PATENT DOCUMENT

Publication Number: US-8559575-B2
Application Number: US-433507-A
Country: US
Kind Code: B2

Title: Microcontroller clock calibration using data transmission from an accurate third party

Abstract:
Systems and methods are provided for calibrating the internal oscillator of a microcontroller from a remote clock source. In some embodiments, an electronic device can request timing information from a third party device using a timing independent signal. The timing information received from the third party device may be used to calibrate the microcontroller clock of the electronic device. In some embodiments, the internal oscillator may be calibrated based on timing information received from multiple third party devices. Once calibrated, the microcontroller may initiate timing dependent communication with other electronic devices using a timing dependent protocol, such as a serial protocol.

Claims:
What is claimed is:  
     
       1. A method of enabling timing dependent communication between a first electronic device and a second electronic device, comprising:
 transmitting a timing independent signal from the first electronic device to a third party device to request transmission of timing information, wherein the timing independent signal is comprised of a voltage change in the timing independent signal that is detectable by the third party device; 
 calibrating a clock source of the first electronic device based on the timing information transmitted from the third party device and received by the first electronic device, wherein the timing information is comprised of a clock signal; and 
 conducting the timing dependent communication between the first electronic device and the second electronic device, wherein a data rate of the timing dependent communication is based on a clock rate of the clock source. 
 
     
     
       2. The method of  claim 1 , wherein the transmitting the timing independent signal comprises pulsing a voltage of the timing independent signal. 
     
     
       3. The method of  claim 1 , wherein the third party device comprises a reliable clock source, and wherein the timing information is derived from the reliable clock source. 
     
     
       4. The method of  claim 3 , wherein the reliable clock source is a crystal oscillator. 
     
     
       5. The method of  claim 1 , further comprising transmitting a timing dependent signal from the first electronic device to the third party device to end transmission of the timing information. 
     
     
       6. The method of  claim 1 , further comprising:
 transmitting a predetermined packet from the first electronic device to the third party device using a timing dependent protocol; 
 ending the calibrating of the clock source in response to the first electronic device receiving an acknowledgement of receipt of the predetermined packet from the third party device; and 
 retransmitting the predetermined packet from the first electronic device to the third party device when the acknowledgement is not received by the first electronic device. 
 
     
     
       7. The method of  claim 1 , wherein the timing independent signal is a first timing independent signal, the method further comprising:
 transmitting a second timing independent signal from the first electronic device to the third party device subsequent to the transmitting the first timing independent signal to request transmission of additional timing information; and 
 recalibrating the clock source based on the additional timing information transmitted from the third party device and received by the first electronic device. 
 
     
     
       8. The method of  claim 7 , wherein the second timing independent signal is automatically transmitted a predetermined amount of time after the transmitting the first timing independent signal. 
     
     
       9. The method of  claim 7 , wherein the second timing independent signal is transmitted after the conducting the timing dependent communication ceases for a timeout period of time. 
     
     
       10. A method of transmitting timing information from a first electronic device to a second electronic device, wherein the first electronic device comprises a reliable clock source, and wherein the second electronic device comprises a microcontroller with an internal oscillator, the method comprising:
 receiving a timing independent signal from the second electronic device with the first electronic device, wherein the timing independent signal is comprised of a voltage change in the timing independent signal that is detectable by the first electronic device; 
 detecting a request for the timing information from the received timing independent signal with the first electronic device; and 
 transmitting the timing information from the first electronic device to the second electronic device, wherein the timing information is derived from the reliable clock source, wherein the timing information is comprised of a clock signal, and wherein the timing information is used by the second electronic device to calibrate the internal oscillator of the microcontroller. 
 
     
     
       11. The method of  claim 10 , wherein the detecting the request comprises detecting a voltage change of the timing independent signal. 
     
     
       12. The method of  claim 10 , further comprising powering the first electronic device using the timing independent signal. 
     
     
       13. A system, comprising:
 a third party device comprising a reliable clock source, wherein the third party device is configured to transmit timing information derived from the reliable clock source in response to receiving a timing independent request, wherein the timing independent signal is comprised of a voltage change in the timing independent signal that is detectable by the third party device, wherein the timing information is comprised of a clock signal; and 
 a first electronic device coupled to the third party device, wherein the first electronic device comprises a microcontroller with an internal oscillator, and wherein the first electronic device is configured to:
 transmit the timing independent request to the third party device; and 
 calibrate the internal oscillator with the timing information received from the third party device. 
 
 
     
     
       14. The system of  claim 13 , wherein the third party device is a wireless headset, and wherein the reliable clock source is a crystal oscillator. 
     
     
       15. The system of  claim 13 , wherein the first electronic device is a docking device adapted to be an accessory device for the third party device. 
     
     
       16. The system of  claim 13 , further comprising a second electronic device for communicating with the first electronic device using a timing dependent protocol. 
     
     
       17. The system of  claim 16 , wherein the second electronic device is a portable media player. 
     
     
       18. An electronic device, comprising:
 a first communication link for transmitting, to a second electronic device, a request for timing information using a timing independent protocol,
 wherein the second electronic device comprises a reliable clock source, wherein the timing independent protocol is comprised of a voltage change in a timing independent signal that is detectable by the second electronic device; 
 
 a second communication link for receiving the timing information, wherein the timing information is comprised of a clock signal; and 
 a microcontroller for controlling operations of the electronic device, wherein an internal oscillator of the microcontroller is calibrated based on the received timing information. 
 
     
     
       19. The electronic device of  claim 18 , wherein the first communication link is adapted for providing power to the second electronic device. 
     
     
       20. The electronic device of  claim 18 , further comprising:
 a regulator for providing a first voltage and a second voltage, wherein the first voltage is different from the second voltage; and 
 a switch for selectively providing one of the first voltage and the second voltage to the first communication link, wherein the request for the timing information is transmitted by changing a state of the switch for a period of time. 
 
     
     
       21. A method of calibrating a clock source of a first electronic device for use in enabling timing dependent communication between the first electronic device and a second electronic device, the method comprising:
 transmitting a request for timing signals from the first electronic device to a plurality of third party devices using a timing independent protocol, wherein the timing independent protocol is comprised of a voltage change in a timing independent signal that is detectable by the plurality of third party devices; 
 receiving a plurality of timing signals with the first electronic device from the plurality of third party devices in response to the request, wherein the plurality of timing signals is comprised of clock signals; 
 deriving timing information with the first electronic device from at least a subset of the timing signals; and 
 calibrating the clock source based on the timing information. 
 
     
     
       22. The method of  claim 21 , wherein the clock source is an internal oscillator of a microcontroller within the first electronic device. 
     
     
       23. The method of  claim 21 , wherein the deriving the timing information comprises averaging the at least a subset of the timing signals. 
     
     
       24. The method of  claim 21 , wherein the deriving the timing information comprises selecting one of the plurality of timing signals based on at least one magnitude of the timing signals and priorities of ports of the first electronic device that obtained each of the plurality of timing signals. 
     
     
       25. The method of  claim 21 , wherein the plurality of timing signals are received with the first electronic device from at least a subset of the plurality of third party devices.

