronic device. 
     
     
       3. The adapter defined in  claim 2  wherein the circuitry that transmits the user input to the electronic device comprises an ultrasonic tone generator that transmits the user input to the electronic device. 
     
     
       4. The adapter defined in  claim 3  wherein the user interface comprises at least one button. 
     
     
       5. The adapter defined in  claim 4  wherein the at least one button comprises a shorting button that momentarily shorts two lines in the cable in response to actuation by the user. 
     
     
       6. The adapter defined in  claim 4  wherein the at least one button comprises a plurality of resistively encoded buttons. 
     
     
       7. The adapter defined in  claim 6  further comprising an impedance detector that provides the user input from the resistively encoded buttons to the ultrasonic tone generator. 
     
     
       8. The adapter defined in  claim 1 , wherein the adapter comprises speakers and a microphone. 
     
     
       9. The adapter defined in  claim 8 , wherein the electronic device comprises circuitry that receives microphone audio from the microphone and that outputs analog audio signals for playback through the speakers. 
     
     
       10. The adapter defined in  claim 1 , wherein the circuitry that transmits identity information of the adapter to the electronic device comprises circuitry that encodes the identity information in ultrasonic tones and transmits the identity information of the adapter to the electronic device. 
     
     
       11. The adapter defined in  claim 10 , wherein the identity information comprises a serial number. 
     
     
       12. The adapter defined in  claim 11 , wherein the identity information allows the electronic device and the adapter to operate together if the identity information of the adapter satisfies security authentication criteria. 
     
     
       13. An adapter configured to be coupled between a headset and an electronic device, comprising:
 an audio jack that receives an audio plug associated with the headset, wherein the audio jack includes at least tip, ring, and sleeve contacts; 
 a user interface that receives user input from a user, wherein the user interface comprises a plurality of resistively encoded buttons; 
 circuitry that transmits the user input to the electronic device, wherein the circuitry that transmits the user input to the electronic device comprises an ultrasonic tone generator that transmits the user input to the electronic device; 
 a cable with an audio plug that is connected to the electronic device; and 
 switching circuitry that is configured in response to voltages received from the electronic device to operate in: 
 a tone mode in which the user input is transmitted to the electronic device as ultrasonic tones generated by the ultrasonic tone generator; and 
 a resistance detection mode in which resistance values associated with the resistively encoded buttons are measured by the electronic device.

Description:
This application claims the benefit of provisional patent application No. 61/020,988, filed Jan. 14, 2008, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This invention relates to electronic devices and accessories for electronic devices. 
     Electronic devices such as computers, media players, and cellular telephones typically contain audio jacks. Accessories such as headsets have mating plugs. A user who desires to use a headset with an electronic device may connect the headset to the electronic device by inserting the headset plug into the mating audio jack on the electronic device. Miniature size (3.5 mm) phone jacks and plugs are commonly used electronic devices such as notebook computers and media players, because audio connectors such as these are relatively compact. 
     Stereo audio connectors typically have three contacts. The outermost end of an audio plug is typically referred to as the tip. The innermost portion of the plug is typically referred to as the sleeve. A ring contact lies between the tip and the sleeve. When using this terminology, stereo audio connectors such as these are sometimes referred to as tip-ring-sleeve (TRS) connectors. The sleeve can serve as ground. The tip contact can be used in conjunction with the sleeve to handle a left audio channel and the ring contact can be used in conjunction with the sleeve to handle the right channel of audio. 
     In devices such as cellular telephones, it is often necessary to convey microphone signals from the headset to the cellular telephone. In arrangements in which it is desired to handle both stereo audio signals and microphone signals, an audio connector typically contains an additional ring terminal. Audio connectors such as these have a tip, two rings, and a sleeve and are therefore sometimes referred to as four-contact connectors or TRRS connectors. When a four-contact connector is used, the sleeve or one of the rings may serve as ground. The tip contact and the outermost ring contact may be used in conjunction with the ground to carry audio for the left and right headset speaker audio channels. The remaining contact (e.g., the sleeve contact) may be used in conjunction with the ground to carry microphone signals. 
     In a typical microphone-enabled headset, a bias voltage is applied to the microphone from the electronic device over the microphone line. The microphone in the headset generates a microphone signal when sound is received from the user (i.e., when a user speaks during a telephone call). Microphone amplifier circuitry and analog-to-digital converter circuitry in the cellular telephone can convert microphone signals from the headset into digital signals for subsequent processing. 
     Some users may wish to operate their cellular telephones or other electronic devices remotely. To accommodate this need, some modern microphone-enabled headsets feature a button. When the button is pressed by the user, the microphone line is shorted to ground. Monitoring circuitry in a cellular telephone to which the headset is connected can detect the momentary grounding of the microphone line and can take appropriate action. In a typical scenario, a button press might be used be used to answer an incoming telephone or might be used skip tracks during playback of a media file. 
     Conventional button arrangements such as these offer limited functionality and may introduce undesirable clicking noises if the button is actuated during normal use of the microphone. 
     It would therefore be desirable to be able to provide improved arrangements for supporting interactions between electronic devices and accessories such as headsets. 
     SUMMARY 
     Electronic devices and accessories for electronic devices are provided. The electronic devices may be computers, handheld computing devices such as smart cellular telephones or media players, or any other suitable computing equipment. These devices typically generate audio signals. The audio signals may be used to drive speakers in accessories such as headsets and other equipment capable of presenting sound to a user. 
     Some electronic devices support operations that involve gathering sound input with a microphone. Accessories with microphones may be used to supply microphone signals to electronic devices with audio input capabilities. For example, accessories with microphones may be used to supply voice signals to a cellular telephone in connection with cellular telephone calls or may be used to supply audio when an audio clip is being recorded by a voice memo application on a device. Speakers may be used to play media files, sound from telephone call, or other suitable audio information. 
     It may be desirable to gather user input with a user input interface that is part of an accessory (i.e., a stand-alone accessory or an adapter). With this type of arrangement, buttons, a touch pad, a touch screen, or other user input interface equipment may be used at the accessory to gather user input. Resistively encoded buttons may be used to gather user input. An impedance detector may be used in the accessory to determine which of the resistively encoded buttons has been pressed. Button activity may also be monitored directly by the electronic device using voltage detection circuitry. The accessory may have an ultrasonic tone generator that conveys ultrasonic tones in response to user input activity such as button press activity. The electronic device may have a tone detector that monitors the user input by receiving and processing the ultrasonic tones. The user input may be used to adjust the functions of the electronic device such as media playback functions, cellular telephone operations, and other suitable functions. 
     If desired, user input can be conveyed from the accessory to the electronic device as ultrasonic tones using a microphone line and ground line that are also being used to convey audio information. For example, in an accessory with buttons, information on button actuation events can be transmitted as ultrasonic signals at the same time that analog microphone signals are conveyed from the accessory to a corresponding amplifier in the electronic device. 
     Ultrasonic tones are not audible to humans, so they can be carried over the microphone and ground path without resulting in audible microphone interference. This allows a user to convey button actuation activity to the electronic device at the same time that the user carries on a telephone call using the microphone in the accessory. Microphone signals corresponding to the user&#39;s voice may be conveyed to the electronic device, while simultaneously conveying button press data to the electronic device. Both analog microphone signals and ultrasonic button actuation data may be transmitted from the accessory to the electronic device simultaneously. The ultrasonic signals will not be audible as audio signals and therefore will not interfere with other audio signals such as music or voice signals. 
     The buttons that are used to produce the ultrasonic signals may be resistively-encoded buttons that provide button press data to a tone generator. The tone generator may, in turn, transmit corresponding ultrasonic tones to the electronic device. These buttons need not short the microphone and ground lines together. As a result, the microphone and ground lines can be left undisturbed by shorting events during button presses. This helps allow a user to make button presses at the same time that the user is carrying on a telephone call. In this type of configuration, button presses are used to control the tone generator and will not short the microphone and ground lines together. Shorting events will therefore not interrupt a telephone call. The ultrasonic tones that are produced by the tone generator in response to the button presses can be conveyed over the microphone and ground lines during the telephone call, but will not be audible to the user because they fall outside the range of human hearing. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative electronic device in communication with an accessory such as a headset and other external equipment in a system in accordance with an embodiment of the present invention. 
         FIG. 2  is a perspective view of an illustrative electronic device such as a portable computer with an audio connector that mates with accessories such as headsets in accordance with an embodiment of the present invention. 
         FIG. 3  is a perspective view of an illustrative handheld electronic device such as a media player, cellular telephone, or hybrid device showing how the handheld electronic device may have an audio connector that mates with accessories such as headsets in accordance with an embodiment of the present invention. 
         FIG. 4  is a cross-sectional side view of illustrative three-contact and four-contact audio connectors that may be used in accordance with embodiments of the present invention. 
         FIG. 5  is a perspective view of an illustrative accessory such as a headset that may be provided with a user input interface such as input-output circuitry containing multiple user-selectable buttons in accordance with an embodiment of the present invention. 
         FIG. 6  is perspective view of an illustrative accessory such as a headset that has been connected to an adapter accessory having an input interface such as an interface with multiple user-selectable buttons in accordance with an embodiment of the present invention. 
         FIG. 7  is a schematic diagram showing illustrative circuitry that may be used in an electronic device and an associated accessory in accordance with embodiments of the present invention. 
         FIG. 8  is a flow chart of illustrative steps involved in using an electronic device and accessory in accordance with an embodiment of the present invention. 
         FIG. 9  is a circuit diagram of an illustrative accessory such as a headset having two speakers in accordance with an embodiment of the present invention. 
         FIG. 10  is a circuit diagram showing illustrative circuitry that may be used to drive audio signals from an electronic device onto associated speaker paths in an accessory such as a headset in accordance with an embodiment of the present invention. 
         FIG. 11  is a circuit diagram of an illustrative accessory such as a headset having two speakers, a microphone, and a switch associated with a button in accordance with an embodiment of the present invention. 
         FIG. 12  is a circuit diagram showing illustrative circuitry that may be used to handle microphone signals and control signals received from an accessory such as the headset of  FIG. 11  and that may be used to drive audio signals onto associated speaker paths in the accessory in accordance with an embodiment of the present invention. 
         FIG. 13  is a flow chart of illustrative steps involved in using an electronic device and accessory such as a headset with a microphone and associated button in accordance with an embodiment of the present invention. 
         FIG. 14  is a circuit diagram of an illustrative accessory such as headset that may have one or more buttons or other user interface equipment for producing encoded resistance values that are processed by an associated electronic device that has resistance detection capabilities in accordance with an embodiment of the present invention. 
         FIG. 15  is a circuit diagram of illustrative circuitry that may be used in an electronic device to provide audio signals to speakers in an accessory such as a headset and that may be used to implement resistance detection capabilities for decoding resistively encoded user input such as button actuation events made using buttons in an accessory of the type shown in  FIG. 14  in accordance with an embodiment of the present invention. 
         FIG. 16  is a circuit diagram of an illustrative accessory such as a headset having resistively encoded buttons and an optional button that shorts two contacts in a four-contact audio connector together when the optional button is actuated in accordance with an embodiment of the present invention. 
         FIG. 17  is a flow chart of illustrative steps involved in decoding resistively encoded button actuation events or other user control events supplied by a user with an accessory such as a headset in accordance with an embodiment of the present invention. 
         FIG. 18  is a circuit diagram of an illustrative accessory such as a headset in which a user interface gathers user input and in which control circuitry such as tone-generator-based control circuitry assists in conveying the user input to a corresponding electronic device in accordance with an embodiment of the present invention. 
         FIG. 19  is a circuit diagram of an illustrative user input device such as a set of resistively encoded button switches or other controls and associated processing circuitry such as an impedance detector that may be used in an accessory such as a headset in accordance with an embodiment of the present invention. 
         FIG. 20  is a circuit diagram of illustrative input interface and control circuitry that may be used to process user input in an accessory such as a headset in accordance with an embodiment of the present invention. 
         FIG. 21  is a circuit diagram of illustrative circuitry that may be used in an electronic device in receiving and processing control signals such as tone-based-control signals from a headset or other accessory in accordance with an embodiment of the present invention. 
         FIG. 22  is a flow chart of illustrative steps involved in using an electronic device and accessory that communicate with one another using tone-based signaling or other suitable communications techniques in accordance with an embodiment of the present invention. 
         FIG. 23  is a diagram showing how different types of accessories may be used with different types of electronic devices in accordance with embodiments of the present invention. 
         FIG. 24  is a circuit diagram of illustrative circuitry that may be used in an electronic device to interface with an accessory such as a headset that includes tone-based encoder circuitry for encoding user input in accordance with an embodiment of the present invention. 
         FIG. 25  is a circuit diagram of an illustrative adjustable power supply circuit that may be used to produce a controllable bias for a microphone line or other conductor associated with an accessory such as a headset in accordance with an embodiment of the present invention. 
         FIG. 26  is a circuit diagram of an illustrative circuit that may be used for monitoring the voltage of a signal from an accessory such as a headset on a conductive path such as a microphone line in accordance with an embodiment of the present invention. 
         FIG. 27  is a table showing illustrative registers that may be used in an electronic device to store information associated with interactions between the electronic device and an accessory such as a headset in accordance with an embodiment of the present invention. 
         FIG. 28  is a circuit diagram of illustrative circuitry that may be used in an accessory such as a headset to process user input and to supply corresponding tone-encoded signals to a corresponding electronic device in accordance with an embodiment of the present invention. 
         FIG. 29  is a table showing illustrative states in which an illustrative set of switches may be placed by control circuitry in various accessories during different modes of operation in accordance with embodiments of the present invention. 
         FIG. 30  is a circuit diagram of illustrative circuitry that may be used in an accessory such as a headset to process user input and to supply corresponding tone-encoded signals to a corresponding electronic device in accordance with an embodiment of the present invention. 
         FIG. 31  is another circuit diagram of illustrative circuitry that may be used in an accessory such as a headset to process user input and to supply corresponding tone-encoded signals to a corresponding electronic device in accordance with an embodiment of the present invention. 
         FIG. 32  is a graph showing illustrative tones that may be conveyed between an accessory such as a headset and an electronic device when the accessory and electronic device are communicating in accordance with an embodiment of the present invention. 
         FIG. 33  is an illustrative table showing tone frequencies that may be used in a tone-based scheme for supporting communications between an accessory such as a headset and an electronic device in accordance with an embodiment of the present invention. 
         FIG. 34  is a circuit diagram of an illustrative tone detector that may be used in circuitry such as circuitry on an electronic device to process incoming tones from an accessory such as a headset in accordance with an embodiment of the present invention. 
         FIG. 35  is a diagram illustrating how tones may be processed by a tone generator of the type shown in  FIG. 33  in accordance with an embodiment of the present invention. 
         FIG. 36  is a flow chart of illustrative operations involved in handling tone-based communications between an accessory such as a headset and an electronic device in accordance with an embodiment of the present invention. 
         FIG. 37  is a flow chart of illustrative operations involved in determining what type of accessory is connected to an electronic device in accordance with an embodiment of the present invention. 
         FIG. 38  is a chart showing illustrative actions that may be taken in an electronic device in response to user input such as user input supplied to an accessory that is connected to the electronic device in accordance with embodiments of the present invention. 
         FIG. 39  is a chart showing additional illustrative actions that may be taken in an electronic device in response to user input such as user input supplied to an accessory that is connected to the electronic device in accordance with embodiments of the present invention. 
         FIG. 40  is a circuit diagram of illustrative circuitry that may be used in an accessory such as a headset without a microphone to process user input and to supply corresponding tone-encoded signals to a corresponding electronic device in accordance with an embodiment of the present invention. 