Description:
FIELD OF THE INVENTION 
     This can relate to clocking for electronic devices and, more particularly, to obtaining a reliable clock signal from a third party electronic device for microcontroller clock calibration. 
     BACKGROUND OF THE DISCLOSURE 
     Currently, there are a wide variety of electronic devices in everyday use. For example, many individuals own cellular telephones and portable media players for on-the-go communication and entertainment. There are even electronic devices that are specifically designed to be accessory devices to other electronic devices, such as wireless Bluetooth™ headsets for cellular telephones. These accessory devices may enhance the functionality, convenience, or aesthetics of another electronic device. For example, a wireless Bluetooth™ headset may enhance the use of a cellular telephone by allowing users to have a hands-free, wireless conversation through their cellular telephone. Similarly, a docking device may be an accessory to a portable media player, where the docking device is used to update and provide power to the portable media player. 
     Two or more electronic devices, such as a device and its accessory device, can communicate using an established protocol. For example, the devices may communicate using a serial interface, such as a universal serial bus (“USB”) interface. For USB protocols and other serial protocols, the transfer of information occurs at an agreed upon data rate. If either device loses its ability to accurately transmit or receive information at that data rate, synchronization may be lost and communication may cease. Therefore, each of the devices typically includes a reliable clock source for use in maintaining data transfer at the agreed upon data rate. Electronic devices often use crystal oscillators as this reliable clock source. 
     Although crystal oscillators are reliable and accurate, they have several disadvantages. First, they are large components. For a portable device, where size is a crucial factor in its design, having such a large component in the device is highly undesirable. Moreover, crystal oscillators are typically expensive components and are also a common source of manufacturing defects in commercial electronic devices. Accordingly, it would be beneficial to be able to provide an approach for a microcontroller-based electronic device to accurately transmit and receive serial data without including an extra clock source. 
     SUMMARY OF THE DISCLOSURE 
     Systems and methods are provided for calibrating the internal oscillator of a microcontroller based on a remote clock source. 
     In one embodiment of the invention, timing dependent communication between a first electronic device and a second electronic device can be enabled. A timing independent signal may be transmitted from the first device to a third party device to request transmission of timing information, and a clock source of the first device can be calibrated based on the timing information transmitted from the third party device and received by the first device. Then, timing dependent communication may be conducted between the first device and the second device, where the data rate of the timing dependent communication is based on a clock rate of the clock source. 
     In another embodiment of the invention, timing information can be transmitted from a first electronic device to a second electronic device. The first device can include a reliable clock source, such as a crystal oscillator, and the second device can include a microcontroller with an internal oscillator. First, a timing independent signal can be received with the first device from the second device. A request for timing information can be detected with the first device from the received timing independent signal. For example, the first device can detect a request by detecting a voltage change of the timing independent signal. Timing information may then be transmitted from the first device to the second device. The timing information may be derived from the reliable clock source of the first device, and the timing information may be used by the second device to calibrate the internal oscillator of its microcontroller. 
     In still another embodiment of the invention, a system is provided that can include a third party device and a first electronic device coupled to the third party device. The third party device can include a reliable clock source and can be configured to transmit timing information that is derived from the reliable clock source in response to receiving a timing independent request. The third party device may be, for example, a wireless headset with a crystal oscillator as its reliable clock source. The first device can include a microcontroller with an internal oscillator. The first device can be configured to transmit the timing independent request to the third party device and calibrate the internal oscillator with the timing information received from the third party device. The first device may be, for example, a docking device adapted to be an accessory device for the third party device. 
     The system may further include a second electronic device that can communicate with the first device. The first and second devices may communicate using a timing dependent protocol, such as a USB protocol. The second device may be, for example, a portable media player, and the first device may be a docking device adapted to be an accessory device for the portable media player. 
     In still another embodiment of the invention, an electronic device is provided that can include a first and a second communication link. The first communication link can be adapted to transmit a request for timing information using a timing independent protocol, and the second communication link can be adapted to receive the timing information. The electronic device can also include a microcontroller that can control operations of the electronic device. An internal oscillator of the microcontroller can be calibrated based on the received timing information. 
     The electronic device may also include a regulator and a switch. The regulator can provide a first voltage and a second voltage different from the first voltage. The switch can selectively provide one of the voltages to the first communication link. The request for timing information may be transmitted from the first communication link by changing a state of the switch for a period of time. 
     In still another embodiment of the invention, a clock source of a first electronic device can be calibrated for use in enabling timing dependent communication between the first electronic device and a second electronic device. A request for timing signals can be transmitted from the first device to a plurality of third party devices using a timing independent protocol. In response to the request, a plurality of timing signals may be received with the first device from the plurality of third party devices, and the first device can derive timing information from at least a subset of the timing signals. For example, the timing information may be derived by averaging the at least a subset of the timing signals. Then, the clock source of the first device may be calibrated based on the timing information. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects and advantages of the invention will become more apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
         FIG. 1  shows a simplified system diagram in accordance with an embodiment of the invention; 
         FIG. 2  shows an illustrative timing diagram for calibrating a microcontroller clock in accordance with an embodiment of the invention; 
         FIG. 3  shows a simplified block diagram of an electronic device in accordance with an embodiment of the invention; 
         FIG. 4  shows a simplified circuit for providing a clock signal for a microcontroller in accordance with an embodiment of the invention; 
         FIG. 5  shows a simplified block diagram of a third party electronic device in accordance with an embodiment of the invention; 
         FIG. 6  shows another illustrative timing diagram for calibrating a microcontroller clock in accordance with an embodiment of the invention; 
         FIGS. 7 and 8  show illustrative flow diagrams for initiating communication with an electronic device in accordance with various embodiments of the invention; 
         FIG. 9  shows an illustrative flow diagram for calibrating a microcontroller clock based on a plurality of timing signals in accordance with an embodiment of the invention; 
         FIG. 10  shows an illustrative flow diagram for transmitting a timing signal for clock calibration in accordance with an embodiment of the invention; and 
         FIGS. 11 and 12  show illustrative flow diagrams for maintaining communication with a second device in accordance with various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     An electronic device in accordance with an embodiment of the invention can include a microcontroller for controlling the basic operation of the electronic device. In some embodiments, the electronic device may be an accessory device, such as a microcontroller-based docking device for one or more other electronic devices. The microcontroller of the electronic device may have an internal oscillator, sometimes referred to herein as a microcontroller clock, that relies on a reliable clock source for maintaining a consistent and accurate frequency. 
     Instead of including a reliable clock source on the electronic device to calibrate the microcontroller clock, a clocking signal may be obtained from another electronic device that has a reliable clock source. The other electronic device may be, for example, a Bluetooth™ wireless device having a crystal oscillator. In the embodiment where the microcontroller-based electronic device is a docking device, this other electronic device may be one of the devices that couples to the docking device. The other electronic device may be sometimes referred to herein as a “third party device,” because, in some embodiments, and for any given period of time, the third party device may not communicate with the electronic device other than to provide a clocking signal. However, the term “third party” is not intended to limit the invention to any particular type of device, or to any particular functionality other than providing a clocking signal. 