         FIG. 41  is a circuit diagram of illustrative circuitry without microphone line shorting buttons that may be used in an accessory such as a headset to process user input and to supply corresponding tone-encoded signals to a corresponding electronic device in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to electronic devices and accessories for electronic devices. 
     The electronic devices may be, for example, devices such as desktop computers or portable electronic devices such as laptop computers or small portable computers of the type that are sometimes referred to as ultraportables. The electronic devices may also be somewhat smaller portable electronic devices such as wrist-watch devices, pendant devices, and other wearable and miniature devices. If desired, the electronic devices may include wireless capabilities. 
     The electronic devices may be handheld electronic devices such as cellular telephones, media players with wireless communications capabilities, handheld computers (also sometimes called personal digital assistants), remote controllers, global positioning system (GPS) devices, and handheld gaming devices. The electronic devices may also be hybrid devices that combine the functionality of multiple conventional devices. Examples of hybrid electronic devices include a cellular telephone that includes media player functionality, a gaming device that includes a wireless communications capability, a cellular telephone that includes game and email functions, and a portable device that receives email, supports mobile telephone calls, has music player functionality and supports web browsing. These are merely illustrative examples. 
     An example of an accessory that may be used with an electronic device is a headset. A headset typically includes a pair of speakers that a user can use to play audio from the electronic device. The accessory may have a user control interface such as one or more buttons. When a user supplies input, the input may be conveyed to the electronic device. As an example, when the user presses a button on the accessory, a corresponding signal may be provided to the electronic device to direct the electronic device to take an appropriate action. Because the button is located on the headset rather than on the electronic device, a user may place the electronic device at a remote location such as on a table or in a pocket, while controlling the device using conveniently located headset buttons. 
     If the electronic device is a media player and is in the process of playing a song or other media file for the user, the electronic device may be directed to pause the currently playing media file when the user presses a button. As another example, if the electronic device is a cellular telephone with media player capabilities and the user is listening to a song when an incoming telephone call is received, actuation of the button by the user may direct the electronic device to answer the incoming telephone call. Actions such as these may be taken, for example, while the media player or cellular telephone is stowed within a user&#39;s pocket. 
     Accessories such as headsets are typically connected to electronic devices using audio plugs (male audio connectors) and mating audio jacks (female audio connectors). Audio connectors such as these may be provided in a variety of form factors. Most commonly, audio connectors take the form of 3.5 mm (⅛″) miniature plugs and jacks. Other sizes are also sometimes used such as 2.5 mm subminiature connectors and ¼ inch connectors. In the context of accessories such as headsets, these audio connectors and their associated cables are generally used to carry analog signals such as audio signals for speakers and microphone signals. Digital connectors such as universal serial bus (USB) and Firewire® (IEEE 1394) connectors may also be used by electronic devices to connect to external equipment such as headsets, but it is generally preferred to connect headsets to electronic devices using standard audio connectors such as the 3.5 mm audio connector. Digital connectors such as USB connectors and IEEE 1394 connectors are primarily of use where large volumes of digital data need to be transferred with external equipment such as when connecting to a peripheral device such as a printer. Optical connectors, which may be integrated with digital and analog connectors, may be used to convey data between an electronic device and an associated accessory, particularly in environments that carry high bandwidth traffic such as video traffic. If desired, audio connectors may include optical communications structures to support this type of traffic. 
     An illustrative system in accordance with an embodiment of the present invention is shown in  FIG. 1 . As shown in  FIG. 1 , system  10  may include an electronic device such as electronic device  12  and an accessory such as accessory  14 . A path such as path  16  may be used to connect electronic device  12  and accessory  14 . In a typical arrangement, path  16  includes one or more audio connectors such as 3.5 mm plugs and jacks or audio connectors of other suitable sizes. Conductive lines in path  16  may be used to convey signals over path  16 . There may, in general, be any suitable number of lines in path  16 . For example, there may be two, three, four, five, or more than five separate lines. These lines may be part of one or more cables. Cables may include solid wire, stranded wire, shielding, single ground structures, multi-ground structures, twisted pair structures, or any other suitable cabling structures. Extension cord and adapter arrangements may be used as part of path  16  if desired. In an adapter arrangement, some of the features of accessory  14  such as user interface and communications functions may be provided in the form of an adapter accessory with which an auxiliary accessory such as a headset may be connected to device  12 . 
     Accessory  14  may be any suitable device that works in conjunction with electronic device  12 . Examples of accessories include audio devices such as audio devices that contain or work with one or more speakers. Speakers in accessory  14  may be provided as an earphone or a headset or may be provided as a set of stand-alone powered or unpowered speakers (e.g., desktop speakers). Accessory  14  may, if desired, include audio-visual (AV) equipment such as a receiver, amplifier, television or other display, etc. Devices such as these may use path  16  to receive audio signals from device  12 . The audio signals may, for example, be provided in the form of analog audio signals that need only be amplified or passed to speakers to be heard by the user of device  12 . An optional microphone in accessory  14  may pass analog microphone signals to device  12 . Buttons or other user interface devices may be used to gather user input for device  12 . The use of these and other suitable accessories in system  10  is merely illustrative. In general, any suitable accessories may be used in system  10  if desired. 
     Electronic device  12  may be a desktop or portable computer, a portable electronic device such as a handheld electronic device that has wireless capabilities, equipment such as a television or audio receiver, or any other suitable electronic equipment. Electronic device  12  may be provided in the form of stand-alone equipment (e.g., a handheld device that is carried in the pocket of a user) or may be provided as an embedded system. Examples of systems in which device  12  may be embedded include automobiles, boats, airplanes, homes, security systems, media distribution systems for commercial and home applications, display equipment (e.g., computer monitors and televisions), etc. 
     Device  12  may communicate with network equipment such as equipment  18  over path  22 . Path  22  may be, for example, a cellular telephone wireless path. Equipment  18  may be, for example, a cellular telephone network. Device  12  and network equipment  18  may communicate over path  22  when it is desired to connect device  12  to a cellular telephone network (e.g., to handle voice telephone calls to transfer data over cellular telephone links, etc.). 
     Device  12  may also communicate with equipment such as computing equipment  20  over path  24 . Path  24  may be a wired or wireless path. Computing equipment  20  may be a computer, a set-top box, audio-visual equipment such as a receiver, a disc player or other media player, a game console, a network extender box, or any other suitable equipment. 
     In a typical scenario, device  12  may be, as an example, a handheld device that has media player and cellular telephone capabilities. Accessory  14  may be a headset with a microphone and a user input interface such as a button-based interface for gathering user input. Path  16  may be a four or five conductor audio cable that is connected to devices  12  and  14  using 3.5 mm audio jacks and plugs (as an example). Computing equipment  20  may be a computer with which device  12  communicates (e.g., to synchronize a list of contacts, media files, etc.). 
     While paths such as path  24  may be based on commonly available digital connectors such as USB or IEEE 1394 connectors, it may be advantageous to use standard audio connectors such as a 3.5 mm audio connector to connect device  12  to accessory  14 . Connectors such as these are in wide use for handling audio signals. As a result, many users have a collection of headsets and other accessories that use 3.5 mm audio connectors. The use of audio connectors such as these may therefore be helpful to users who would like to connect their existing audio equipment to device  12 . Consider, as an example, a user of a media player device. Media players are well known devices for playing media files such as audio files and video files that contain an audio track. Many owners of media players own one or more headsets that have audio plugs that are compatible with standard audio jacks. It would therefore be helpful to users such as these to provide device  12  with such a compatible audio jack, notwithstanding the availability of additional ports such as USB and IEEE 1394 high speed digital data ports for communicating with external devices such as computing equipment  20 . 
     Illustrative examples are shown in  FIGS. 2 and 3 . In the example of  FIG. 2 , device  12  is a portable computer. Portable computer  12  of  FIG. 2  has a display such as display  30  and user input equipment such as touch pad and keys  32 . As shown in  FIG. 2 , device  12  may have an audio jack such as jack  26  for receiving a mating audio plug. Device  12  may also have digital ports such as serial and parallel digital data ports (i.e., port  28 ). 
     In the example of  FIG. 3 , device  12  is shown as having a screen such as screen  30  and a user input device such as user interface device  32 . Device  32  may be, for example, a click wheel, a touch pad, keys, switches, or other suitable buttons, a touch screen, etc. Screen  30  may be, for example, a touch screen that covers a large fraction of the front face of device  12 . Audio jack  26  may be provided to allow a user to connect a headset or other accessory to device  12 . Additional connectors such as connector  28  may also be provided. Connector  28  may be a 30-pin connector, a USB port, etc. 
     If desired, connectors such as audio connector  26  in  FIGS. 2 and 3  may be the sole input-output connector on a given device  12 . Additional connectors may also be provided (e.g., one, two, three, or more than three additional connectors). Such additional connectors may be suitable for handling audio, digital signals, etc. 
     Illustrative audio connectors that may be used to interconnect device  12  and accessory  14  are shown in  FIG. 4 . As shown in  FIG. 4 , audio connectors  46  may include audio plugs such as plugs  34  and  36  that mate with corresponding audio jacks such as audio jacks  38  and  40 . Connectors  46  may be used at any suitable location or locations within path  16  ( FIG. 1 ). For example, audio jacks such as jacks  38  and  40  can be formed within the housing of device  12 , as shown in the examples of  FIGS. 2 and 3  and plugs such as plugs  34  and  36  can be formed on the end of a cable that is associated with a headset or other accessory  14 . As shown in  FIG. 4 , cable  70  may be connected to audio plug  34  via strain-relief plug structure  66  and cable  72  may be connected to audio plug  36  via strain-relief plug structure  68 . Structures such as structures  66  and  68  may be formed with an external insulator such as plastic (as an example). 
     Audio plug  34  is an example of a four-contact plug. A four-contact plug has four conductive regions that mate with four corresponding conductive regions in a four-contact jack such as jack  38 . As shown in  FIG. 4 , these regions may include a tip region such as region  48 , ring regions such as rings  50  and  52 , and a sleeve region such as region  54 . These regions surround the cylindrical surface of plug  34  and are separated by insulating regions  56 . When plug  34  is inserted in mating jack  38 , tip region  48  may make electrical contact with jack tip contact  74 , rings  50  and  52  may mate with ring regions  76  and  78 , and sleeve  54  may make contact with sleeve terminal  80 . In a typical configuration, there are four wires in cable  70 , each of which is electrically connected to a respective contact. Ring  52  may serve as ground. Tip  48  and ring  52  may be used together to handle a left audio channel (e.g., signals for a left-hand speaker in a headset). Ring  50  and ring  52  may be used for right channel audio. In accessories that contain microphones, ring  52  and sleeve  54  may be used to carry microphone audio signals from the accessory to electronic device  12 . Because this type of wiring scheme is commonly used in other devices, contacts such as contact  54  and the associated line in cable  70  (i.e., one of lines  88 ) are sometimes referred to as the microphone contact and microphone line, even when no microphone is present in accessory  14 . Plugs and accessories with this configuration have tip, outer ring, inner ring, and sleeve contacts that are respectively associated with left audio, right audio, ground, and microphone signals. If desired, plugs and jacks with other signal assignment schemes may be used. For example, sleeve  80  may be used for ground and ring  52  may be used as a microphone contact, etc. 
     Plug  36  of  FIG. 4  is an example of a three-contact audio connector. Tip  60  mates with region  82  in jack  40 . Ring  62  on plug  36  mates with ring region  84  in jack  40 . Sleeve region  64  electrically connects to region  86  in jack  40  when plug  36  is inserted in jack  40 . Regions  60 ,  62 , and  64  are separated by regions  58 . There is generally no microphone line in wires  90 , because tip  60  and ring  62  are used for left and right speaker signals. 
     As indicated by dashed lines  42 , it is physically possible to insert a four-connector plug such as plug  34  into a three-connector jack such as jack  40 , although doing so will short ring  52  of plug  34  to sleeve  54  of plug  34 , thereby preventing normal use of microphone contact  54  and the associated microphone line in lines  88  of cable  70 . Similarly, as indicated by dashed lines  44 , it is possible to physically insert a three-contact plug such as plug  36  into a four-contact jack such as jack  38 , although this will short regions  78  and  80  and will therefore not allow these regions to operate independently. If desired, audio connectors may be used that have more than four contacts or that have fewer than three contacts. For clarity, however, aspects of the invention will sometimes be described in the context of examples based on three-contact and four-contact audio connectors. 
     The  FIG. 4  examples are merely illustrative audio connectors that may be used to interconnect device  12  and accessory  14 . In general, audio connectors such as audio connectors  46  may be formed from any suitable plugs (male connectors) and any suitable jacks (female connectors) or any other suitable mating connectors. Moreover, connectors  46  may be placed at any suitable locations along path  16 . With a typical arrangement, a jack is mounted within device  12  and a mating plug is connected to accessory  14  by a cable. This is, however, merely illustrative. A jack may be mounted in accessory  14  and a plug may be connected to device  12  via a cable. As another example, jacks may be used in both device  12  and accessory  14  and a double-ended cable (i.e., a cable with male connectors on either end) may be used to connect device  12  with accessory  14 . Adapters may also be used. For example, an adapter may be plugged into device  12  (e.g., using a digital port). The adapter, which may be considered to be a type of accessory  14 , may be provided with a jack into which a plug from a headset or other equipment may be inserted to complete path  16 . In this type of scenario, the adapter may contain circuitry for performing functions that would otherwise be performed by buttons and circuitry on the headset. 
     An illustrative accessory is shown in  FIG. 5 . Accessory  14  of  FIG. 5  is a headset with a microphone. Speakers  92  may be provided in the form of over-the-ear speakers, ear plugs, or ear buds (as examples). Dual-conductor wires such as wires  94  may be used to connect speakers  92  to user interface main unit  96 . Unit  96  may include a microphone  98 . In some applications, microphone  98  may not be needed and may therefore be omitted from accessory  14  to lower cost. In other applications, such as cellular telephone application, voice recording applications, etc., microphone  98  may be used to gather audio signals (e.g., from the sound of a user&#39;s voice). 
     Unit  96  may include user input devices such as user input interface  100 . In the  FIG. 5  example, unit  96  includes three buttons. If desired, more buttons, fewer buttons, or non-button user input devices may be included in accessory  14 . Moreover, it is not necessary for these devices to be mounted to the same unit as microphone  98 . The  FIG. 5  arrangement is merely illustrative. If desired, unit  96  may be connected within one of the branch paths  94 , rather than at the junction between path  108  and paths  94 . This may help position a microphone within unit  96  closer to the mouth of a user, so that voice signals can be captured accurately. An illustrative headset with buttons and a microphone that may be located in this way and that may be used as an accessory  14  for electronic device  12  is described in commonly-assigned concurrently-filed patent application Ser. No. 12/203,866 and being entitled “Accessory Controller for Electronic Devices” (Wey-Jiun Lin et al.), which is hereby incorporated by reference herein in its entirety. An example of another multi-button headset on which accessory  14  may be based is described in commonly-assigned concurrently-filed patent application Ser. No. 12/203,872 and being entitled “In Cable Micro Input Devices” (Kurt Stiehl et al.), which is hereby incorporated by reference herein in its entirety. 
     In an illustrative three-button arrangement, a first of the three buttons such as button  102  may be pressed by a user when it is desired to advance among tracks being played back by a music application or may be used to increase a volume setting. A second of the three buttons, such as button  104  may be pressed when it is desired to stop music playback, answer an incoming cellular telephone call made to device  12  from a remote caller, or when it is desired to make a menu selection. A third of the three buttons such as button  106  may be selected when it is desired to move to an earlier track or when it is desired to lower a volume setting. Multiple clicks, click and hold operations, and other user input patterns may also be used. The up/down volume, forward/reverse track, and “answer call” examples described in connection with  FIG. 5  are merely illustrative. In general, the action that is taken in response to a given command may be adjusted by a system designer through modification of the software in device  12 . 
     As shown in  FIG. 5 , a cable such as cable  108  may be integrated into accessory  14 . At its far end, cable  108  may be provided with a connector such as audio connector  110 . In the  FIG. 5  example, accessory  14  has two speakers  92  and a microphone (microphone  98 ). Connector  110  may therefore be of the four-contact variety. In accessories in which microphone  98  or one of the speakers is omitted, signals can be carried over a three-contact connector. If desired, connectors with additional contacts may also be used (e.g., to carry auxiliary power, to carry control signals, etc.). Audio connectors with optical cores can be used to carry optical signals in addition to analog electrical signals. If desired, microphone  98  may be connected at a location along one of the wires leading to speakers  92 , as this may help position microphone  98  adjacent to the mouth of a user. 
     Accessory  14  may be provided with circuitry that helps convey signals from user input interface  100  to device  12  over path  16 . In general, any suitable communications format may be used to convey signals (e.g., analog, digital, mixed arrangements based on both analog and digital formats, optical, electrical, etc.). These signals may be conveyed on any suitable lines in path  16 . To avoid the need to provide extra conductive lines in path  16  and to ensure that accessory  14  is as compatible as possible with standard audio jacks, it may be advantageous to convey signals over existing lines (e.g., speaker, microphone, and ground). In particular, it may be advantageous to use the microphone and ground lines (e.g., the lines connected to contacts such as sleeve  54  and ring contact  52  in audio plug  34  of  FIG. 4 ) to convey signals such as user input signals and control signals between accessory  14  and electronic device  12 . 
     With one suitable communications arrangement, buttons such as buttons  102 ,  104 , and  106  may be encoded using different resistances. When a user presses a given button, device  12  can measure the resistance of user input interface  100  over the microphone and ground lines and can thereby determine which button was pressed. With another suitable arrangement, a button may be provided that shorts the microphone and ground wires in cable  108  together when pressed. Electronic device  12  can detect this type of momentary short. With yet another suitable arrangement, button presses within interface  100  may be converted to ultrasonic tones that are conveyed over the microphone and ground line. Electronic device  12  can detect and process the ultrasonic tones. 
     If desired, electronic device  12  can support communications using two or more of these approaches. Different approaches may be used, for example, to support both legacy hardware and new hardware, to support different types of software applications, to support reduced power operation in certain device operating modes, etc. 
     Ultrasonic tones lie above hearing range for human hearing (generally considered to be about 20,000 Hz). In a typical arrangement, the ultrasonic tones might fall within the range of 75 kHz to 300 kHz (as an example). Ultrasonic tones at frequencies of less than 75 kHz may be used, but may require more accurate circuitry to filter from normal microphone audio signals. Ultrasonic tones above 300 kHz may become susceptible to noise, because the conductors in many headset cables are not design to handle high-frequency signals. The cables can be provided with shielding and other structures that allow high speed signaling to be supported, or, more typically, lower tone frequencies may be used. 
     Ultrasonic tones may be formed using any suitable oscillating waveform such as a sine wave, saw (triangle) wave, square wave, etc. An advantage of saw and sine waves is that these waveforms contain a narrower range of harmonics than, for example, square waves. As a result, ultrasonic tones based on sine or saw waves may exhibit relatively narrow bandwidth. This may simplify detection and reduce the likelihood of audio interference. 
     Ultrasonic tones will not be audible to human hearing and therefore represent a form of out-of-band transmission. Arrangements that rely on ultrasonic tones in this way can avoid undesirable audible pops and clicks that might otherwise be associated with a button arrangement that momentarily shorts the microphone line and ground line together upon depression of a button and thereby momentarily disrupts normal operation of the microphone signal path. 
     In configurations in which the microphone and ground are shorted together upon button actuation events, it will generally not be possible to transmit audio information such as microphone signals while the microphone and ground line are shorted. An advantage of using devices that do not short the microphone and ground lines together such as devices that use ultrasonic tones to convey button actuation information (and that may therefore omit shorting switches between the microphone and ground lines) is that this allows audio information such as microphone signals to be transmitted in a continuous uninterrupted fashion. Even if a user is currently carrying on a telephone conversation, the user may press buttons that are ultrasonically encoded without interrupting the telephone conversation. Each time a button is pressed, the button press event results in the transmission of a corresponding ultrasonic tone, but does not short the microphone and ground lines. The other party to the user&#39;s telephone conversation will therefore be able to hear the user&#39;s voice without interruption. The microphone and ground lines can be used to convey microphone signals, while the user is able to control the operation of the user&#39;s device without concern about disturbing the conversation. 
     The ability to simultaneously make button presses and to carry on uninterrupted conversation is generally not present in conventional devices that rely on momentarily shorting of the microphone line to ground. This is because the shorting operation in conventional devices blocks transmission of microphone signals, whereas the ultrasonic tones that are used to represent button press events fall out of the human hearing range and can therefore be simultaneously transmitted with microphone signals without being audible to a user. 
     Circuitry may be provided within accessory  14  (e.g., within main unit  96 ) to handle operations associated with communicating between accessory  14  and device  12 . For example, circuitry may be provided in accessory  14  to transmit ultrasonic tones and to receive signals from device  12 . If desired, this circuitry may be provided in an accessory that takes the form of an adapter. 
     An illustrative arrangement that is based on an adapter is shown in  FIG. 6 . As shown in  FIG. 6 , headset  14  may have an audio plug  116  that plugs into a mating audio jack  114  on adapter  112  (itself a type of accessory  14 ). Plug  116  and jack  114  may be audio connectors such as audio connectors  46  of  FIG. 4 . Adapter  112  may include electrical paths that pass audio signals from device  12  to speakers in headset  14  and that pass microphone signals from microphone  98  to device  12 . Adapter  112  may also include the circuitry that handles communications with device  12  over path  16  that would otherwise be included within an accessory such as accessory  14  of  FIG. 5 . It is therefore not necessary for headset  14  in the  FIG. 6  arrangement to include this circuitry. In the  FIG. 6  example, headset  14  includes speakers  92  and microphone  98 , but need not include any buttons, because buttons  102 ,  104 , and  106  are included on accessory  112 . Accessory  112  may have a cable such as cable  108  with an audio connector  118  for plugging into a mating audio jack on device  12 . Adapter-type arrangements such as the arrangement of  FIG. 6  allow a user to add button functionality to an accessory such as a headset that does not include buttons. This may be particularly advantageous if a user already owns several different styles of buttonless headset, yet desires to use buttons such as buttons  102 ,  104 , and  106  to control electronic device  12  remotely. If desired, accessory  112  may be provided with a microphone. 
     Electronic device  12  and accessory  14  may communicate over paths such as path  16  using any suitable techniques. For example, device  12  may present one or more direct current (DC) voltages on suitable lines in path  16  (e.g., across the microphone and ground line pair). These DC voltages may bias any microphone that is present in accessory  14  and may serve as control signals. In turn, accessory  14  may communicate with device  12  using ultrasonic tones. Accessory  14  may also have resistively encoded buttons or other controls. In this type of arrangement, device  12  can bias the resistive network associated with the resistively encoded buttons and can sense the resulting voltage. Information on button activity can also be conveyed from accessory  14  to device  12  using a switch that momentarily shorts the microphone and ground lines in path  16  to each other. Shorts in accessory  14  lead to a detectable zero-voltage condition across these lines that can be detected by device  12 . 
     Arrangements such as these allow device  12  to discover which type of accessory  14  is attached to device  12  and allow user inputs to be conveyed from accessory  14  to device  12  during normal operation. If desired, other communications techniques may be used. For example, device  12  and accessory  14  may communicate using a bidirectional high-speed digital link. The link may be compliant with standard protocols such as the USB protocol (as an example). Digital data can also be conveyed using other buses (e.g., an RS-232 bus, a High-Definition Multimedia Interface (HDMI) bus, other parallel and serial buses, etc. If desired, device  12  may be provided with an ultrasonic transmitter so that device  12  may transmit ultrasonic tones to a mating ultrasonic receiver in accessory  14 . Accessory  14  may be provided with power supply circuitry that supplies various DC voltages to device  12  as a form of communication. Resistance coding may be used in device  12  (e.g., to allow accessory  14  to determine what type of device  12  is in communication with accessory  14 ). These arrangements, other suitable arrangements, and combinations of such arrangements may be used to support communications over path  16 . 
     In environments in which both device  12  and accessory  14  are able to transmit information over path  16 , handshaking schemes may be used. Handshaking may be used upon device power-up, when an accessory is plugged into device  12 , whenever accessory  14  transmits user input to device  12 , or at any other suitable time. Handshakes may take the form of confirmatory signals that indicate that devices are operating properly or that echo transmitted data to confirm signal integrity. Bidirectional exchanges of handshake-type information may also be used to identify equipment and to implement security features. For example, whenever an accessory is connected to device  12 , device  12  may query the attached accessory to determine the type of accessory that is in use and to verify that the accessory is authorized (e.g., with an appropriate security code or other identifier). 
     Support for extensive communications capabilities typically involves additional cost and complexity, so a designer of electronic devices such as device  12  and accessories such as accessory  14  may need to make tradeoffs. For some applications, it may be desirable to forego extensive bidirectional communications support in the interests of reducing weight, cost, complexity, and power consumption requirements. For other applications, issues such as security and data integrity may be more important. In environments such as these, the inclusion of more extensive communications circuitry in device  12  and accessory  14  may be justified. 
     A generalized diagram of an illustrative electronic device  12  and accessory  14  is shown in  FIG. 7 . In the  FIG. 7  example, device  12  and accessory  14  are shown as possibly including numerous components for supporting communications and processing functions. If desired, some of these components may be omitted, thereby reducing device cost and complexity. The inclusion of these components in the schematic diagram of  FIG. 7  is merely illustrative. 
     Device  12  may be, for example, a computer or handheld electronic device that supports cellular telephone and data functions, global positioning system capabilities, and local wireless communications capabilities (e.g., IEEE 802.11 and Bluetooth®) and that supports handheld computing device functions such as internet browsing, email and calendar functions, games, music player functionality, etc. Accessory  14  may be, for example, a headset with or without a microphone, a set of stand-alone speakers, audio-visual equipment, an adapter (e.g., an adapter such as adapter  112  of  FIG. 6 ), an external controller (e.g., a keypad), or any other suitable device that may be connected to device  12 . Path  16  may include audio connectors such as connectors  46  of  FIG. 4  or other suitable connectors. 
     As shown in  FIG. 7 , device  12  and accessory  14  may include storage  126  and  144 . Storage  126  and  144  may include one or more different types of storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory), volatile memory (e.g., static or dynamic random-access-memory), etc. 
     Processing circuitry  128  and  146  may be used to control the operation of device  12  and accessory  14 . Processing circuitry  128  and  146  may be based on processors such as microprocessors and other suitable integrated circuits. These circuits may include application-specific integrated circuits, audio codecs, video codecs, amplifiers, communications interfaces, power management units, power supply circuits, circuits that control the operation of wireless circuitry, radio-frequency amplifiers, digital signal processors, analog-to-digital converters, digital-to-analog converters, or any other suitable circuitry. 
     With one suitable arrangement, processing circuitry  128  and  146  and storage  126  and  144  are used to run software on device  12  and accessory  14 . The complexity of the applications that are implemented depends on the needs of the designer of system  10 . For example, the software may support complex functionality such as internet browsing applications, voice-over-internet-protocol (VOIP) telephone call applications, email applications, media playback applications, operating system functions, and less complex functionality such as the functionality involved in encoding button presses as ultrasonic tones. To support communications over path  16  and to support communications with external equipment such as equipment  18  and  20  of  FIG. 1 , processing circuitry  128  and  146  and storage  126  and  144  may be used in implementing suitable communications protocols. Communications protocols that may be implemented using processing circuitry  128  and  146  and storage  126  and  144  include internet protocols, wireless local area network protocols (e.g., IEEE 802.11 protocols—sometimes referred to as Wi-Fi®), protocols for other short-range wireless communications links such as the Bluetooth® protocol, protocols for handling 3G communications services (e.g., using wide band code division multiple access techniques), 2G cellular telephone communications protocols, serial and parallel bus protocols, etc. In a typical arrangement, more complex functions such as wireless functions are implemented exclusively or primarily on device  12  rather than accessory  14 , but accessory  14  may also be provided with some or all of these capabilities if desired. 
     Input-output devices  130  and  148  may be used to allow data to be supplied to device  12  and accessory  14  and may be used to allow data to be provided from device  12  and accessory  14  to external destinations. Input-output devices  130  and  148  can include devices such as non-touch displays and touch displays (e.g., based on capacitive touch or resistive touch technologies as examples). Visual information may also be displayed using light-emitting diodes and other lights. Input-output devices  130  and  148  may include one or more buttons. Buttons and button-like devices may include keys, keypads, momentary switches, sliding actuators, rocker switches, click wheels, scrolling controllers, knobs, joysticks, D-pads (direction pads), touch pads, touch sliders, touch buttons, and other suitable user-actuated control interfaces. Input-output devices  130  and  148  may also include microphones, speakers, digital and analog input-output port connectors and associated circuits, cameras, etc. Wireless circuitry in input-output devices  130  and  148  may be used to receive and/or transmit wireless signals. 
     As shown schematically in  FIG. 7 , input-output devices  130  may sometimes be categorized as including user input-output devices  132  and  150 , display and audio devices  134  and  152 , and wireless communications circuitry  136  and  154 . A user may, for example, enter user input by supplying commands through user input devices  132  and  150 . Display and audio devices  134  and  152  may be used to present visual and sound output to the user. These categories need not be mutually exclusive. For example, a user may supply input using a touch screen that is being used to supply visual output data. 
     As indicated in  FIG. 7 , wireless communications circuitry  136  and  154  may include antennas and associated radio-frequency transceiver circuitry. For example, wireless communications circuitry  136  and  154  may include communications circuitry such as radio-frequency (RF) transceiver circuitry formed from one or more integrated circuits, power amplifier circuitry, passive RF components, antennas, and other circuitry for handling RF wireless signals. Wireless signals can also be sent using light (e.g., using infrared communications). 
     The antenna structures and wireless communications devices of devices  12  and accessory  14  may support communications over any suitable wireless communications bands. For example, wireless communications circuitry  136  and  154  may be used to cover communications frequency bands such as cellular telephone voice and data bands at 850 MHz, 900 MHz, 1800 MHz, 1900 MHz, and 2100 MHz (as examples). Wireless communications circuitry  136  and  154  may also be used to handle the Wi-Fi® (IEEE 802.11) bands at 2.4 GHz and 5.0 GHz (also sometimes referred to as wireless local area network or WLAN bands), the Bluetooth® band at 2.4 GHz, and the global positioning system (GPS) band at 1575 MHz. 
     Although both device  12  and accessory  14  are depicted as containing wireless communications circuitry in the  FIG. 7  example, there are situations in which it may be desirable to omit such capabilities from device  12  and/or accessory  14 . For example, it may be desired to power accessory  14  solely with a low-capacity battery or solely with power received through path  16  from device  12 . In situations such as these, the use of extensive wireless communications circuitry may result in undesirably large amounts of power consumption. For low-power applications and situations in which low cost and weight are of primary concern, it may therefore be desirable to limit accessory  14  to low-power consumption wireless circuitry (e.g., infrared communications) or to omit wireless circuitry from accessory  14 . Moreover, not all devices  12  may require the use of extensive wireless communications capabilities. A hybrid cellular telephone and media player device may benefit from wireless capabilities, but a highly portable media player may not require wireless capabilities and such capabilities may be omitted to conserve cost and weight if desired. 