     The electronic device can request transmission of timing information (e.g., a clocking signal) for microcontroller clock calibration from the third party device. The electronic device may request timing information, for example, when it is no longer able to partake in timing-based communication or “timing dependent communication” (e.g., USB) with other devices. Alternatively, the electronic device may request timing information periodically irrespective of its ability to partake in timing dependent communication. In some embodiments, the electronic device may provide power to the third party device, and may transmit a request for timing information by changing the voltage on the power line from, for example, 5 volts to 3.3 volts for a period of time sufficient for the third party device to detect the change. This or any other voltage-based out-of-band signaling technique does not depend on the data rate of the transmitted request, and therefore may be referred to as “timing independent.” A timing independent protocol can be advantageous, as accurate transmission of the request does not rely on the microcontroller clock having an accurate frequency. 
     In response to detecting the voltage change, the third party device may begin transmitting timing information. The transmission rate of the timing information may depend on the rate of the reliable clock source of the third party device. The electronic device can use the received timing information to calibrate the internal oscillator of its microcontroller. After proper calibration, the microcontroller clock may be sufficiently accurate to perform any suitable timing dependent functions. For example, the electronic device can initiate timing dependent communication with a second electronic device, which may be the third party device or a different electronic device. After a predetermined period of time, or if proper communication ceases unexpectedly, the electronic device may again request timing information from the third party device. 
     In some embodiments, the electronic device may derive timing information for its microcontroller clock from multiple signals received from different third party devices. The electronic device can, for example, average the received signals to produce the timing information. In addition, one or more of the signals can be selected based on any suitable criteria, such as the magnitude of the received signals or the priority of the ports from which the signals were received. 
     Methods and systems for calibrating the internal oscillator of a microcontroller based on a remote clock source are provided and described with reference to  FIGS. 1-12 . 
       FIG. 1  shows a simplified block diagram of system  100 , and is intended to represent any collection of two or more electronic devices that are configured to communicate with one another in accordance with the invention. In the illustrated embodiment, system  100  includes three electronic devices: third party device  102 , accessory device  104 , and portable electronic device  106 . 
     Third party device  102  can be any suitable electronic device that includes a clock source  108 . Clock source  108  may be a crystal oscillator or any other source that can reliably provide an accurate clock signal at a fixed or controllable frequency. Third party device  102  can use the signal provided by clock source  108  to derive and transmit a clocking signal to accessory device  104 , and to enable its own timing dependent communication. For example, third party device  102  can be a wireless headset that uses its reliable clock source for USB communication. In this embodiment, third party device  102  may have any of the components, features, or functionalities of the wireless headsets discussed in commonly assigned U.S. Patent Application Publication No. 2008/0164934 , published Jul. 10, 2008 (hereinafter “the headset application”), which is hereby incorporated herein by reference in its entirety. 
     Accessory device  104  can be any suitable microcontroller-based electronic device, and can include a microcontroller  110  for controlling the basic operation of the device. In some embodiments, accessory device  104  may be a docking device-type accessory for portable electronic device  106  and/or third party device  102 . For example, accessory device  104  may have any of the components, features, or functionalities of the docking devices discussed in co-pending, commonly assigned U.S. Patent Application Publication No. 2008/0167088 , published Jul. 10, 2008 (hereinafter “the docking application”), which issued Dec. 27, 2011 as U.S. Pat. No. 8,086,281, and which is hereby incorporated herein by reference in its entirety. 
     Microcontroller  110  of accessory device  104  may be any suitable microcontroller that can use a clock source external to microcontroller  110  to calibrate its internal oscillator  111 . As described above, accessory device  104  can obtain timing information from third party device  102 , and can calibrate microcontroller  110  based on the received clocking signal. Thus, the signal used by microcontroller  110  to calibrate its internal oscillator  111  may be based on the reliable clock signal of clock source  108 . This signal obtained from third party device  102  may be sometimes referred to herein as “timing information.” Because the timing information is timing dependent, the timing information may also be referred to as a “timing dependent signal” or simply a “timing signal.” 
     Portable electronic device  106  can be any suitable electronic device capable of communicating with accessory device  104 . Portable electronic device  106  may communicate with accessory device  104  to obtain updates, information, or power. For example, portable electronic device  106  may be a portable media player (e.g., an iPod™) or a cellular telephone (e.g., an iPhone™) that can obtain power, media file downloads, software updates, user preference settings, synchronization settings, or any other suitable information from accessory device  104 . In some embodiments, portable electronic device  106  may not be “portable,” but may be designed for use in a fixed location. 
     As shown in  FIG. 1 , third party device  102 , accessory device  104 , and electronic device  106  may be coupled via communications links. In particular, third party device  102  may communicate with accessory device  104  via communications link  112 , and accessory device  104  may communicate with electronic device  106  via communications link  114 . Communications links  112  and  114  can include any number and combination of suitable wired or wireless paths for enabling timing independent or timing dependent communication. Communications links  112  and  114  can include power lines, serial data lines, coaxial cables, standard cables for given communications protocols, or space for wireless data transmission. Although not shown in  FIG. 1 , in some embodiments, third party device  102  may directly communicate with portable electronic device  106  through another communications link. Alternatively, third party device  102  may indirectly communicate with portable electronic device  106  through accessory device  104  and communications links  112  and  114 . 
     In some embodiments, accessory device  104  may communicate with portable electronic device  106  using a serial interface or another timing dependent interface. Thus, communication between these devices may rely on the ability of accessory device  104  and portable electronic device  106  to maintain an agreed upon data rate. Because accessory device  104  may not include a reliable clock source (e.g., a crystal oscillator) for clocking microcontroller  110 , the internal oscillator of microcontroller  110  may be susceptive to deviation from its normal frequency, thereby potentially preventing reliable communication between the two devices. To maintain reliable communication, accessory device  104  may request timing information from third party device  102  for use in recalibrating its microcontroller clock  111 . However, especially in cases where communication ceases between accessory device  104  and portable electronic device  106 , timing dependent communication also may not be possible between third party device  102  and accessory device  104 . Thus, in various embodiments of the invention, accessory device  104  can be configured to transmit to third party device  102  a timing independent request for timing information. Third party device  102  may be configured to detect the timing independent request, and, in response, may transmit timing information to accessory device  104 . An example of this approach is illustrated in  FIG. 2 . 
       FIG. 2  shows timing diagram  200  that illustrates one approach for accessory device  104  to reliably request and obtain timing information from third party device  102 , and will be described with continuing reference to  FIG. 1 . Timing diagram  200  illustrates three signals: the first (“request”) representing a request for timing information that may be transmitted from accessory device  104  to third party device  102 , the second (“RX”) representing timing information that may be received by accessory device  104  from third party device  102 , and the third (“TX”) representing a timing dependent signal that may be transmitted by accessory device  104  at the completion of clock calibration. For clarity, the description below of timing diagram  200 , and any other timing diagrams described herein, assumes that a signal transmitted from one device is the same signal received by the other device, and vice versa. 