     Transceiver circuitry  120  and  138  may be used to support communications between electronic device  12  and accessory  14  over path  16 . In general, both device  12  and accessory  14  may include transmitters and receivers. For example, device  12  may include a transmitter that produces signal information that is received by receiver  142  in accessory  14 . Similarly, accessory  14  may have a transmitter  140  that produces data that is received by receiver  124  in device  12 . If desired, transmitters  122  and  140  may include similar circuitry. For example, both transmitter  122  and transmitter  140  may include ultrasonic tone generation circuitry (as an example). Receivers  124  and  142  may each have corresponding tone detection circuitry. Transmitters  122  and  140  may also each have DC power supply circuitry for creating various bias voltages, digital communications circuitry for transmitting digital data, or other suitable transmitter circuitry, whereas receivers  124  and  142  may have corresponding receiver circuitry such as voltage detector circuitry, digital receivers, etc. Symmetric configurations such as these may allow comparable amounts of information to be passed in both directions over link  16 , which may be useful when accessory  14  needs to present extensive information to the user through input-output devices  148  or when extensive handshaking operations are desired (e.g., to support advanced security functionality). 
     It is not, however, generally necessary for both device  12  and accessory  14  to have identical transmitter and receiver circuitry. Device  12  may, for example, be larger than accessory  14  and may have available on-board power in the form of a rechargeable battery, whereas accessory  14  may be unpowered (and receiving power only from device  12 ) or may have only a small battery (for use alone or in combination with power received from device  12 ). In situations such as these, it may be desirable to provide device  12  and accessory  14  with different communications circuitry. 
     As an example, transmitter  122  in device  12  may include adjustable DC power supply circuitry. By placing different DC voltages on the lines of path  16  at different times, device  12  can communicate relatively modest amounts of data to accessory  14 . This data may include, for example, data that instructs accessory  14  to power its microphone (if available) or to respond with an acknowledgement signal. A voltage detector and associated circuitry in receiver  138  of accessory  14  may process the DC bias voltages that are received from device  12 . In this type of scenario, transmitter  140  in accessory  14  may include an ultrasonic tone generator that supplies acknowledgement signals and user input data (e.g., button press data) to device  12 . A tone detector in receiver  124  may decode the tone signals for device  12 . 
     Applications running on the processing circuitry of device  12  may use the decoded user input data as control signals. As an example, a cellular telephone application may interpret the user input as commands to answer or hang up a cellular telephone call, a media playback application may interpret the user input as commands to skip a track, to pause, play, fast-forward, or rewind a media file, etc. Still other applications may interpret user button-press data or other user input as commands for making menu selection, etc. 
     Illustrative steps involved in using electronic device  12  and accessory  14  are shown in  FIG. 8 . At step  156 , a user may connect accessory  14  to device  12 . For example, a user may insert a male audio connector such as one of the audio plugs of  FIG. 4  into a mating female connector in device  12  such as one of the audio jacks in  FIG. 4 . (If an adapter such as adapter  112  of  FIG. 6  is being used, the user plugs adapter  112  into device  12  and plugs accessory  14  into adapter  112 .) The process of attaching accessory  14  to device  12  involves creating a wired path (e.g., path  16 ) in which contacts in the audio connector of the accessory mate with corresponding contacts in the audio connector of the device and thereby connect the conductive lines of path  16  between device  12  and accessory  14 . 
     Once device  12  and accessory  14  have been electrically interconnected in this way, the electronic device and the accessory may interact at step  158 . In general, the interactions of step  158  may include transmission and reception by device  12  and accessory  14  of any suitable signals (e.g., using the transceiver circuitry  120  and  138  of  FIG. 7 ). In one suitable arrangement, device  12  supplies various DC bias voltages to accessory  14  over the microphone line and the ground line. In response accessory may transmit an ultrasonic acknowledgement tone. If desired, information on the identity of the accessory  14  (e.g., its type, serial number, part number, associated user identity, or other suitable information) may be conveyed to device  12  by encoding information in ultrasonic tones. The point in the biasing process at which an ultrasonic tone is conveyed to device  12  may also be used as an indicator of accessory identity information or other suitable information. If desired, device  12  and accessory  14  may be prevented from operating together until suitable handshaking or security authentication criteria have been satisfied. 
     Device  12  may, in response to information received from accessory  14 , or in response to the needs of an application running on device  12 , make bias voltage adjustments after accessory  14  has been identified or proper operation confirmed. Such bias voltage adjustments may, for example, be used to place accessory  14  and device  12  in one or more desired modes of operation. These modes may include, for example, a tone mode in which user input is conveyed using ultrasonic tones and a resistance detection mode in which user input is conveyed using a resistively encoded button actuation arrangement. 
     Once device  12  has completed all desired start-up operations (e.g., accessory discovery, confirmation operations, authentication, etc.), processing may proceed to step  160 . During the operations of step  160 , a user may operate device  12  and accessory  14  in a normal user mode of operation. At this time, the user may supply input to accessory  14  using input-output devices  148 . As an example, the user may press or otherwise actuate a button or other switch, the user may press an appropriate portion of a touch screen or touch-sensitive button, the user may actuate a joystick, or may make any other user input. This user input may be transmitted to device  12  and received by transceiver circuitry  120 . At the same time, if accessory  14  has a microphone, sound input may be gathered and conveyed to device  12  over path  16  (e.g., over the microphone and ground lines). A corresponding microphone amplifier in processing circuitry  128  may be used to receive this audio signal. Device  12  may also supply output to the user. For example, device  12  may play audio signals through speakers in accessory  14  using speaker lines (and the associated ground line) in path  16 . Other output (e.g., video, status information, etc.) may also be conveyed to the user and presented using input-output devices  148 . 
     During the operations of step  160 , the mode of operation of device  12  and accessory  14  may change. For example, if a user switches between using a telephone application (in which a microphone is required to capture the user&#39;s voice) to a media playback operation (in which the microphone is not used), device  12  and accessory  14  may switch from a tone mode (in which user input is conveyed as ultrasonic tones from accessory  14  to device  12 ) to a resistance detection mode (in which device  12  monitors the resistance of a resistor network associated with buttons on accessory  14  to determine which buttons are pressed). There may be advantages to using one mode over the other. For example, one mode of operation (e.g., the resistance detection mode) may consume less power or may be compatible with a wider range of accessories  14 . Mode adjustments may be made when different applications are launched on device  12 , when a given application exercises a different set of features, or at any other suitable time. In some situations, different operating modes may be invoked when a user removes an accessory of one type and connects an accessory of a different type. 
     If desired, each different type of device  12  may be configured to operate properly with only a particular corresponding type of accessory. More generally, it can be advantageous to allow various devices and accessories to operate with one another. In environments such as these, the functionality that is available to the user may vary depending on which capabilities are available in the device and accessory. 
     A basic accessory is shown in  FIG. 9 . In the example of  FIG. 9 , accessory  14  has two speakers  92 . Accessory  14  of  FIG. 9  may be, for example, a stereo headset of a set of accessory speakers. Conductive lines  94  may be used to connect speakers  92  to left and right terminals L and R and ground terminal G. These terminals may be associated with appropriate contacts in a plug such as plug  36  of  FIG. 4 . Because the illustrative accessory of  FIG. 9  does not contain a microphone or buttons, the user of this accessory is only able to receive audio and is not able to supply audio or button press information. 
     Corresponding circuitry  162  that may be used in device  12  to supply audio signals to speakers  92  of accessory  14  of  FIG. 9  is shown in  FIG. 10 . As shown in  FIG. 10 , circuitry  162 , which may be part of processing circuitry  128  of  FIG. 7 , may have digital-to-analog converter  164  and amplifiers  166  and  168 . Digital-to-analog converter circuitry  164  may receive digital input from a processor using digital input  170  and may supply corresponding analog audio signals to terminals L, R, and G using amplifiers  166  and  168  and lines  172 . Terminals L, R, and G may be associated with contacts in an audio connector as described in connection with  FIG. 4 . 
     Circuitry such as circuitry  162  may be used in device  12  whenever it is desired to provide speakers in accessory  14  with audio signals. In complex devices  12 , additional circuitry may be used (e.g., to gather microphone signals and user input signals corresponding to button actuation events from an appropriate accessory  14 ). In simpler devices  12 , some or all of this additional circuitry may be omitted. 
       FIG. 11  is a circuit diagram of an illustrative configuration for accessory  14  that includes a microphone. As shown in  FIG. 11 , lines  94  may be used to connect terminals M, G, R, and L to speakers  92 , microphone  174 , and switch  176 . Switch  176  may be associated with a user-actuated button. Line  94 A is connected to microphone terminal M and may therefore sometimes be referred to as a microphone line. Line  94 B is connected to ground terminal G and may be referred to as a ground line. With the arrangement of  FIG. 11 , audio signals may be driven onto the left speaker using the L and G terminals and may be driven onto the right speaker using the R and G terminals. Microphone signals (e.g., the user&#39;s voice or other audio input) may be conveyed to terminals M and G from microphone  174  using lines  94 A and  94 B. When a user actuates switch  176 , lines  94 A and  94 B may be momentarily electrically connected to each other. This creates a low-impedance path from terminal M to terminal G that bypasses microphone  174 . If microphone  174  is in use during the button actuation event, a click or pop may arise on the microphone line due to current surges associated with the momentary short. The presence of the momentary short may be detected using circuitry in device  12 , which may then take appropriate action. 
     Illustrative circuitry  178  that may be used in device  12  when interfacing with an accessory of the type shown in  FIG. 11  is shown in  FIG. 12 . Circuitry  178  may be, for example, part of processing circuitry  128  of  FIG. 7 . As shown in  FIG. 12 , circuitry  178  may include a power supply  180 . Power supply  180  may be a fixed or adjustable power supply and may impose a bias voltage on line  184  via resistor  182  (which may serves as a microphone signal load resistor). This bias voltage may be placed across the microphone terminal M and ground terminal G, thereby biasing microphone line  94 A relative to ground line  94 B in a corresponding accessory such as accessory  14  of  FIG. 11 . When a user shorts lines  94 A and  94 B in an accessory such as accessory  14  of  FIG. 11 , the M and G terminals in circuitry  178  will likewise be shorted together. The lack of any appreciable voltage drop across terminals M and G can be detected using comparator  186 . Comparator  186  may have a first input such as input  190  that is connected to microphone terminal M in circuitry  178  via line  184  and may have a second input such as input  188  that receives a reference voltage VREF (e.g., 200 mV). Whenever the difference in voltage between the M and G terminals falls below VREF, comparator  186  may adjust its output voltage on output  192  (e.g., by taking a logic low signal on output  192  to a logic high signal or vice versa). This change in the output of comparator  186  may be processed by downstream processing circuitry in device  12  (e.g., to instruct device  12  to take an appropriate action in an applicable software application). 
     Microphone amplifier  194  in circuitry  178  of  FIG. 12  may be used to amplify microphone signals in normal operation (i.e., when switch  176  of  FIG. 11  is not closed). These signals may be received over path  16  from accessory  14  using the amplified microphone terminal M and ground terminal G. A corresponding amplified microphone audio signal may be supplied to analog-to-digital converter  196  over path  198 . Analog-to-digital converter  196  may digitize the analog microphone signal. A corresponding digitized version of the microphone signal may be supplied on output path  200  for subsequent processing (e.g., for wireless transmission to a remote location as part of a cellular telephone call, etc.). Digital-to-analog converter  164  may be used to convert digital audio signals to analog audio signals that are driven onto left and right speaker terminals L and R by amplifiers  166  and  168 . If desired, the components of circuitry  178  may be integrated onto one or more integrated circuits. For example, microphone amplifier  194  may be provided as part of the same integrated circuit as analog-to-digital converter  196  (as an example). As another example, digital-to-analog converter  164  and analog-to-digital converter  196  may be supplied as part of the same integrated circuit. Other configurations may also be used (e.g., in which all of circuitry  178  is included on a single chip). 
     A flow chart of illustrative operations involved in processing user input gathered using a button such as button  176  in an accessory of the type shown in  FIG. 11  is shown in  FIG. 13 . At step  202 , a user may connect the headset or other accessory to device  12 . At step  204 , when contact is made between the mating terminals of the female and male portions of the audio connector, electronic device  12  may bias microphone line  94 A in accessory  14 . For example, microphone line  94 A may be raise to a voltage of 2.7 volts above ground line  94 B. This bias may supply microphone  174  with power and may make it possible for device  12  to detect shorts between lines  94 A and  94 B that result from actuation of button  176 . 
     When a user actuates button  176 , terminals M and G in device circuitry such as circuitry  178  of  FIG. 12  are shorted together. When this change is detected (at step  206 ), the state of output  192  is adjusted by comparator  186 . Because the output of comparator  186  is reflective of the occurrence of a button actuation event, processing circuitry in device  12  can conclude that button  176  has been pressed and may take appropriate action. 
     If desired, more than one user-actuated button may be provided in accessory  14 . To distinguish between actuation events that involve different buttons, each button may generate a different resulting signal. The different signals may be different digital codes, different analog signals, etc. At device  12 , the signals that are generated by a given button actuation event may be processed to determine which button was pressed (and for how long). Device  12  may then take appropriate action. 
     With one suitable arrangement, buttons (or other user input interface devices) may use a resistive-encoding scheme. With this type of arrangement, actuation of different buttons results in different resistance values within an appropriate portion of the circuitry of accessory  14 . As an example, consider the arrangement of  FIG. 14 . In the  FIG. 14  example, accessory  14  has speakers  92 , but no microphone (as an example). Buttons in accessory  14  control corresponding switches. For example, a first button may control switch  176  and second, third, and fourth buttons may control respective switches  210 . The buttons may have a mechanical lock-out feature that allows only a single button or other suitable number of buttons to be pressed simultaneously or device  12  may analyze simultaneous button presses based on known rules (e.g., by accepting only the button that is pressed first, by associating particular actions with particular combinations of button presses, etc.). In a typical arrangement, only a single button is pressed at a time. 
     When a button is actuated, the configuration of the resistor network formed by resistors  208  changes. As a result, the resistance between terminals M and G (or other suitable audio connector terminals in accessory  14 ) changes. The resulting resistance between terminals M and G can be measured and acted upon by device  12 . 
     Schemes of the type shown in  FIG. 14  in which buttons are associated with various resistors are said to use resistance encoding. With resistively encoded button arrangements, device  12  can determine which buttons are actuated by analyzing the resistance across terminals M and G. If, for example, the leftmost switch  210  in  FIG. 14  is closed, the leftmost resistor  208  will be switched into place. If the middle switch  210  is closed, the leftmost and middle resistors  208  will be switched into place. Closing the rightmost switch  210  will switch all three of the  FIG. 14  resistors into place. Resistors  208  may all have the same resistance or may have different resistances, provided that the resulting resistor network allows device  12  to discriminate between different button presses. 
     In the  FIG. 14  example, button  176  is of the “shorting” variety described in connection with  FIG. 11 . This type of button may be included in the resistive network formed by resistors  208  if desired. Device  12  can discriminate between actuation of button  176  and actuation of buttons  210 , because only actuation of button  176  will result in a short circuit between terminals M and G. Button  176  is optional. Moreover, any suitable number of resistively encoded buttons such as buttons  210  may be provided if desired. The example of  FIG. 14  includes three buttons, but, as indicated by dots  212 , more than three resistively encoded buttons may be included in accessory  14  if desired. Arrangements with fewer resistively encoded buttons may also be used. 
     When accessory  14  has resistively encoded switches, device  12  may be provided with circuitry such as circuitry  214  of  FIG. 15 . When it is desired to determine which resistively encoded switch has been actuated by a user, circuitry  214  may use power supply  180  to supply a known bias voltage across terminals M and G. The bias voltage may, for example, be supplied through resistor  182 . The known bias voltage across the M and G terminals in device  12  results in a known voltage drop between lines  94 A and  94 B in  FIG. 14 . In this type of arrangement, the resistance of resistor  182  and the resistance of the components between lines  94 A and  94 B form a voltage divider. The voltage drop across terminals M and G in circuitry  214  of  FIG. 15  will change depending on the resistance produced between lines  94 A and  94 B by buttons  210  and their associated resistors. 
     Voltage detector  216  may monitor the resulting voltage on terminal M relative to ground terminal G and may produce corresponding digital output signals on output  218  for processing by processing circuitry on device  12 . When the voltage drop across the M and G terminals is high, device  12  can conclude that no buttons have been depressed. When the voltage drop measured by voltage detector  216  is zero or close to zero, device  12  may conclude that switch  176  has been pressed (if such a switch is used). Intermediate values of voltage can be correlated with particular switch actuation patterns in resistively encoded switches  210 . 
     In the  FIG. 14  arrangement, resistors  208  are connected along line  94 A and each switch  210  is connected along this resistive ladder at a respective tap point. This is merely an illustrative example of a suitable resistor network that may be used for resistively encoding switches in accessory  14 . Another illustrative arrangement is shown in  FIG. 16 . In the  FIG. 16  example, each resistively encoded switch  210  has a respective series-connected resistor  208 . The resistance values of resistors  208  in arrangements of the type shown in  FIG. 16  are preferably each different, allowing discrimination between switches. If the user presses the first switch, the resistance between lines  94 A and  94 B will be resistance R 1 , if the user presses the second switch, the resistance will be equal to R 2 , and if the user presses the third switch, the resistance will be R 3 . An optional shorting switch such as switch  176  may be connected in parallel with resistively encoded switches  210  if desired. The resistance value resulting from user actuation of desired switches  210  in accessory  14  of  FIG. 16  may be measured using any suitable resistance measuring circuitry such as the biasing power supply and voltage detector circuits of  FIG. 15 . 
     A flow chart of illustrative steps involved in using circuitry of the type shown in  FIG. 15  to determine which of multiple resistively encoded switches in an accessory such as accessory  14  of  FIG. 14  or accessory  14  of  FIG. 16  has been pressed is shown in  FIG. 17 . At step  220 , the resistive network associated with the buttons in accessory  14  may be biased using an appropriate bias voltage. The bias voltage may, for example, be generated by power supply  180  in device  12 , as described in connection with  FIG. 15 . 
     When a user presses a button in the user input interface portion of accessory  14 , the resistance bridging a given pair of lines in path  16  such as the microphone and ground lines is altered. In circuits such as circuit  214  of  FIG. 15 , the bridging resistance between lines  94 A and  94 B in the accessory forms a voltage divider in combination with the resistance of resistor  182 . The fraction of the bias voltage supplied by power supply  180  that falls across terminals M and G in circuit  214  is therefore determined according to Ohm&#39;s law and can be measured using voltage detector  216  (step  222 ). A corresponding digital signal that identifies which button was pressed may be supplied on output line  218  (step  224 ). Simultaneous button presses can result in different detectable resistances (e.g., intermediate resistance values). Device  12  may respond accordingly (e.g., by taking an appropriate action in response to the set of buttons that is pressed, by ignoring multiple simultaneous button presses, etc.). 
     The embodiments of accessory  14  illustrated in  FIGS. 9 ,  11 ,  14 , and  16  are merely illustrative. For example, features of these different accessory arrangements may be combined in other topologies if desired. A circuit diagram of a generalized accessory  14  that may be used in system  10  is shown in  FIG. 18 . In the example of  FIG. 18 , accessory  14  has been provided with a four-contact audio connector such as jack  34  of  FIG. 4 , having terminals M, G, L, and R. This is, however, merely illustrative. Accessory  14  may be provided with any suitable connector. 
     As shown in  FIG. 18 , accessory  14  may have speakers  92 . There may, in general, be no speakers  92 , one speaker  92 , two speakers  92 , or any other suitable number of speakers in a given accessory. Accessories such as headsets typically include two speakers, so accessory  14  is sometimes described herein as including two speakers as an example. 
     Accessory  14  may also have circuitry  226 . Circuitry  226  may include one or more optional microphones such as microphone  230  or other audio transducer equipment. Microphone  230  may be implemented using any suitable powered or unpowered microphone technology. For example, microphone  230  may be an electret microphone or a microphone formed using microelectromechanical systems (MEMS) technology. Microphone  230  may also be based on other suitable arrangements (e.g., dynamic microphones, condenser microphones, piezoelectric microphones, etc.). 
     User input interface  232  may be used to gather input from a user. In a typical arrangement, user input interface  232  may include buttons. This is, however, merely illustrative. User input interface  232  may include a touch screen, a touch pad, a touch-sensitive button, buttons that make up a portion of a keypad, a joystick, a camera, a proximity sensor, a temperature sensor, an accelerometer, an ambient light sensor, or any other suitable device for gathering input (e.g., input gathered from a user that is associated with a user interaction with accessory  14 ). 
     Control circuitry  228  may be used in processing the user input that has been gathered and may be used in transmitting the user input to device  12  over path  16 . If, as an example, user input interface  232  includes a touch screen sensor, control circuitry  228  may be used to determine the location on the sensor that has been touched by a user. Control circuitry  228  may then transmit corresponding information to device  12  that indicates the nature of the user&#39;s input. As another example, user input interface  232  may include an array of buttons. When a user presses a given button, control circuitry  228  may be used to determine which button has been pressed. Control circuitry  228  may communicate this information to device  12 , so that device  12  may take appropriate actions. 
     With one suitable arrangement, control circuitry  228  may include ultrasonic tone generator circuitry that may be used to transmit user input information to device  12  in the form of ultrasonic tones. This is, however, merely illustrative. Any suitable format may be used for transmitting information on user input to device  12 . Moreover, circuitry  226  may, if desired, include circuits of the types described in connection with  FIGS. 11 ,  14 , and  16  in which button activity is conveyed to device  12  by momentarily shorting the microphone and ground lines or by using resistively encoded buttons (as examples). In accessories that contain multiple different types of button configurations such as these, device  12  and accessory  14  may switch between different modes of operation depending, for example, on which applications or application features are being exercised by device  12  at a given point in time. Different modes of operation may also be applicable when particular accessories or particular devices are used. For example, in one mode of operation, an electronic device may monitor the microphone and ground lines in accessory  14  directly to attempt to detect events corresponding to actuation of button  176  or buttons  210 , whereas in another mode of operation, the electronic device may use an internal tone detector to determine whether the accessory is attempting to transmit user input in the form of ultrasonic tones. 
     If desired, the buttons or other user interface used in accessory  14  may avoid the use of buttons that momentarily short the microphone and ground lines together. Instead, buttons may, for example, be used to control an ultrasonic tone generator that sends button press information to an electronic device over the microphone and ground lines in the form of ultrasonic pulses. With this type of scheme, button press events will not momentarily short the microphone and ground lines together, so pops, clicks, and dead time that might be associated with switches of the type that short the microphone and ground lines together may be avoided. This allows continuous uninterrupted use of the microphone and ground lines (e.g., for carrying on a telephone call while button presses are being made). The use of ultrasonic tones may also help avoid interference with telephone calls, because ultrasonic tones on the microphone line will fall outside the range of human hearing and will therefore not be audible to users. Multiple buttons can be represented by using more than one ultrasonic tone. User interface  232  may therefore contain one button, two buttons, three buttons, or more than three buttons. 
       FIG. 19  shows an illustrative arrangement for an accessory such as accessory  14  showing how circuitry  228  may include impedance detector  236  and associated control circuit  238 . In this type of arrangement, user input interface  232  may include any suitable resistively encoded components. As shown in  FIG. 19 , for example, user input interface  232  may include an array of resistively encoded buttons  210 . Using the illustrative resistive network topology of  FIG. 19 , each of resistors  208  may have a different resistance value. Impedance detector  236  may be connected to buttons  210  and the resistor network formed by resistors  208 . When a user actuates a given one of switches  210 , impedance detector  236  may detect the resulting resistance (i.e., R 1 , R 2 , R 3 , or R 4  in this example) between impedance detector  236  and line  234  (e.g., the ground line or the microphone line as examples). Impedance detector  236  may then inform control circuit  238  of the identity of the switch that has been actuated by the user. Control circuit  238  may transmit this information to device  12  (e.g., using transmitter  140  of  FIG. 7 ). If a user presses more than one button simultaneously, the resulting resistance detected by impedance detector  236  may be an intermediate resistance value such as R 1 *R 2 /(R 1 +R 2 ) if the “R 1 ” and “R 2 ” buttons are pressed. Device  12  may respond to simultaneous button presses such as these by taking an appropriate action in response to the particular set of buttons that is pressed, by ignoring multiple simultaneous button presses, etc. 
     In arrangements in which accessory  14  includes control circuitry such as control circuit  238 , it is not necessary to use resistance encoding for buttons  210 . An arrangement in which control circuitry  228  has been implemented without the resistors  208  of  FIG. 19  is shown in  FIG. 20 . In arrangements of the type shown in  FIG. 20 , each switch  210  has two terminals. Each terminal  240  is connected to control circuit  228  and each terminal  242  is connected to a suitable circuit node (e.g., line  234 , which may be, for example, a line that has been biased to a particular voltage such as a microphone line or ground line). When one of switches  210  is closed, control circuit  228  can detect which of the terminals  240  has been electrically connected to line  234 . In response, control circuit  228  can transmit information to device  12  indicative of which switch has been selected (e.g., using a tone generator or other transmitter circuitry such as transmitter  140  of  FIG. 7 ). If multiple buttons are pressed simultaneously, device  12  may take an appropriate action such as a particular action associated with the combination of buttons that have been pressed. Device  12  may also be configured to ignore simultaneous button press events. 
     Illustrative circuitry  244  that may be used in device  12  to interface with an accessory that contains a tone generator is shown in  FIG. 21 . There may be one or more circuits such as circuitry  244  in a given electronic device. For example, a laptop with two such circuits may be provided to allow two users to listen to media, each having their own separate volume control. 
     As shown in  FIG. 21 , circuitry  244  may include a power supply  180  for biasing microphone line M through resistor  182  in relation to ground line G. Power supply  180  may be an adjustable voltage supply that device  12  uses to bias the microphone line in accessory  14  to one or more different levels. Accessory  14  may, if desired, include circuitry that is responsive to the different bias voltages (e.g., to place accessory  14  into different modes, to direct accessory  14  to send an acknowledgement signal, or to cause accessory to take other suitable actions in response to the received bias from power supply  180 ). 
     Tone detector  246  may be coupled to microphone terminal M, as shown in  FIG. 21 . When accessory  14  transmits ultrasonic tones over path  16 , tone detector  246  may receive those tones on input  248 . After processing (e.g., to identify the nature of the incoming tone signal), tone detector  250  may generate a suitable output on output  250 . Output  250  may, for example, be used to provide digital signals to downstream processing circuitry so that device  12  can identify which buttons have been pressed and can identify what other tone-based information has been received from accessory  14 . 
     Illustrative steps involved in using circuitry such as circuitry  244  of  FIG. 21  in device  12  to communicate with accessory  14  (e.g., an accessory of the type shown in  FIG. 18 ) are shown in  FIG. 22 . 
     At step  252 , a user may supply input to accessory  14  using user input interface  232 . The user may, for example, actuate a switch or other user interface device to supply accessory  14  with user input. 
     Circuitry  228  in accessory  14  may process the user input (step  254 ). For example, circuitry  228  may use an impedance detector or other suitable circuit to identify which button was pressed in a resistively encoded button array (as an example). 
     At step  256 , a tone generator or other suitable control circuitry  228  may be used to transmit the user input to device  12  over path  16 . 
     At step  258 , electronic device  12  may use tone detector  246  to receive the transmitted tone information. This information may be processed to identify the user input. For example, incoming tones may be processed to recover user button press data or other user input that is indicative of a user&#39;s desire to control device  12 . In response, device  12  may take appropriate action (step  260 ). For example, if the user is playing back a media file with device  12  and device  12  receives a user input indicative of user actuation of stop button  104  ( FIG. 5 ), device  12  can stop the playback of the media file. 
     The diagram of  FIG. 23  indicates how various different electronic devices  12  can operate in conjunction with various different accessories  14 . In the  FIG. 23  example, there are three electronic devices. 
     Electronic device  12 A may have circuitry such as circuitry  162  of  FIG. 10  to drive speakers such as the speakers described in connection with accessory  14  of FIG.  9 , but does not include circuitry for handling microphone signals or button presses. 
     Electronic device  12 B may have circuitry such as circuitry  178  of  FIG. 12  for detecting momentary shorts between a microphone terminal and a ground terminal, as described in connection with accessory  14  of  FIG. 11 , but does not have circuitry for processing ultrasonic tones. 
     Electronic device  12 C may have circuitry such as circuitry  244  of  FIG. 21  to detect tones and, if desired, may have additional circuitry such as voltage detector  216  of  FIG. 15  for detecting resistances associated with resistively encoded buttons and circuitry such as comparator  186  of  FIG. 12  for detecting momentary shorts between a microphone and ground line. 
     Accessories such as accessories  14 A,  14 B,  14 C, and  14 D may be plugged into devices such as devices  12 A,  12 B, and  12 C. The functionality of the resulting combined system (i.e., a given one of the electronic devices and a given one of the accessories) depends on which system is considered. 
     Consider, as an example, a scenario in which headset  14 A is plugged into device  12 C. Headset  14 A does not have buttons or a microphone and may have the functionality of accessory  14  of  FIG. 9 . When connected to device  12 C, device  12 C will not be able to receive or process incoming tones and will not be able to detect electrical shorts. Nevertheless, device  12 C will be able to drive audio onto the speakers of accessory  14 A, through audio connectors  46 . 
     As another example, consider accessory  14 B. Accessory  14 B may be, for example, a headset such as headset  14  of  FIG. 9  with a microphone of the type shown in  FIG. 11 . When connected to device  12 C, the microphone in accessory  14 B can supply audio signals that are processed by a corresponding microphone amplifier in device  12 C, but because no buttons are available on accessory  14 B, device  12 C will not, in this scenario, be able to process or respond to button presses. 
     Accessory  14 C may be, for example, a one-button accessory such as accessory  14  of  FIG. 11 . Device  12 C may have a comparator such as comparator  186  that is able to detect when button  176  of accessory  14 C is depressed. Audio may be driven onto the speakers of accessory  14 C and microphone input from microphone  174  may be processed by microphone amplifier  194  of  FIG. 12 . 
     The remaining scenarios illustrated in  FIG. 23  involve accessory  14 D. Accessory  14 D may be, for example, an accessory of the type described in connection with  FIG. 18 . As described in connection with  FIG. 18 , accessory  14 D may have user input interface  232 . User input interface  232  may include speakers  92 , a button such as button  176  that bridges the microphone and ground lines in accessory  14 D, resistively encoded buttons, a microphone, and an ultrasonic tone generator. 
     When connected to a relatively simple device such as audio-out-only electronic device  12 A, the speakers in accessory  14 D may be used, but the buttons and microphone will be unavailable. 
     When connected to device  12 B, the microphone in accessory  14 D may be used and button presses made using button  176  may be processed. If device  12 B has voltage detector circuitry such as voltage detector  216  of  FIG. 15 , device  12 B may be able to directly detect actuation of various resistively encoded buttons. If device  12 B does not have voltage detector circuitry, but only has tone detection circuitry, device  12 B will not be able to directly detect actuation of resistively encoded buttons but can detect ultrasonic tones (i.e., ultrasonic tones generated in response to user input). 