     Accessory device  104  may initiate a request for timing information at some time, t 1 . Accessory device  104  may automatically initiate a request, for example, when it is no longer able to communicate using a timing independent protocol, after a predetermined period of time, or when a previous attempt at clock calibration is unsuccessful. At time t 1 , accessory device  104  may enter into a state, which may sometimes be referred to herein as a “CALIBRATION state,” that can occur whenever accessory device  104  recalibrates its microcontroller clock  111 . In CALIBRATION state, accessory device  104  may suspend any timing dependent functions, such as timing dependent communication with portable electronic device  106 . 
     To request transmission of timing information from third party device  102 , accessory device  104  may transmit a timing independent signal to third party device  102  at time t 1 . Therefore, as described above, even if an internal oscillator of microcontroller  110  cannot maintain timing dependent communication, a request for timing information can still be reliably transmitted. In some embodiments, accessory device  104  may initiate the request at time t 1  by switching the request voltage from a first voltage V 1  to a second voltage V 2  for a period of time sufficient for third party device  102  to detect the change. The voltage change can create voltage pulse  202 . For example, if third party device  102  detects the voltage change at some time t 2 , pulse  202  may be of sufficient length to initiate transmission of timing information from third party device  102 . This timing independent approach illustrates one form of timing independent communication that may be sometimes referred to herein as “level-based signaling.” In other embodiments, transmission-based signaling, where third party device  102  may be configured to detect a transition in the request signal, may be used to communicate a request for timing information. 
     As described above, at time t 2 , third party device  102  may determine that a request for timing information has been received. In response to receiving the request, third party device  102  may begin transmitting timing information  204  at time t 2 . Timing information  204  can be any suitable signal that enables accessory device  104  to calibrate its microcontroller clock  111 . In some embodiments, and as shown in  FIG. 2 , timing information  204  may be a clock signal with a 50% duty ratio. In other embodiments, timing information  204  may be a clock signal with a different duty ratio or a signal of another suitable sequence or pattern. Third party device  102  may transmit timing information  204  to accessory device  104  until a time, t 4 . The amount of time timing information  204  is transferred, or t 4 -t 2 , may be a period of time sufficient for accessory device  104  to complete microcontroller clock calibration. For example, if accessory device  104  completes microcontroller clock calibration at time t 3 , timing information  204  is transmitted for a sufficient amount of time. 
     With continuing reference to  FIGS. 1 and 2 , accessory device  104  may calibrate internal oscillator  111  of microcontroller  110  using timing information  204  received from third party device  102 . Accessory device  104  may enable clock calibration to occur while in CALIBRATION state—that is, from time t 1  to a some later time, t 3 . The time between t 1  and t 3  may be a predetermined amount time programmed or hardwired into accessory device  104 , after which proper clock calibration is assumed to have been completed. If clock calibration is completed successfully, accessory device  104  may be capable of performing timing dependent tasks. 
     At time t 3 , accessory device  104  may enter into a new state, which may sometimes be referred to herein as a “COMMUNICATION state.” In this state, accessory device  104  may disable microcontroller clock calibration, and may initiate or resume any timing dependent tasks. For example, accessory device  104  can initiate timing dependent communication with portable electronic device  106 , and can begin transmitting timing dependent data  206  to portable electronic device  106 . Alternatively, accessory device  104  may begin exchanging information with a different device, check connections between various devices coupled to accessory device  104 , establish connections between various devices, provide updates to various devices, facilitate transfer of data between various devices, etc. These and other tasks that can be performed by accessory device  104  are described in greater detail in the docking application, for example. 
     It should be understood that system  100  of  FIG. 1  and timing diagram  200  of  FIG. 2  are merely illustrative. In fact, system  100  can include any suitable number of electronic devices with any suitable number of communications links coupling them. Also, the above described embodiments of third party device  102 , accessory device  104 , and portable electronic device  106 , as well as their functions as described in connection with  FIG. 2 , are merely illustrative. Devices  102 ,  104 , and  106  can each be any suitable electronic device capable of communicating with one or more other devices, and do not necessarily have a device-accessory relationship. For example, and where appropriate, each of third party device  102 , accessory device  104 , portable electronic device  106 , and any other device in communication with system  100  (not shown) may be any suitable portable or stationary electronic device, including but not limited a laptop computer, a desktop computer, an audio player (e.g., a Walkman™, compact disc player, etc.), a video player, a media player (e.g., an iPod™, etc.), a set top box, a portable video game system (e.g., Sony&#39;s PSP™, Nintendo&#39;s Game Boy™, etc.), an electronic book, a cellular telephone, a wireless telephone, a hand held computer, a global positioning system (“GPS”) device, a personal digital assistant (“PDA”) (e.g., Palm&#39;s Pilot™, etc.), a wireless headset for a telephone, a satellite radio, a remote control, an automobile key fob, a printer, an automobile radio, an automobile computing system, an automobile cigarette lighter (or other mobile power source, such as an airplane cigarette lighter), a camera, an accessory device for a computer (e.g., a wireless mouse, wireless keyboard, etc.), a watch, a surge protector, an AC/DC converter, etc. 
       FIGS. 3-5  show illustrative embodiments of electronic devices capable of operating in accordance with the invention. In particular,  FIG. 3  shows an illustrative block diagram of a microcontroller-based electronic device capable of transmitting a request for timing information.  FIG. 4  shows an illustrative block diagram of a clock calibration circuit for processing received timing information, which can be implemented on the electronic device of  FIG. 3 . Finally,  FIG. 5  shows an illustrative block diagram of a third party device capable of detecting a request for timing information and transmitting timing information in response to detecting the request. 
     Referring first to  FIG. 3 , a simplified and illustrative block diagram of an accessory device  300  is shown. Accessory device  300  may be a more detailed, yet still simplified view, of accessory device  104  of  FIG. 1 . Accessory device  300  can include port  302 , port  304 , port  306 , regulator  308 , switch  310 , and microcontroller  312 . Device  300  can be implemented using a single integrated circuit or, for example, a multi-chip module including two or more separate integrated circuits. Also, as described above, although device  300  is referred to as an “accessory” device, this is merely one embodiment of device  300 . Device  300  can be any suitable type of electronic device with any suitable relationship to the other electronic devices it may communicate with. 
     The block diagram of accessory device  300  is merely illustrative. For clarity, the components of accessory device  300  will be described below mainly in terms of the ability of device  300  to request timing information and to calibrate its microcontroller clock based on the received timing information. However, it should be understood that accessory device  300  can have many features and functionalities, and any additional components, such as those described in the docking application. Moreover, each component of accessory device  300  may have any of the features or embodiments described in connection with one or more corresponding components in the docking application. 
     Ports  302 ,  304 , and  306  of accessory device  300  can each be any suitable type of wired or wireless port (e.g., a female USB connector, a male 30-pin connector, and a symmetrical 4-pin connector, respectively) that enables other electronic devices to be coupled to accessory device  300 . Ports  302 ,  304 , and  306  can respectively couple powering device  324 , portable electronic device  326 , and third party device  328  to accessory device  300 . Powering device  324 , coupled via port  302 , may be any suitable type of electronic device discussed above, such as an AC adapter or a computer, that can provide power, among other things, to accessory device  300  via power supply line  316 . Portable electronic device  326 , coupled via port  304 , can be similar in functionality to portable electronic device  106  of  FIG. 1 . That is, accessory device  300  may be operable to provide power, information, or updates to portable electronic device  326  via power supply line  318 , for example. Third party device  328 , coupled via port  306 , may be similar in functionality to third party device  102  of  FIG. 1 . For example, third party device  328  may operate in conjunction with accessory device  300  to produce timing waveforms similar to those shown in timing diagram  200  of  FIG. 2 . 
     It should also be understood that, in some embodiments or in some operating scenarios, power may be provided to device  300  from portable electronic device  326  or third party device  328  in addition to or instead of from power device  324 . Also, although only three ports are shown in  FIG. 3 , it should be understood that accessory device  300  may include any suitable number of ports that may couple any suitable number of devices to accessory device  300 . 
     Microcontroller  312  may have the same or similar features and functionality as microcontroller  110  of  FIG. 1 . Microcontroller  312  may operate based on an internal oscillator  311  that relies on another clock source external to the microcontroller for maintaining an accurate frequency. For example, microcontroller  312  can be any suitable commercial 8-bit, 16-bit, or larger microcontroller (e.g., Intel&#39;s 8088, etc.) with one or more clock inputs for accepting timing information. Accessory device  300  may include one or more components that enable proper operation of microcontroller  312  (e.g., additional storage units for use as instruction or data memory). Accessory device  300  can obtain timing information from third party device  328 . In some embodiments, accessory device  300  may obtain timing information from other devices as well. 
     Microcontroller  312  can facilitate the transfer of information and power among the devices coupled to accessory device  300 . In particular, microcontroller  312  may facilitate the transfer of power from powering device  324  (via power supply line  316 ) to third party device  328  and portable electronic device  326  (via power supply line  314  and power supply line  318 , respectively). Also, microcontroller  312  may be configured to transmit and receive information to and from and between portable electronic device  326  and third party device  328  via transmit/receive (TX/RX) line  322  and TX/RX line  320 , respectively. TX/RX lines  320  and  322  can be bidirectional links or can include one or more separate transmit and receive links. 
     Information exchanged via TX/RX lines  320  and  322  can be exchanged using a timing dependent protocol (e.g., a serial protocol, such as a USB protocol), where the information data rate may be based on the internal oscillator  311  of microcontroller  312 . Thus, the reliability of data transfer, and the ability to transfer data at all, may depend on the consistency and accuracy of the internal oscillator  311  of microcontroller  312 . Because accessory device  300  may not include a reliable clock source for maintaining an accurate microcontroller clock frequency, accessory device  300  may request timing information from third party device  328 , or any other device coupled to accessory device  300 , whenever recalibration of the microcontroller clock  311  is necessary. 
     As described above, accessory device  300  may request timing information using a timing independent approach (e.g., a level-based or transition-based approach) to ensure that a request can be accurately transmitted even when the internal oscillator  311  of microcontroller  312  is inaccurate. In particular, accessory device  300  may transmit a request by changing power voltage V x  provided to third party device  328  from a first voltage, V 1  to a second voltage, V 2 , or vice versa. For example, V 1  may be the voltage typically provided to third party device  328  to power or charge the third party device  328 . Microcontroller  312  can transmit a request by lowering the voltage typically provided to third party device  328  to a significantly lower voltage, V 2 , for a predetermined period of time, for example. Microcontroller  312  can initiate this request by controlling switch  310  to select between outputs of regulator  308 . In this way, microcontroller  312  can generate the request waveform shown in timing diagram  200  of  FIG. 2 . 
     Regulator  308  can regulate power obtained from powering device  324 , or from any combination of devices coupled to accessory device  300 , to obtain various voltages, such as voltages V 1  and V 2 . These voltages may be used to power third party device  328  and portable electronic device  326 , and may be used to transmit timing independent requests for timing information. V 1  and V 2  can be any standard power voltage, such as 3V, 3.3V, or 5V, or any nonstandard power voltage. For simplicity, it will be assumed that V 1  is greater than V 2 . Thus, for example, V 1  may be 5 volts and may be the voltage typically supplied to third party device  328 , while V 2  may be 3.3 volts. In some embodiments, regulator  308  may generate these two voltages by taking the voltage of power supply line  316  as V 1 , and stepping down V 1  to obtain V 2 . Alternatively, regulator  308  may take the voltage of power supply line  316  as V 2  and may boost V 2  to obtain V 1 , or regulator  308  may derive both V 1  and V 2  from the voltage at power supply line  316  using some other suitable technique. Regulator  308  can be implemented using any suitable approach (e.g., a linear regulator, a buck/boost regulator, or any other PWM-based regulator, etc.), and is therefore not limited to any particular implementation. 
     Switch  310  of  FIG. 3  can provide one of V 1  and V 2  as power supply voltage V x  for third party device  328 , and can be modeled as a single pole, double throw switch, for example. That is, in one state, switch  310  may couple the V 1  output of regulator  308  to power supply line  314 , and in another state, switch  310  may couple the V 2  output of regulator  308  to power supply line  314 . Thus, to initiate a request for timing information, microcontroller  312  can change the state of switch  310  using, for example, control line  330 . This can allow microcontroller  312  to apply a pulse on voltage supply line  314  to third party device  328  for a time sufficient for third party device  328  to detect the level change (for level-based signaling) or the voltage transition (for transition-base signaling). It should be understood that switch  310  can be implemented using any suitable technique (e.g., a transistor-based switch), and is therefore not limited to any particular implementation. 
     Microcontroller  312  can receive the timing information from third party device  328  via TX/RX line  320 , for example. Thus, in some embodiments, TX/RX line  320  may support both the transfer of data as well as the transfer of timing information. In these embodiments, TX/RX line  320  may be coupled not only to the data input/outputs (“I/Os”) of microcontroller  312 , but also directly or indirectly to the clock inputs of microcontroller  312 . In other embodiments, third party device  328  and accessory device  300  may include a separate communication link (not shown) dedicated to the transfer of timing information. Calibrating microcontroller  312  via timing information received from third party device  328  or portable electronic device  326 , or both, will be described in greater detail below in connection with  FIG. 4 . 
     In some embodiments, timing information may be requested from multiple devices, instead of only from third party device  328 . For example, timing information may be requested from both third party device  328  and portable electronic device  326 . Microcontroller  312  may control switch  310  to transmit the same timing independent request via both power supply line  314  and power supply line  318 . Thus, each request can be signaled to both third party device  328  and portable electronic device  326  substantially concurrently. Alternatively, switch  310  may be controlled to selectively signal requests to one or more particular devices. For example, switch  310  may provide a first voltage value V x1  for third party device  328  and a second voltage value V x2 , for portable electronic device  326 . Microcontroller  312  may selectively change one of these voltage signals to initiate a request with one of these devices. Microcontroller  312  may be configured to select a particular device to receive a request for any suitable reason. For example, microcontroller  312  may be configured to transmit a request to only those ports that have devices coupled to them. Similarly, microcontroller  312  may be configured to send a request to a device coupled to either the highest or lowest priority port. Port priorities and other determinations that microcontroller  312  may use to control one or more voltages V x  are discussed in greater detail in the docking application, for example. 
     Accessory device  300  may transmit timing independent requests for timing information via power supply line  314 . This technique may be advantageous because an extra communication link dedicated to transmissions of timing independent requests is not necessary. Moreover, many devices (e.g., the wireless headsets described in the headset application) may already be capable of detecting changes in their power supply voltage, and would not require a substantial amount of extra circuitry to detect requests for timing information. However, it should be understood that in other embodiments, a different communication link can be used to transmit requests (e.g., TX/RX link  320 ) or an extra communication link can be implemented that is dedicated to the transmission of these requests. 
     Referring now to  FIG. 4 , an illustrative block diagram of clock calibration circuit  400  is shown for providing timing information to a microcontroller. Calibration circuit  400  can be implemented as part of accessory device  300  of  FIG. 3  to provide appropriate timing information to microcontroller  312 . Clock calibration circuit  400  can include selection circuit  402 , tri-state buffer  404 , and clock circuit  406 . 