     When an accessory such as accessory  14 D is connected to an electronic device such as electronic device  12 C, both the device and accessory are able to fully exercise a variety of functions. In particular, because device  12 C has audio driver circuitry and microphone amplifier circuitry, device  12 C will be able to drive audio signals onto speakers in accessory  14 D and will be able to receive incoming microphone signals. Momentary shorts between the microphone line and ground line that result from actuation of buttons such as button  176  ( FIG. 11 ) in accessory  14 D may be detected by device  12 C using a comparator such as comparator  186  of  FIG. 12  (which may be part of a voltage detection circuit such as voltage detector  216  of  FIG. 15 ). Actuation of resistively encoded buttons may be detected by directly detecting resistance changes between the microphone and ground lines in accessory  14 D (e.g., using a voltage detector such as voltage detector  216  of  FIG. 15 ) or may be detected by receiving and processing ultrasonic tones that accessory  14 D transmits to device  12 C in response to button actuation events. 
     If desired, the microphone may be omitted from accessory  14 D. When an accessory of this type is connected to a device such as audio-out-only electronic device  12 A, the speakers in accessory  14 D may be used, but the buttons will be unavailable. There is no microphone present, so no microphone is used. 
     When an accessory such as a microphoneless accessory  14 D is connected to device  12 B, microphone functions associated with device  12 B will not be used. However, button presses made using a button such as button  176  in a microphoneless accessory such as accessory  14 D may be processed. Moreover, if voltage detector circuitry such as voltage detector  216  of  FIG. 15  is used in device  12 B, device  12 B may be able to directly detect actuation of various resistively encoded buttons on the microphoneless accessory. If device  12 B does not have voltage detector circuitry, but only has tone detection circuitry, device  12 B will not be able to directly detect actuation of resistively encoded buttons in a microphoneless accessory, but can detect ultrasonic tones such as ultrasonic tones generated in response to user input. 
     When an accessory such as accessory  14 D that does not have a microphone is connected to an electronic device such as electronic device  12 C, device  12 C will be able to drive audio signals onto speakers in accessory  14 D, but will be unable to receive incoming microphone signals. Momentary shorts between a “microphone” line and a ground line that result from actuation of buttons such as button  176  ( FIG. 11 ) in an accessory  14 D without a microphone may be detected by device  12 C using a comparator such as comparator  186  of  FIG. 12 . As in scenarios in which accessory  14 D contains a microphone, when accessory  14 D does not include a microphone, actuation of resistively encoded buttons in accessory  14 D may be detected by directly detecting resistance changes between the microphone and ground lines in accessory  14 D using a voltage detector such as voltage detector  216  of  FIG. 15  or may be detected by receiving and processing ultrasonic tones that accessory  14 D transmits to device  12 C in response to button actuation events. 
     As these various scenarios illustrate, the use of standard audio connectors such as connectors  46  of  FIG. 4  may allow a variety of different types of accessories to be connected to different electronic devices. When a device is connected to an accessory that supports fewer features that the device supports, certain features may not be available to the user. Similarly, when an accessory is connected to a device that supports fewer features than the accessory supports, all accessory features may not be available to the user. When, however, devices and accessories have comparable feature support, the functions of the devices and accessories may be more fully utilized. An advantage of this type of arrangement is that devices such as device  12 C that have numerous features may be used with a wide variety of accessories, even if those accessories do not fully support the features of device  12 C. In a similar fashion, an accessory such as accessory  14 D that supports numerous features may be used with a wide variety of electronic devices, even if those electronic devices do not fully support the features of accessory  14 D. 
       FIG. 24  is a circuit diagram of illustrative circuitry that may be used in an electronic device  12  that supports features such as tone mode detection. As shown in  FIG. 24 , circuitry  262  may include power supply circuitry  180 . Power supply  180  may be a DC power supply that uses switching circuit  264  to supply an adjustable DC power supply voltage on its output. Filter  266  may use resistor  182  to supply the output voltage from power supply  180  to node M, where it may be used as a microphone contact bias voltage for biasing microphone line  94 A in accessory  14 . With one suitable arrangement, power supply  180  may be adjusted to provide voltages of 0 volts (ground), 2.0 volts, or 2.7 volts on terminal M or may be place in an open circuit configuration in which terminal M floats. A raw power supply voltage AVDD of more than 2.7 volts or other suitable voltage level may be supplied to the AVDD terminal of  FIG. 24 . If desired, power supplies with more adjustable output voltage levels or fewer adjustable output voltage levels may be used. 
     Incoming microphone signals from accessory  14  may be amplified using microphone amplifier  194 . As shown in  FIG. 24 , microphone amplifier  194  may, for example, be implemented as part of a larger integrated circuit such as an audio codec  284 . Resistor  268 , which may be, for example, a 10 kilo-ohm resistor, may be used in optimizing current protection in circuitry  262 . Filter  266 , and, in particular, the capacitor in filter  266 , may be used to remove high frequency noise from microphone terminal M. Resistor  182  may form a load for the microphone circuit when the microphone of accessory  14  is in use. 
     Voltage detection circuitry  216  may be used to measure the voltage across terminals M and G. Audio driver circuitry  166  and  168  may be used to drive audio signal onto the speakers in accessory  14 . 
     An electromagnetic interference (EMI) filter such a filter  270  may be used to help make circuitry  262  immune to the undesired effects of electromagnetic interference. 
     Tone detector  246  may receive ultrasonic tones from microphone line M and may provide corresponding digital output on output line  250  that indicates what type of tones have been received. Control circuitry  274  may help to process the tone signal data from line  250 . 
     Control circuitry  274  may include a level shifter such as level shifter  276  that serves as an interface between the relatively higher voltages that may be used in circuitry  262  and the relatively lower voltages that may be used elsewhere in device  12 . Communications circuitry in control circuitry  274  such as I 2 C communications circuitry  272  may be used to help circuitry  274  communicate with other circuitry on device  12 . Circuitry  272  may be used to send and receive digital data over bus  278 , which may be, for example, a two-wire I 2 C bus. Circuitry  274  may have an enable input  280  that receives an enable signal EN. The enable signal EN may be deasserted when, for example, an application that is running within device  12  desires to disable accessory functions to save power. Interrupt line  282  may be asserted when control circuitry  274  generates an interrupt signal INT. Processing circuitry such as processing circuitry  128  of  FIG. 7  may periodically examine the state of interrupt line  282 . When the interrupt is asserted, processing circuitry  128  may examine the states of registers within control circuitry  274  to determine what type of activity in circuitry  262  has resulted in the assertion of the interrupt. This activity might be, for example, detection of an incoming ultrasonic tone, etc. 
     Illustrative power supply circuitry  180  that may be used in circuitry  262  is shown in  FIG. 25 . As shown in  FIG. 25 , power supply circuit  180  may have fixed power supply  286  and fixed power supply  288 . Switch SW 1  may be closed when it is desired to route the output voltage from supply  286  to output node M. Switch SW 2  may be closed when it is desired to route the output voltage from supply  288  to output M. Driver  290  and feedback path  296  may be used to regulate the output voltage V LDO  on node  298 . The voltage on the “+” input of device  290  serves as an adjustable reference voltage. In the example of  FIG. 25 , V LDO  will be 2.7 volts when SW 1  is closed and SW 2  is open and will be 2.0 volts when SW 1  is open and SW 2  is closed. Switch SW 3  may be closed and switch SW 4  may be opened when it is desired to route the selected output V LDO  to microphone node M. When it is desired to ground terminal M, switch SW 4  may be closed and switch SW 3  may be opened. Terminal M may be placed in a floating condition in which terminal M is disconnected from the ground and power supply output by opening both switch SW 3  and switch SW 4 . 
     Illustrative voltage detection circuitry  216  that may be used in circuitry  262  of  FIG. 24  is shown in  FIG. 26 . Comparator  186  may receive the voltage on the M terminal on input  190  and may receive a reference voltage VREF (e.g., 0.2 volts or other suitable value close to 0 volts) on input  188 . Comparator  186  may compare the voltage levels on inputs  188  and  190  and may assert a corresponding output signal on line  192  whenever the voltage on microphone line M falls below VREF, indicating that a user has depressed a shorting button such as button  176  (e.g., in  FIG. 14 ). 
     Comparator  312  may receive the microphone line voltage from terminal M on input  314  and an adjustable reference voltage VR on input  316 . The magnitude of voltage VR may be controlled by controlling the digital control signals on control lines  306 . These control signals may be supplied to switch  300  by control circuitry  274 . The inputs to switch  300  may be obtained from a voltage divider such a voltage divider  302 . Each node  304  of the resistor tree in voltage divider  302  establishes a separate reference voltage derived from voltage V LDO  on node  298 . In response to the control signals received on lines  306 , switch  300  routes a selected one of these voltages to output  308  for use as the reference voltage VR on input  316 . Comparator  312  compares the microphone voltage on terminal M to the selected value of the reference voltage and produces a corresponding output  310  that is indicative of whether the microphone line M is at a higher or lower voltage than the selected reference voltage. By adjusting switch  300 , control circuitry  274  ( FIG. 24 ) can accurately measure the magnitude of the voltage on microphone line M, thereby obtaining information from accessory  14  on the state of the microphone in accessory  14 . In the example of  FIG. 26 , switch  300  supports 16 different inputs. If desired, finer control may be provided by using a switch with a larger number of inputs. Switches with fewer inputs may also be used if desired. 
     Comparator  312  may receive the microphone line voltage from terminal M on input  314  and an adjustable reference voltage VR on input  316 . The magnitude of voltage VR may be controlled by controlling the digital control signals on control lines  306 . These control signals may be supplied to switch  300  by control circuitry  274 . The inputs to switch  300  may be obtained from a voltage divider such a voltage divider  302 . Each node of the resistor tree in voltage divider  302  establishes a separate reference voltage derived from voltage V LDO  on node  298 . In response to the control signals received on lines  306 , switch  300  routes a selected one of these voltages to output  308  for use as the reference voltage VR on input  316 . Comparator  312  compares the microphone voltage on terminal M to the selected value of the reference voltage and produces a corresponding output  310  that is indicative of whether the microphone line M is at a higher or lower voltage than the selected reference voltage. By adjusting switch  300 , control circuitry  274  ( FIG. 24 ) can accurately measure the magnitude of the voltage on microphone line M, thereby obtaining information from accessory  14  on the state of the microphone in accessory  14 . In the example of  FIG. 26 , switch  300  supports 16 different inputs. If desired, finer control may be provided by using a switch with a larger number of inputs. Switches with fewer inputs may also be used if desired. 
     As indicated schematically by registers R in control circuitry  274  of  FIG. 24 , one way in which circuitry  262  may interface with other processing circuitry on device  12  is through the periodic adjustment of register values. When, for example, a particular ultrasonic tone is detected, control circuitry  274  may adjust the contents of a corresponding register in control circuitry  274  and may, if desired, assert the interrupt line  282  to inform processing circuitry on device  12  of the need to inspect the new contents of registers R. Any suitable number of registers may be used in control circuitry  274  (e.g., one, two, more than two, tens of registers, more than tens of registers, etc.). 
     Illustrative registers that may be used in registers R of control circuitry  274  are shown in  FIG. 27 . As indicated by the text in the register boxes of  FIG. 27 , a variety of status conditions may be represented by the state of register bits. A TX ACK bit may be set high, for example, when it is desired to set a timer for enabling detection of an incoming ultrasonic acknowledgement tone (e.g., a tone of a particular length such as 6 ms). The “short detect only mode” bit may be set high to place device  12  in a low power standby mode of operation (e.g., a mode in which only detector  186  is being used and in which only button presses from shorting buttons such as button  176  of  FIG. 11  are recognized). The “resistor button detect enable” bit may be set high when it is desired to use the resistance decoding functions of comparators C 1 , C 2 , C 3 , and C 4  of  FIG. 26  to support direct detection of user actuation of resistively encoded buttons (e.g., by analyzing the resistance bridging lines  94 A and  94 B in accessory  14 ). The V LDO  CTRL 0  and V LDO  CTRL 1  bits may be used to control the magnitude of V LDO  by controlling the states of switches SW 1 , SW 2 , SW 3 , and SW 4  of circuit  180 , as described in connection with  FIG. 25 . The MIC DETECT bits may represent the values of the control signals applied to the four input lines  306  of switch  300  in voltage detector  26  of  FIG. 26 . The “mic detect true” bit may be set high when control circuitry  274  has detected the presence of a microphone in accessory  14  during an initial accessory discovery process. 
     An illustrative accessory  14  that may be used to support tone mode operations and resistance detection mode operations in conjunction with circuitry  262  of  FIG. 24  is shown in  FIG. 28 . As shown in  FIG. 28 , accessory  14  may have speakers  92  that are driven by audio output circuits such as audio drivers  166  and  168 . Buttons may be associated with switches S 0 , S 1 , S 2 , S 3 , and S 4 . Switch S 0  may be used to momentarily short microphone line M to ground line G and may be used when device  12  is in “short detect only mode” or when a device that only supports short detect button decoding operations is used. If desired, switch S 0  may be omitted. In configurations in which switch S 0  is omitted, button press events (e.g., events in which switch S 0  is closed to short the M and G terminals together) are avoided, so that audio signal transmission between accessory  14  is not interrupted by button actuation activity. 
     Switches S 1 , S 2 , S 3 , and S 4  may be resistively encoded using resistors  208 . The resistive network made up of resistors  208  may be configured using any suitable topology, as described in connection with  FIGS. 14 and 16 . The arrangement of  FIG. 28  is merely illustrative. Impedance detector  236  may be used to detect which of switches S 1 , S 2 , S 3 , and S 4  has been actuated. In resistance detection mode, device  12  may measure the voltage drop between microphone line M and ground line G, thereby directly measuring the resistance of the switches. This allows device  12  to determine which of switches S 1 , S 2 , S 3 , and S 4  has been actuated without using impedance detector  236 . In tone mode, impedance detector  236  may provide information on which switch has been actuated to adjustable tone generator  318 , which, in turn, may transmit appropriate tones to device  12  for detection by tone detector  246  ( FIG. 24 ). The tones may be transmitted over the microphone and ground lines. The tones may be ultrasonic tones that fall out of the range of human hearing and are therefore not disruptive to user activities such as telephone call activities. 
     Voltage detector and latch circuitry  320  may respond to various bias voltages that are applied to microphone line M by device  12 . This allows device  12  to control the operation of accessory  14  via path  16 . The bias voltages may be generated by power supply circuitry  180  ( FIG. 25 ) in response to control signals from control circuitry  274  ( FIG. 24 ). A bias voltage on the microphone line may help to power a microphone in accessory  14 . Time-dependent changes in the bias voltage may be used as a way to control accessory  14  and may therefore be considered to form a type of data transfer between device  12  and accessory  14 . At the same time that a bias voltage is being supplied to accessory  14  by device  12  using the microphone and ground lines, device  12  may be monitoring microphone signals on the microphone and ground lines that result from capturing the user&#39;s voice or other sound at accessory  14 . 
     Shunt regulator  338  may be used with resistor  328  to regulate the voltage on node N 1 . Shunt regulator  338  may operate as a Zener diode, pinning the voltage on node N 1  at a desired value over a wide range of operating currents. This regulated voltage may be used to power microphone  336  through switch SWA when switches SWA and SWC are closed. As indicated by paths  319 , shunt regulator  338  may be used to power adjustable tone generator  318  and impedance detector  236 . This prevents noise in the form of fluctuating currents in adjustable tone generator  318  and impedance detector  236  from being added onto microphone line M through resistor  328  and thereby prevents audible noise from being added to the microphone signal. Resistor  334  sets the magnitude (gain) of the microphone signal that is coupled onto node M from microphone  336 . In the arrangement shown in  FIG. 28 , microphone  336  is being implemented using a MEMS module. Capacitor  332  is a DC blocking capacitor that allows alternating current (AC) signals from microphone  336  to pass to microphone terminal M, while preventing the DC bias voltage on node M from adversely affecting the bias of amplifier AM in the MEMS module of microphone  336 . The MEMS module may include a microphone unit and a voltage multiplier that work in conjunction with amplifier AM to provide microphone output signals in response to received sound from a user. If desired, other types of microphones may be used such as electret microphones (see, e.g., the arrangement of  FIG. 30 ). 