     Selection circuit  402  can derive a timing dependent signal useful for producing timing information from various inputs, illustrated in  FIG. 4  as inputs CLK 1  through CLKN, for example. Two clocks, CLK 1  and CLK 2 , may be provided, for example, from third party device  328  and portable electronic device  326  of  FIG. 3  via communications lines  320  and  322 , respectively. Thus, selection circuit  402  may be configured to allow a subset of signals received from other devices to affect the timing information eventually provided to the microcontroller (e.g., microcontroller  312  of  FIG. 3 ). In some embodiments, selection circuit  402  may be implemented as a single pole, double throw switch, and may select one of the CLK inputs to output as the timing independent signal. Selection circuit  402  may make this selection based on the value of a SELECT input  410 , for example. In other embodiments, selection circuit  402  may average two or more of the clock inputs. For example, selection circuit  402  may average the signal values of CLK 1  and CLK 2  to obtain a new timing dependent signal. In still other embodiments, selection circuit  402  may be operable to either select a single CLK input or average multiple CLK inputs based on, for example, the value of SELECT input  410 . 
     SELECT input  410 , which may control the selection operation of selection circuit  402 , may be derived from the microcontroller (e.g., microcontroller  312  of  FIG. 3 ). The microcontroller can select a particular operation based on any suitable factors. In some embodiments, the microcontroller may enable the selection of one or more clocks of greatest magnitude, or may enable selection based on quality of the input clock signals. Alternatively, the microcontroller may choose one or more clocks based on the ports that the CLK input signals originated from. 
     With continuing reference to  FIG. 4 , tri-state buffer  404  can be configured according to an ENABLE input  412  to allow a signal to pass through buffer  404 , for example, only when the electronic device (e.g., accessory device  104  of  FIG. 1 ) is in a CALIBRATION state. For the example of  FIG. 2 , tri-state buffer  404  may be enabled to pass its input data to its output between time t 1  and time t 3 , (e.g., the period of time that timing information may be requested). While the electronic device is in a COMMUNICATION state, on the other hand, tri-state buffer  404  may be configured to output high impedance. In this way, tri-state buffer  404  can prevent a different type of signal (e.g., a data signal, a timing independent signal), or a signal transmitted at a frequency other than the desired frequency, from affecting the internal oscillator of the microcontroller. ENABLE input  412 , which may control the state of tri-state buffer  404 , may be controlled by the microcontroller. Thus, at the time that the microcontroller requests timing information, the microcontroller can also enable tri-state buffer  404 . Then, once the microcontroller determines that its internal oscillator has finished recalibrating, it can disable tri-state buffer  404 . 
     Clock circuit  406  of calibration circuit  400  can include any suitable circuitry to convert the timing signal provided by selection circuit  402  to one or more Xtal input(s)  408  in a format expected by the clock input of the microcontroller (e.g., microcontroller  312  of  FIG. 3 ). In some embodiments, clock circuit  406  may change the characteristics (e.g., voltage or current) of the timing signal. Also, clock circuit  406  may include any passive components, such as resistors or inverters, that would have been necessary even if a reliable clock source were present in the microcontroller. In some embodiments, clock circuit  406  may improve the quality of the signal provided by selection circuit  402 . For example, clock circuit  406  may include an operational amplifier-based comparator for improving the edges of the signal provided by tri-state buffer  404 . 
     Referring now to  FIG. 5 , an illustrative block diagram of a third party device  500  is shown in accordance with an embodiment of the invention. In some embodiments, the block diagram of  FIG. 5  is a more detailed, yet still simplified, view of third party device  102  of  FIG. 1  or third party device  328  of  FIG. 3 . Thus, third party device  500  can be coupled to accessory device  300  of  FIG. 3 , for example, by coupling port  514  of device  500  to port  306  of accessory device  300  of  FIG. 3 . Third party device  500  can include power bus  502 , battery  504 , detector  506 , clock source  508 , processing circuitry  510 , communications circuitry  512 , and I/O lines Vcc/Vdd and TX/RX. 
     The block diagram of third party device  500  is merely illustrative. For clarity, the components of third party device  500  will be described below mainly in terms of their ability to detect requests for timing information and to provide timing information in response to detecting these requests. However, it should be understood that third party device  500  can have many functions and functionalities, and any additional components, such as those described in the headset application. Moreover, each component of third party device  500  may have any of the features or embodiments described in connection with one or more corresponding components in the headset application. 
     Processing circuitry  510  can be any suitable combination of hardware, software, or firmware, and any accompanying components (e.g., memory elements) necessary for controlling the operation of third party device  500 . Although processing circuitry  510  is shown as a single component, third party device  500  may instead have multiple processing circuitries that each have their own specialized functions. 
     In some embodiments, processing circuitry  510  can provide information to and process information obtained from an accessory device coupled through port  514 , such as accessory device  300  of  FIG. 3 . Processing circuitry  510  may communicate with an accessory device using a timing dependent protocol (e.g., a serial protocol), where the data rate of communication is based on a clock signal provided by clock source  508 , for example. Clock source  508  can be any suitable clock source that provides a reliable clock signal, such as a crystal oscillator, and may be the same or a similar clock source as described above in connection with clock source  108  of third party device  102  of  FIG. 1 . 
     Third party device  500  may include communications circuitry  512  to accurately exchange information with an accessory device coupled via port  514 . In some embodiments, communications circuitry  512  may include an encoder to convert information provided by processing circuitry  510  to information suitable for transmission from device  500 , or to convert the information to a standard transmission format (e.g., USB). Similarly, communications circuitry  512  can include any necessary circuitry for interpreting information obtained from the coupled accessory device, such as detectors, error control decoders, or USB decoders, for example. 
     As described above, a third party device, such as third party device  500 , can be a portable electronic device. For example, third party device  500  can be a wireless headset. Third party device  500  can include a battery  504  to provide power to the other components of device  500  (e.g., processing circuitry  510 , communication circuitry  512 , etc.). Battery  504  may be any suitable portable powering device, such as a lithium ion battery, for example. 
     Power can also be provided to third party device  500  via one or more power supply lines. In particular, when an accessory device is coupled to device  500  via port  514 , for example, third party device  500  can draw power from the accessory device using one or more power supply lines. For example, when third party device  500  is connected to accessory device  300  of  FIG. 3 , third party device  500  can obtain power from power supply line  314  of accessory device  300 . The power supply line can be used to provide power of any suitable voltage (e.g., 5V, 3.3V, V 1 , V 2 , etc.). The power provided by the power supply line can be transported to different areas of third party device  500 , and to the various components of third party device  500 , by power bus  502 , for example. Power bus  502  can be any suitable power line for transporting power across third party device  500 . In some embodiments, power bus  502  can be coupled to battery  504  and can be used to recharge battery  504 . 
     The components of third party device  500  may be selectively powered by either power bus  502  or battery  504 , or both. In some embodiments, power bus  502  can provide power to some or all of the other components of third party device  500  when power can be drawn from a device coupled to port  514 . For example, power bus  502  can be used to power one or more of the components of third party device  500  (e.g., to all components but communications circuitry  512 , which may be powered instead by battery  504 ). If power cannot be drawn from another device, the components of device  500  may instead be powered by battery  504 . The determination as to which source may power the components of third party device  500  can be based on the detection results of detector  506 . In other embodiments of the invention, each of the components of third party device  500  may be powered by battery  504  regardless of whether power can be drawn from another device. In such embodiments, the power provided to power bus  502  may be used solely to recharge battery  504 . 
     With continuing reference to  FIG. 5 , detector  506  can be coupled to power bus  502  and can monitor the voltage on power bus  502 . Detector  506  can provide a signal to processing circuitry  510  when an expected power voltage (e.g., V 1 ) on power bus  502  changes to a different voltage (e.g., V 2 ). As described above, timing information may be transmitted in response to detecting such a voltage change. To perform this detection, detector  506  can include any necessary components or circuitry, such as one or more voltage comparators or analog-to-digital converters (“ADCs”). For example, to detect when the voltage on power bus  502  drops from V 1  to V 2 , a voltage comparator can be used to detect when the voltage dips below a certain voltage, (e.g., below a voltage V, where V 1 &gt;V&gt;V 2 ). In some embodiments, detector  506  can be powered by battery  504  to provide a substantially constant power source while monitoring the voltage on power bus  502 . 
     Processing circuitry  510  can be configured to react to a particular voltage change on power bus  502  detected by detector  506  (e.g., from V 1  to V 2 ). Thus, when detector  506  detects a request for timing information, processing circuitry  510  can react by having timing information sent via the TX/RX line. For example, if processing circuitry  510  includes a microprocessor, a detected voltage change on power bus  502  may trigger an interrupt sequence to be initiated. This interrupt sequence may include instructions to output timing information via TX/RX line through communications circuitry  512 . 
     Third party device  500  and accessory device  300  may be operable to communicate according to timing diagram  200  of  FIG. 2 , thereby enabling accessory device  300  to recalibrate its microcontroller clock  311  based on a reliable clock source of third party device  500 . Alternatively, another suitable handshaking protocol may be used between the two devices. One such alternative protocol is illustrated by timing diagram  600  of  FIG. 6 . 
       FIG. 6  will be described in connection with accessory device  300  and third party device  500 . Timing diagram  600  illustrates four waveforms: the first representing the voltage of power supply line  314  provided from accessory device  300  to third party device  500  (V x ), the second representing the information received by accessory device  300  from third party device  500  via TX/RX line  320  (RX_third), the third representing the information transmitted by accessory device  300  and received by third party device  500  via TX/RX line  320  (TX_third), and the fourth representing information transmitted by accessory device  300  to another electronic device (e.g. portable electronic device  326 ) via TX/RX line  322  (TX). 
     At time t 5 , accessory device  300  can request timing information from third party device  500  by changing power supply voltage V x , from a first voltage, V 1 , to a second voltage, V 2 , for example. In particular, at this time, microcontroller  312  of accessory device  300  can enter into a CALIBRATION state, and can be configured to change the state of switch  310  to create pulse  602 . Microcontroller  312  of accessory device  300  can generate pulse  602  for a period of time sufficient for third party device  500  to detect the change. 
     Once third party device  500  detects the voltage change at time t 6  (e.g., via detector  506 ), third party device  500  may begin transmitting timing information  604  to accessory device  300 . The timing information may be used by device  300  to calibrate internal oscillator  311  of its microcontroller  312 . 
     At time t 7 , microcontroller  312  of accessory device  300  may transmit a packet of information to third party device  500  using a timing dependent format. The packet may therefore be transmitted at a rate dependent on the internal oscillator  311  of microcontroller  312 . This timing dependent packet is illustrated in timing diagram  600  as TX_PKT  606 , where TX_PKT  606  may be any suitable digital sequence or pattern of any suitable length. The sequence or pattern transmitted by accessory device  300  may be known and expected by third party device  500 . If third party device  500  is able to accurately interpret TX_PKT  606  at time t 8 , proper clock calibration can be assumed. In response to accurately receiving TX_PKT  606 , third party device  500  may stop transmitting timing information  604 , and may instead transmit acknowledgment (“ACK”)  608  to accessory device  300 . 
     Upon receiving acknowledgement  608  of proper clock calibration from third party device  500  at time t 9 , accessory device  300  may switch from CALIBRATION state to COMMUNICATION state. Accessory device  300  can initiate timing dependent communication with third party device  500  or with any other suitable electronic device (e.g., portable electronic device  326  of  FIG. 3 ). In particular, accessory device  300  may transmit data  610  to another device using a suitable timing dependent protocol (e.g., USB). Other tasks device  300  may perform are discussed above in connection with timing diagram  200  of  FIG. 2 . 
     In some scenarios, proper clock calibration may not have completed by t 7 . In this case, timing dependent communication may not be possible between third party device  500  and accessory device  300 . Therefore, at time t 8 , if third party device  500  is not able to accurately interpret TX_PKT  606  transmitted from accessory device  300 , third party device  500  may not send acknowledgement  608 . In this scenario, accessory device  300  may continue to calibrate its clock according to timing information  604 , and can transmit TX_PKT  606  again at a later time. Third party device  500  may send an acknowledgement once a subsequent TX_PKT is received accurately. Thus, accessory device  300  may continue to calibrate its microcontroller clock and send packets to third party device  500  as many times as is necessary (unless a timeout is implemented) to enable timing dependent communication. 
     Referring now to  FIG. 7 , an illustrative flow diagram of a process  700  is shown for enabling time dependent communication (e.g. using a serial protocol) between a first electronic device (e.g., accessory device  300  of  FIG. 3 ) and a second electronic device (e.g., portable electronic device  326 ). The first electronic device can execute the steps of process  700 . 
     At step  702 , the first electronic device may transmit a signal to a third party device (e.g., third party device  500  of  FIG. 5 ). The signal can be transmitted to request transmission of timing information from the third party device. In some embodiments, the signal may be timing independent and may be provided by changing the power supply voltage of the third party device. For example, the voltage of the power line may be changed from a normal voltage of V 1  (e.g., 5V) to a voltage substantially less than V 1  (e.g., 3.3V). Changing the voltage in this way may allow for transition-based signaling or level-based signaling, neither of which is necessarily dependent on the rate of communication. 
     After transmitting a request for timing information, the first electronic device may receive the requested timing information. The timing information may be received after a period of time corresponding to the time it takes for the third party device to detect the request, process the request, and transmit the timing information. The timing information can be of any suitable form, such as a clock signal with a suitable duty ratio (e.g., 50%, etc.) or a timing dependent signal with a suitable signaling pattern. At step  704 , a microcontroller clock of the first device can be calibrated using the received timing information. For example, the timing information or a processed version of the timing information received from the third party device may be provided to one or more clock inputs of the microcontroller. 
     With continuing reference to  FIG. 7 , at step  706 , the first electronic device can initiate timing dependent communication with another electronic device (e.g., the second electronic device). The timing dependent communication can be of any suitable format or standard, such as a USB standard. Also, the second electronic device can be any other device, including but not limited to the third party device. For example, timing dependent communication can be initiated with portable electronic device  326  of  FIG. 3 , which may be an iPod™ or iPhone™. The timing dependent communication can be initiated at a rate based on the rate of the internal microcontroller clock of the first electronic device. Thus, the ability of the timing dependent communication may depend on the success of the calibration of the internal oscillator at step  704 . 
     Any normal, timing dependent or timing independent functions can be performed after the steps of process  700  are completed. These may involve performing any tasks that would have been performed even if clock calibration from a third party device were not necessary. In some embodiments, information may be exchanged with the other devices. In fact, any suitable tasks may be performed at this point, including checking connections between various devices, establishing connections between various devices, providing updates to various devices, or facilitating the transfer of data between various devices, for example. 
     It should also be understood that process  700  of  FIG. 7  and any other process described below are merely illustrative. In fact, any of the shown steps of process  700  or other processes may be omitted or modified, and any additional steps may be performed without departing from the scope of the invention. 
     Referring now to  FIG. 8 , an illustrative flow diagram of a process  800  is shown for calibrating a microcontroller of a first electronic device (e.g., microcontroller  312  of accessory device  300  of  FIG. 3 ) using timing information from a third party device (e.g., third party device  500  of  FIG. 5 ). The steps of the flow diagram are alternative steps for those shown in  FIG. 7 , and can also enable the first electronic device to communicate with a second electronic device (e.g., portable electronic device  326  of  FIG. 3 ) using a timing dependent protocol. These steps differ from those of process  700  at least because process  800  ensures that proper calibration of the microcontroller clock has occurred before initiating communication with the second electronic device. 
     Similar to step  702  and step  704  described above, at step  802  and step  804 , the first electronic device can transmit a request for transmission of timing information to the third party device and can calibrate its microcontroller clock based on the timing information received from the third party device. 