     As shown in  FIG. 28 , circuitry  320  may include a comparator  322 . When the voltage on line M exceeds a reference voltage (e.g., a reference voltage obtained from a bandgap voltage reference in accessory  14 ), the output of comparator  322  goes high and sets the output of latch  324  high. The output of latch  324  may be conveyed to the control input of switch SWB over control line  340 . An inverted version of the latch output may be conveyed to the control input of switch SWC via control line  326  and may be conveyed to the control input of switch SWA via control line  330 . When it is desired to operate in a resistance detection mode, switch SWB may be closed, thereby connecting the network of resistors  208  and switches  210  between terminals M and G. In this situation, switches SWA and SWC may be open to disable microphone  336 . When it is desired to operate in a tone detection mode, switch SWB may be open and switches SWA and SWC may be closed, thereby disconnecting the resistively encoded switches from terminals M and G and biasing microphone  336  for operation. 
     In  FIG. 30 , an illustrative accessory circuit that is based on an electret microphone rather than a MEMS microphone is shown. 
       FIG. 31  shows another illustrative arrangement that may be used for the circuitry of accessory  14 . In the circuitry of  FIG. 31 , resistor RC and capacitor CC may serve as a tone coupling circuit. This tone coupling circuit helps properly attenuate tone signals transmitted from tone generator  318  to node M. The tone coupling circuit also serves as a high pass filter that allows ultrasonic tones from tone generator  318  to be merged onto microphone line M, which also carries regular audio signals (e.g., signals from roughly 20 Hz to 20 kHz in frequency) from microphone  336 . Resistor RB sets the DC bias for microphone line M. Resistor RG and capacitor CG set the AC gain for amplifier AM in microphone module  336  (e.g., a MEMS module). Amplifier AM may operate in constant current mode. 
     Voltage detector and latch  320  may activate at a suitable threshold voltage. When, for example, the voltage on microphone line M is 2.7 volts (i.e., greater than a threshold of 2.3 volts), voltage detector and latch  320  may generate control signals that turn on switches SBW 1  and SWB 2  and that turn off switch SWA. When the voltage on microphone line M falls below this level, switches SWB 1  and SWB 2  may be turned off and switch SWA may be turned on. When switches SWB 1  and SWB 2  are turned off in this way, transistor T 1  is turned off. This lets node NF float and turns off microphone  336 . 
     As with the arrangement of  FIG. 28 , the arrangements of  FIGS. 30 and 31  may, if desired, be configured so that disruptions to the microphone signals on the microphone line are avoided. This may be accomplished by omitting or avoiding the use of switches such as switch S 0  that short the microphone and ground lines together when pressed. Although such switches may be helpful in controlling legacy devices, in situations in which the microphone and ground lines are in use to carry audio signals such as voice signals captured from a microphone during a telephone call, the use of such momentary shorting switches may cause pops, clicks, and dead time. When switch S 0  is omitted or not used, these disruptions to the microphone signals may be avoided. 
     Audio disruptions can also be avoided by the use of ultrasonic tones to convey button press information, because ultrasonic tones are not audible to humans and therefore do not create audible interference when carried over the microphone line. At the same time that ultrasonic button press information is being conveyed from the accessory to the electronic device over the microphone line and at the same time that the electronic device is supplying a DC bias for the microphone over the microphone line, the microphone line may be used to convey audio information from the accessory to the electronic device without interference. 
       FIG. 29  shows the behavior of switches SWA, SWB 1 , and SWB 2  in circuits of the type shown in  FIG. 31  in response to high and low latch output values. When an appropriate accessory is present, such as a headset with speakers and an active microphone, the accessory may be placed in tone mode by setting switch SWA off and by turning switches SWB 1  and SWB 1  on, as indicated in the first row of the table of  FIG. 29 . The second row of the  FIG. 29  table indicates that the same type of accessory may be placed in a resistance detection mode in which only direct detection of the states of resistively encoded switches S 1 -S 4  is being performed by device  12 , by taking the latch state low. The same resistance detection mode may be invoked when, for example, the accessory connected to device  12  only has speakers and no microphone, as indicated in the third row of the table of  FIG. 29 . The fourth row in the  FIG. 29  table indicates the states into which the switches SWA, SWB 1 , and SWB 2  may be placed when it is desired to operate in tone mode to accommodate an accessory with speakers but without a microphone. 
     Any suitable technique may be used to communicate using ultrasonic tones. With one suitable arrangement, each button (e.g., each of the resistively encoded switches S 1 , S 2 , S 3 , and S 4  in the  FIG. 31  example) may be associated with a unique ultrasonic tone frequency. A calibration frequency and a button release frequency may also be used. During power-up, an acknowledgement tone may be transmitted. The acknowledgement tone, which may be provided in conjunction with a calibration tone, may be provided at any suitable frequency that may be produced by tone generator  318  (e.g., at a frequency different from that of the calibration frequency, at a frequency lower than that of the calibration frequency, at a frequency independent of any button press frequency, at a frequency different from the button release frequency, at a frequency that is the same as one of the button frequencies or the button release frequency, at a frequency that is used only for acknowledgements, using multiple acknowledgement frequencies in the form of a code such as a code formed of three 2 ms tones each of a different frequency, using other sequences of more than one tone frequency, using tone frequency sequences containing tones of different lengths, etc.). 
     Illustrative tones that may result from typical button activity are shown in  FIG. 32 . At time t a , a user may depress a button. Tone generator  318  may transmit a calibration frequency at time t a  for time period t 1  (e.g., for 1 ms). After the calibration frequency transmission is complete, tone generator  318  may transmit a tone associated with the button actuated by the user. This tone may be transmitted starting at time t b  and may have a duration of t 2  (e.g., 2 ms). When the user releases the button at time t c , another calibration tone may be transmitted for duration t 3  (e.g., 1 ms). This may be followed by an ultrasonic tone at time td of duration t 4  that indicates that the button has been released. Tone generator  318  may generate these tones from a clock in accessory  14  (e.g., by dividing a 2 MHz local clock to obtain an appropriate ultrasonic frequency). Typical ultrasonic frequencies for the tones produced by tone generator  318  may be, for example above 20 kHz (to avoid interference with audio signals on the microphone line) and below about 1 MHz (to avoid noise issues and to ensure proper transmission of the signals along the wires of the accessory). Illustrative ranges for suitable tone frequencies include 25 kHz-1 MHz, 25-500 kHz, 50-500 kHz, and 75-300 kHz (as examples). Higher frequencies may be used for the ultrasonic tones if desired. Lower frequencies may be used when, for example, the presence of an audio tone on the microphone line is acceptable to the user. 
     The use of the clock in accessory  14  to generate the tones for tone generator is represented schematically by the clock CLK in tone generator  318  of  FIG. 31 . If desired, other arrangements may be used (e.g., by synchronizing the clocks of device  12  and accessory  14 ). An advantage of using unsynchronized clocks is that this may reduce design complexity and lower costs. 
     A table showing illustrative frequency assignments that may be used for the ultrasonic tones is presented in  FIG. 33 . If more buttons are used, unique ultrasonic tones may be assigned to those buttons if desired. 
       FIG. 34  shows illustrative tone detector circuitry such as tone detector  246  of  FIG. 21  that may be used in processing received ultrasonic tones in device  12 . As shown in  FIG. 34 , tone detector  246  may receive an oscillating signal such as a sawtooth or sinusoidal signal over path  16  (e.g., across microphone line M and ground G). This signal may be converted to a square wave signal using limiter circuit  342 . Tone detector  246  may use pulse counting circuitry  346  to process the incoming tones. Counter circuitry  348  such as registers that maintain count values may be used by pulse counting circuitry  346  to analyze received tones. Pulse counting and timing circuitry  346  may by clocked using a device clock on input  350  that is local to device  12  and that runs asynchronously with respect to the clock CLK in accessory  14 . 
     The use of the calibration tones transmitted by tone generation circuitry  318  and the pulse counting and timing circuitry  346  of tone detector  246  may allow ultrasonic tone communications to be used reliably, even in environments in which the clocks of device  12  and accessory  14  are asynchronous. 
     An illustrative processing approach that may be used by tone detector  246  in analyzing incoming ultrasonic tones is shown in  FIG. 35 . As shown in the top portion of  FIG. 35 , tone detector may initially receive a button tone at the calibration frequency (i.e., a tone corresponding to time t a  of  FIG. 32 ). The pulses of the calibration tone can be counted to a count value of N 1  (e.g., a predetermined value such as 64 in the  FIG. 35  example). This first counting process establishes a window size WS. As shown in the middle portion of  FIG. 35 , this window size may be measured in the clock domain of device  12  by simultaneously counting using the device clock. The count value reached by the device clock in window size WS may be referred to as count C 1 . After count C 1  has been established, the pulses of the button tone may be processed (i.e., the tone associated with the transmission of time t b  of  FIG. 32  or, in the case of a button release event, the transmissions associated with time td). As shown in the lower portion of  FIG. 35 , the button tone processing operation may involve counting the pulses of the unknown tone for a duration equal to time window WS. The length of time window WS can be determined by counting with the device clock to count value C 1  (or counting down from C 1  with the device clock). The resulting count N 2  for the unknown pulse can then be compared to the calibration count N 1 . The ratio of N 2  to N 1  represents a calibrated version of the transmitted ultrasonic tone and can be compared to the entries in a table of known values such as the table of  FIG. 33  to identify the button activity that has occurred in accessory  14 . 
     Illustrative steps involved in this type of tone detection procedure are shown in  FIG. 36 . At step  352 , tone detector circuitry in device  12  such as tone detector  246  of  FIG. 34  may begin receiving a calibration tone (e.g., at time t a  of  FIG. 32 ). 
     At step  354 , counting circuitry  346  may count to N 1  cycles (e.g., a known number of cycles such as 64 cycles). Timing circuits in circuitry  346  may be used to start the counting process within the middle portion of the t 1  duration of the calibration pulse. The counting process establishes time window WS. At the same time that counting circuitry  346  is counting to N 1  pulses of the incoming tone, the device clock is being used to keep track of a count value C 1  corresponding to the number of device clock pulses during window WS. The value of N 1  and the value of C 1  that is reached when counting the device clock pulses until the count N 1  of the incoming tone pulses is reached may be stored in count registers  348 . 
     At step  356 , tone detector  246  may start receiving the button tone (i.e., the ultrasonic tone of time t b  or time td of  FIG. 32 ). This may correspond to a button press or button release event (as examples). 
     At step  358 , the time window WS may be reconstructed by counting to the value of C 1  using the device clock. At the same time that the device clock is being used to recreate time window WS, tone detector  246  may use pulse counting circuitry  346  to count the number N 2  of pulses in the incoming tone. 
     The values of N 1  and N 2  may be used to identify the button tone at step  360 . In particular, tone detector  246  or other suitable processing circuitry may compute the value of N 2 /N 1 , which represents the calibrated version of the transmitted ultrasonic tone. The calibrated version of the transmitted tone may then be used in conjunction with a table of the type shown in  FIG. 33  to identify the type of button activity that has been detected. Techniques such as this may also be used to detect tones that have been transmitted from device  12  to accessory  14  (e.g., in system such as those described in connection with  FIG. 7  in which tones may be transmitted bidirectionally). As indicated schematically by line  362 , the operations of steps  352 ,  354 ,  356 ,  358 , and  360  may be repeated to process additional button actuation events. 
     As described in connection with  FIG. 23 , electronic devices and accessories of different configurations may be used together. In this type of environment, it may not be known in advance which capabilities are present in the electronic device and accessory. A discovery process may therefore be used to ascertain the capabilities of components in system  10 . For example, device  12  may perform accessory identification operations to determine which type of accessory  14  is connected to device  12  and which circuitry in accessory  14  is available for use. Discovery operations may be performed, for example, whenever a new accessory is connected to device  12 , upon launching applications that are running on device  12 , when initiated by a user, or at any other suitable time. 
     Steps involved in an illustrative accessory identification process that may be used by electronic device  12  to ascertain the capabilities of an accessory that has been connected to the device are shown in  FIG. 37 . 
     At step  364 , as device  12  awaits insertion of the audio plug of the accessory (e.g., a headset, adapter, or other accessory equipment), device  12  may ground microphone terminal M. For example, in power supply circuitry  180  of  FIG. 25 , device  12  may close switch SW 4  to short the microphone terminal M to ground. Electronic device  12  may have a sensor such as a mechanical switch (e.g., mechanical switch SWM of  FIG. 24 ) that is tripped when the audio plug of accessory  14  is inserted into the mating audio jack of electronic device  12 . During step  364 , device  12  may monitor the state of the mechanical switch. When the user inserts the plug of the accessory into device  12 , the presence of the plug may be reflected by a change in the electrical state of the mechanical switch. This allows device  12  to detect the presence of the accessory (step  366 ). Once the insertion of the accessory plug has been detected, device  12  may initiate accessory identification operations. 
     At step  368 , device  12  may, if desired, wait for a predetermined amount of time (e.g., 300 ms) to ensure that the user has fully inserted the accessory audio plug into the audio jack of device  12 . 
     At step  370 , device  12  may activate its tone detection capabilities (e.g., using tone detector  246 ). 
     At step  372 , device  12  may use power supply  180  to adjust the bias voltage on microphone line M. Device  12  may, for example, set the output voltage of power supply  180  to a nominal value of 2.7 volts. The use of a 2.7 volt bias to bias microphone in accessories may be advantageous, because this bias voltage may be compatible with a relatively wide range of microphone types. Nevertheless, the 2.7 bias voltage that is generated in the illustrative operations of step  372  is merely an example. Other bias voltage levels may be used if desired. 
     The 2.7 volt DC bias voltage (or other suitable voltage) that is supplied by power supply circuit  180  of device  12  may serve as a control signal for accessory  14 . Accessories such as accessory  14  of  FIG. 31  may have voltage detector and latch circuitry  320  that is responsive to the amount of applied voltage on microphone contact M. As a result, accessory  14  may be directed to take various actions by applying particular DC bias voltages or sequences of DC bias voltages on line M. 
     With one suitable arrangement, voltage detector and latch circuitry  320  may place the circuitry of accessory  14  in tone mode at step  374  when a bias voltage is detected on line M that is greater than a particular threshold (e.g., a 2.3 volt threshold voltage). For example, voltage detector and latch circuitry  320  may respond to received voltages on line M that exceed the threshold voltage by opening switch SWA and closing switches SWB 1  and SWB 2 . In accessories with tone mode and microphone capabilities, such as accessory  14  of  FIG. 31 , this will activate the microphone circuitry and will disconnect the resistively encoded switches from the microphone line so that button activity will be conveyed to device  12  as tones, rather than as changes in microphone line to ground line resistances. 
     To ensure that device  12  and accessory  14  work properly together, it may be desirable for accessory  14  to send confirmation information to device  12  in response to detection of the 2.7 volt DC bias from device  12 . Confirmation information may be provided, for example, in the form of an acknowledgement signal. In arrangements of this type, device  12  may await an acknowledgement signal from accessory  14  at step  376 . 
     Device  12  may maintain a local timer. The TX ACK bit in the registers of control circuitry  274  ( FIG. 27 ) may be set high to set the local timer (e.g., to start an appropriate timeout period of 6 ms). The timer may be initialized after raising the output voltage from power supply  180  to 2.7 volts or other voltage that is expected to elicit an acknowledgement from accessory  14 . If no acknowledgement is received from accessory  14  at tone detector  246  within the predetermined timeout period (e.g., 6 ms), device  12  may conclude that accessory  14  does not have properly operating tone-based acknowledgement capabilities, may set the registers of control circuitry  274  to reflect this status, and may generate a corresponding interrupt on line  282  ( FIG. 24 ) to indicate that the timer has expired without receive of an acknowledgement from accessory  14  (step  378 ). Software running on device  12  (e.g., an application that may desire to use the buttons of an accessory) may query the registers of control circuitry  274  to determine why the interrupt was generated (i.e., to discover that the interrupt was generated because the timer expired without receiving an acknowledgement from the accessory indicating that tone capabilities were present). 
     At step  380 , device  12  may use comparator  186  of voltage detection circuitry  216  ( FIG. 26 ) to determine whether microphone line M and ground G are shorted together. 
     If contacts M and G are shorted together, device  12  may verify this condition at step  388 . If a user inserts the audio plug of accessory  14  into the mating audio jack in device  12  slowly, the microphone and ground contacts M and G may be momentarily shorted due to inadvertent momentary contact between the contacts in the plug and metal portions of the jack. During step  388 , comparator  186  may again be used to determine whether the microphone and ground lines are shorted or whether the short detected at step  380  was only momentary (e.g., due to a partial plug insertion). 
     If, at step  380 , it was determined that the microphone contact M and ground G were not shorted together, switch  300  in voltage detection circuit  216  may be adjusted to set VR to an appropriate level (e.g., 2.5 volts) to detect whether a microphone is present in the accessory. At step  382 , voltage detection circuit  216  may be used to determine whether there is a microphone present in the accessory. If there is no microphone in the accessory (e.g., because the user has inserted an extension cable into the jack), the voltage on microphone terminal M will remain near 2.7 volts (i.e., greater than 2.5 volts). If, however, there is a microphone present in the accessory, current drawn through the microphone will pull the voltage on terminal M below 2.5 volts. This reduced voltage will be detected by comparator  312  ( FIG. 26 ), confirming the presence of the microphone. 
     If it is determined at step  382  that a microphone is present, device  12  can conclude that the accessory has a microphone and no tone mode capabilities. For example, device  12  may conclude that the accessory is a headset with a shorting button  176  and a microphone  174  of the type shown in  FIG. 11 . This may be verified during the operations of step  384 . For example, control circuitry  274  use power supply  180  to set its output voltage to 0 volts and, subsequently, to 2.0 volts (as an example). When this procedure is followed, accessories such as the tone-mode enabled accessory  14  of  FIG. 31  will not enter tone mode, because the microphone line bias voltage (e.g., 2.0 volts) will not rise above the threshold associated with voltage detector and latch  320  (e.g., 2.3 volts). The accessory will therefore not be placed in tone mode and the microphone line will not be pulled low. In this situation, the “microphone detect true” bit in the register circuitry of  FIG. 27  will not be set. On the other hand, in accessories such as headsets of the type shown in  FIG. 11 , raising the voltage to 2.0 volts will result in a measured microphone line voltage of about 2.0 volts and will cause the “microphone detect true” bit to be set by control circuitry  274  ( FIG. 24 ). If device  12  reaches step  384  and verification is successful, device  12  can conclude that accessory  14  is of the type shown in  FIG. 11 . 
     If it is determined at step  382  that no microphone is present in accessory  14 , device  12  may direct power supply  180  to bias the microphone line M in accessory  14  at 2.7 volts (step  386 ). Accessory  14 , which may be a headset with resistively encoded buttons of the type shown in  FIG. 14 , may be used to control device  12 . 
     If it is determined at step  380  that the microphone line is shorted, verification operations may be performed at step  388 . For example, the state of output  192  of comparator  186  in voltage detection circuitry  216  may be checked to ensure that the voltage on line  190  is below VREF (i.e., below 0.2 volts). If verification operations at step  388  are successful, device  12  may conclude that accessory  14  is a headset of the type shown in  FIG. 9  having a plug such as plug  36  of  FIG. 4  with a sleeve  64  that is shorting regions  78  and  80  of jack  38  in device  12 . 
     If, at step  376 , an acknowledgement tone signal is successfully detected within the acknowledgement time window (e.g. 6 ms), processing may proceed to step  390 . During the operations of step  390 , device  12  may set a register in control circuitry  274  to reflect that the acknowledgement signal has been received from accessory  14  and may generate an interrupt. The processing circuitry of device  12  may, in response to the interrupt, conclude from the contents of the register circuitry that confirmation information from accessory  14  has been successfully received (i.e., because the tone generator  318  of accessory  14  transmitted an acknowledgement tone to confirm the presence of tone mode capabilities in accessory  14  in response to the operations of step  374 ). 
     At step  392 , device  12  can determine whether a microphone is present in accessory  14 . Voltage detection circuitry  216  may be used to evaluate the voltage on microphone terminal M. If a microphone is present, the voltage on terminal M will be relatively low due to the current drawn by the microphone. In this situation, device  12  may conclude that accessory  14  is of the tone-mode-capable type shown in  FIG. 31  and has a microphone. If no microphone is present, the voltage on terminal M will be relatively high and device  12  can conclude that accessory  14  is of the type shown in  FIG. 31 , but without a microphone present. 
     As this example demonstrates, the various DC voltages produced by power supply  180  in device  12  can serve as control signals for accessory  14 . Accessory  14  can detect these DC voltages and can respond. In “smart” accessories that support tone-mode functions, tone generation circuitry may be used to send confirmatory information to device  12  (e.g., in the form of an ultrasonic acknowledgement tone). Voltage detection circuitry in device  12  may then be used to determine whether the accessory has a microphone. In accessories that do not support advanced tone-mode functions, device  12  can use tone detector  246 , power supply circuitry  180 , and voltage detector circuitry  216  to analyze the accessory and determine its capabilities. 
     Once the discovery process is complete, an application such as a media playback application, cellular telephone application, operating system function, or other suitable software implemented on device  12  can take appropriate action. For example, if it is determined that no tone mode capabilities are present, device  12  can operate in resistance detection mode (if resistively encoded buttons are present) or can await button presses from a shorting button such as button  176 . If it is determined that no microphone is present, certain functions may be blocked (e.g., functions requiring the user&#39;s voice). Other functions may not be blocked (e.g., functions associated with media playback operations). If desired, applications in device  12  may change the operating mode of device  12 . For example, an application running on device  12  might place device  12  in a resistance detection mode when microphone functions are not needed, thereby potentially saving power, even if device  12  has tone mode capabilities. During resistance detection mode, button presses create changes in the impedance between microphone line M and ground G that could be disruptive if a microphone were in active use. The resistance detection mode is therefore generally preferred only when the microphone is not being used. In situations in which the microphone is being used or in which tone mode operations consume less power, tone mode operation may be preferred. 
     Any suitable applications may be implemented on device  12 . For example, device  12  may run software that handles functions associated with wired and wireless communications, games, productivity, finance, entertainment, media, and other functions. Illustrative applications that may be implemented on processing circuitry  128  of device  12  and that may use the functionality of accessory  14  in system  10  include media player applications, radio applications, voice memo applications (e.g., applications that include recording functionality for voice or other sounds), voice or other sound recording playback applications, and exercise applications (e.g., applications that perform fitness-related functions such as keeping track of fitness information, playing media in a way that is suitable when a user is jogging or is working out at a fitness facility, etc.). These applications may be implemented using processing circuitry  128  of  FIG. 7  (as an example). 
     During normal operation of device  12  and accessory  14 , user input such as button press activity information may be conveyed from accessory  14  to device  12  in real time. Processing circuitry  128  may analyze the user input and take appropriate actions. The actions that are taken by device  12  in response to particular user input generally depend on which software is operating on device  12 . For example, device  12  may always or nearly always run an operating system, so user input related to operating system control functions may be processed continuously or nearly continuously. Other user input may result in different actions, depending on context. For example, selection of a “+” button may result in a track skip operation if a user is interacting with a media playback application in a particular mode of operation, whereas selection of the same “+” button may result in an increase in volume for the audio being driven into accessory  14  when the user is interacting with a cellular telephone application. 
     To simplify operations, it may be desirable to limit the range of allowable button presses that can be made by a user. In this type of arrangement, multiple button clicks within a short period of time or user button activity involving simultaneous selection of two buttons may be ignored. With other suitable arrangements, more complex button activity may be allowed (e.g., multiple button clicks, selection of multiple buttons, etc.). 
     If desired, multiple button presses may be handled as follows (as an example). Initially, device  12  can note which button was pressed upon detection of a first button press from the user. If a second button press is detected before a button release tone is received, the second button press may be ignored. On any button release when a button is active, device  12  may assume that a release of the pressed button was intended. 
     Collections of one or more button presses may sometimes be referred to as multi-button commands or user gestures.  FIG. 38  presents a table of illustrative user commands that may be associated with the user input interface on accessory  14 . In the examples of  FIG. 38 , the user interface has been assumed to include three buttons: a “+” button, a center button, and a “−” button, as described in connection with  FIG. 5 . This is, however, merely illustrative. Any suitable number of buttons may be used on accessory  14  to gather user input if desired. 
     As shown in the table of  FIG. 38 , user gestures may involve selection of particular buttons and timing information. When selecting buttons, a user may select a single button, two buttons, or more than two buttons. With respect to timing, a user has several options. For example, a user may press and immediately release a button (sometimes referred to as a “click”). The user may also press and hold the button for an extended period of time (e.g., for a fraction of a second or more than a second). Another possibility relates to multiple selections of the same button. In this type of situation, the user might, for example, press and release the same button twice in rapid selection (sometimes referred to as a “double click”). Triple clicks or even more complex clicking patterns may also be recognized (e.g., to select a previous track). Moreover, combinations of single button presses, multiple button presses, single and double clicks, and hold events may be used as user gestures if desired. As just one example, a double-click and hold command may be recognized as a unique user gesture by device  12  in a voice recording application, as indicated by the last column of the table of  FIG. 38 . 
     The table of  FIG. 38  lists several illustrative applications that may be implemented on electronic device  12  such as a media player application, a radio application, a voice memo record application, a voice memo playback application, and an exercise application. These are merely illustrative applications that may be implemented on device  12 . In general, any suitable applications may be run on the hardware of device  12  such as business productivity applications, games, communications applications, entertainment applications, etc. The user gestures that are shown in  FIG. 38  are also merely illustrative. For example, other combinations of user inputs may be made using buttons in accessory  14 . If desired, user commands may be formed partly using button actuation events and partly using other user input (e.g., sound). For example, a user may supply a voice command while performing a click and hold operation. User input based solely on voice commands or other non-button input may also be provided. 
     As each user command is entered (e.g., using a user gesture composed of button actuation events), a specific corresponding set of ultrasonic tone signals is transmitted to electronic device  12  over the audio jack. Clicks may be represented by distinct ultrasonic tones, depending on which button was pressed. Holds may be represented by repeated transmission of button-specific ultrasonic tones or by special “hold” tones. Still other arrangements may be used in which, for example, a double click is represented by a particular tone and a triple click is represented by another tone. Different commands may be represented by tones of different corresponding frequencies or commands may be represented using codes made up of multiple tones of different frequencies, different tone patterns, different tone durations, etc. 
     Schemes such as these in which different complex user gestures are converted into particular tones or tone-based codes are generally more burdensome on the processing circuitry of accessory  14  than schemes in which each button press results in corresponding unique ultrasonic tones. For this reason, it may be desirable to use an arrangement in which each button press that is detected (e.g., by an impedance detector) results in the production of a corresponding ultrasonic tone by the ultrasonic tone generator. However, schemes in which more button and other user input processing is performed at accessory  14  before transmitting instructions to device  12  as ultrasonic tone information may be used if desired. 
     Although unique user inputs typically result in unique instructions for device  12 , identical commands can result in different actions. This is because the actions taken by electronic device  12  typically depend on context, as illustrated by  FIG. 38 . If, for example, a user is operating a media player application, a click of the center button will pause media playback, whereas a click of the center button will mute radio playback if a radio application is active. The ultrasonic tones that are sent to the electronic device in response to user input on the accessory form specific instructions for the electronic device. When the electronic device receives these instructions over the audio jack, the action taken by the electronic device typically depends on which software applications are operating on the device. 
     Additional user gestures that may be used in system  10  are shown in  FIG. 39 . As shown in  FIG. 39 , volume up and down operations and a play/pause operations may be controlled using button clicks. Additional functions may be controlled using gestures such as a click and hold gesture, a double-click gesture, a gesture formed by making a click followed by a click and hold, or a triple click gesture. The functions that are controlled in this way may be, for example, media playback functions such as music playback functions, playlist navigation functions, etc. 
     When a headset that has a single button and a microphone or a single button but no microphone is used, electronic device  12  may recognize center button presses and can distinguish between click, click &amp; hold, double click, click+click &amp; hold, and triple click gestures. 
     When a headset that has three buttons and a microphone or a headset with three buttons and no microphone is used, electronic device  12  may recognize button presses from the volume up (V+), center, and volume down (V−) buttons, may distinguish between click, click &amp; hold, double click, click+click &amp; hold, and triple click gestures, and may recognize and ignore multiple simultaneous button presses. 
     If desired, distinguishable audio feedback for different button presses may be generated by electronic device  12  and played for the user. 
     During media playback, an accessory with three buttons may allow the user to increase the playback volume. Clicking once on the V+ button may increment the volume one step and a press and hold of the V+ button may cause the playback volume to ramp up. The user may reduce the playback volume by clicking once on the V− button to decrement the volume one step. The user may press and hold the V− button to cause the volume to ramp down. 
     Play and pause operations may be performed using the center button. Clicking once on the center button will cause the media playback to pause if media was playing and resume if media playback was paused. 
     Media playback may also be advanced. In particular, double-clicking on the center button on accessory  14  will produce a “next” command to advance media playback to a next song, chapter, or photo. 
     Playlists may be navigated using user gestures. For example, a click &amp; hold gesture using the center button will advance a user to the next playlist. If there is only one playlist present, a click &amp; hold of the center button will not result in any action being taken. If a click &amp; hold gesture is made while on the last of a list of playlists, electronic device  12  will advance to the first playlist in the list. 
       FIG. 40  shows illustrative circuitry that may be used in an accessory such as accessory  14  when the microphone is omitted. Accessories of the type shown in  FIG. 41  may be used with electronic devices that do not have cellular telephone capabilities or other functions that use microphone signals or may be used in a reduced-functionality mode with a cellular telephone or other such device that contains microphone signal processing circuitry. 
     If desired, potential interference with microphone signals can be avoided using an accessory of the type shown in  FIG. 41 . In this type of arrangement, the momentary shorting button S 0  that might otherwise be connected between the microphone and ground lines has been omitted. As a result, button press events do not result in shorts between the microphone and ground lines. The microphone line is therefore not disrupted, even if buttons are pressed repeatedly while the microphone line is in use to control the electronic device. Moreover, ultrasonic signals may be supplied by tone generator  318 , so that button press data is transmitted using frequencies out of the normal range of human hearing. This makes button data transmission operations inaudible to users of accessory  14 , even though the tone data is transmitted over the microphone line. 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.

Metadata:
Filing Date: 20080903
Publication Date: 20150310
Grant Date: 20150310
Priority Date: 20080114
Inventors: SANDER WENDELL B.
TERLIZZI JEFFREY
FARRAR DOUGLAS M.
JOHNSON TIMOTHY
SANDER BRIAN
CONNER BRIAN J.
DOROGUSKER JESSE L.
Assignee: APPLE INC
CPC Classifications: [{"code": "H04M1/72558", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04M1/6058", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04M1/72527", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/05", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/72442", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04M1/05", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/6058", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04R3/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04R1/1041", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/6058", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04M1/05", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/72442", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R2499/11", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 40850148