     Then, at step  806 , the first electronic device can transmit a predetermined packet of data to the third party device. The predetermined packet can be of any suitable length and of any suitable pattern. The predetermined packet may be chosen such that it is unlikely to be interpreted correctly by the third party device unless the packet is transmitted at an accurate data rate. Thus, if the third party device can correctly receive the packet, accurate microcontroller clock calibration can be assumed. At step  808 , the first electronic device can determine whether communication with the third party device is possible. This determination may involve receiving an acknowledgement from the third party device if the third party device is able to correctly interpret the predetermined packet. If communication is possible, the first electronic device can initiate timing dependent communication with another electronic device at step  810 . 
     If, according to the determination at step  808 , communication is not yet possible, process  800  may move back to step  804 , and the first electronic device may again calibrate its microcontroller clock at step  804 . Alternatively, process  800  may instead return to step  802 , and the first electronic device may again request transmission of timing information from the third party device. The first electronic device may determine that communication is not possible at step  808 , for example, if no acknowledgement is received from the third party device within a predetermined amount of time. Thus, using the steps of flow diagram  800 , the internal oscillator of the microcontroller may continually be calibrated until, at step  808 , it is accurate enough for timing dependent communication. 
     Referring now to  FIG. 9 , an illustrative flow diagram of a process  900  is shown for deriving timing information from at least a subset of a plurality of received timing dependent signals. A suitable electronic device, such as accessory device  300  of  FIG. 3 , can execute the steps of process  900 . At step  902 , a plurality of timing dependent signals may be received. The plurality of timing dependent signals may be received from a plurality of different electronic devices. For example, accessory device  300  can receive a plurality of timing dependent signals from portable electronic device  326  and third party device  328  of  FIG. 3 . At step  904 , timing information can be derived from at least a subset of the timing dependent signals. In some embodiments, some or all of the timing dependent signals may be averaged. In other embodiments, one of the timing dependent signals may be selected based on any suitable criteria. For example, a timing dependent signal may be selected based on which signal has the largest peak magnitude or based on which signal is obtained from the highest or lowest priority port. Then, at step  906 , the first electronic device can calibrate its microcontroller clock using the timing information derived from the plurality of timing dependent signals. 
     Referring now to  FIG. 10 , an illustrative flow diagram of a process  1000  is shown for transmitting timing information for clock calibration in accordance with an embodiment of the invention. Process  1000  can be executed by a third party device in order to provide timing information to another electronic device. For example, process  1000  can be executed by third party device  500  of  FIG. 5  to provide timing information to accessory device  300  of  FIG. 3 , thereby allowing accessory device  300  to calibrate its microcontroller clock. 
     At step  1002  of process  1000 , the third party device may receive a signal from an electronic device (e.g., accessory device  300  of  FIG. 3 ). The signal may be a timing independent signal and may be a level-triggered or transition-triggered signal, which may be sent as a request for timing information. In some embodiments, the timing independent signal may be detected in the form of a voltage change on a power line or another suitable communication link. 
     In response to receiving the timing independent signal, the third party device can suspend its current activity at step  1004 . For example, if the third party device is communicating with another electronic device or is running any suitable program, the third party device can suspend operation of these functions. Then, at step  1006 , the third party device may send timing information to the electronic device that requested timing information. The electronic device may use this timing information to calibrate its microcontroller clock. 
     At step  1008 , the third party device can determine whether calibration of the electronic device&#39;s microcontroller clock is complete. The third party device may make this determination based on a packet sent by the electronic device, or may assume that calibration is complete once a predetermined period of time passes. If, at step  1008 , the third party device determines that clock calibration is not complete, the third party device can continue to send timing information to the electronic device at step  1006 . Alternatively, process  1000  can move back to step  1002 , and the third party device can wait for a new signal from the electronic device that requests completion of clock calibration. If, according to the determination at step  1008 , clock calibration has completed, the third party device can, at step  1010 , resume any of the activities that it may have previously suspended at step  1004 . 
     It should be understood that the flow diagram of process  1000  illustrated in  FIG. 10  is merely illustrative. For example, in some embodiments, the third party device may not need to suspend any activities in order to transmit timing information. Alternatively, rather than suspending current activities, timing information may be transmitted once any current operations are completed. 
       FIGS. 11 and 12  show illustrative flow diagrams of processes  1100  and  1200 , respectively, for maintaining communication between a first electronic device (e.g., accessory device  300  of  FIG. 3 ) and a second electronic device (e.g., portable electronic device  326  of  FIG. 3 ) in accordance with various embodiments of the invention. Processes  1100  and  1200  can be executed by the first electronic device, such as accessory device  300  of  FIG. 3 . Thus, these flow diagrams illustrate two approaches for continually recalibrating the internal oscillator of the first electronic device&#39;s microcontroller (e.g., internal oscillator  311  of microcontroller  312  of  FIG. 3 ) to ensure that timing dependent communication remains possible with the second electronic device. 
     Referring first to  FIG. 11 , the flow diagram of process  1100  is shown for maintaining communication with the second electronic device by periodically recalibrating the internal oscillator of a microcontroller irrespective of microcontroller clock accuracy. At step  1102 , the first electronic device can calibrate or recalibrate its microcontroller clock. Calibrating the microcontroller clock may involve any of the steps described above in connection with  FIGS. 7-10 , for example. After clock calibration, the first electronic device can start communicating or resume communicating with the second electronic device at step  1104 . After a predetermined amount of time (e.g., after one second, five seconds, one minute, etc.), the flow of process  1100  may again return to step  1102 . That is, the microcontroller clock may again be recalibrated regardless of the accuracy of the internal oscillator. This approach may advantageously ensure that the internal oscillator remains accurate and reliable, and that communication capabilities do not cease. 
     Referring now to  FIG. 12 , another flow diagram is shown that illustrates a process for allowing a first electronic device to initiate and/or maintain communication with a second electronic device. In process  1200 , the microcontroller of the first electronic device is recalibrated only when necessary. 
     At step  1202 , the first electronic device may initiate communication with the second electronic device. The communication may be based on a timing dependent protocol, such as a USB protocol. At step  1204 , the first electronic device may determine whether communication is possible with the second electronic device. In some embodiments, the determination can be made based on whether the second electronic device responds appropriately to any information transmitted at step  1202 , such as with a return acknowledgment or with any requested information. If, at step  1204 , the first electronic device determines that communication is not possible, the first electronic device may recalibrate its microcontroller clock at step  1206 . Recalibrating the microcontroller clock may involve any of the steps described above in connection with  FIGS. 7-11 . Once recalibrated, communication with the other device may again be initiated at step  1202 . 
     If, based on the determination at step  1204 , communication is possible with the second electronic device, the first electronic device may continue to communicate or start to communicate at step  1208  with the second electronic device. In some embodiments, actual data transfer between the two devices may begin at step  1208 , as proper communication has been established. Then at step  1210 , the first electronic device can determine whether a timeout in communication has occurred with the second electronic device. For example, a timeout in communication may occur when the second electronic device does not respond to information requests sent by the first electronic device within a predetermined amount of time. If the first electronic device determines that a timeout in communication has not occurred, the first electronic device can continue communicating with the first electronic device at step  1208 . Otherwise, process  1200  can move to step  1206 , and the first electronic device can recalibrate its internal microcontroller clock. Thus, when communication ceases, the first electronic device can assume that its microcontroller clock has lost accuracy, and can recalibrate its microcontroller clock to regain communications capabilities. 
     The foregoing describes systems and methods for calibrating the internal oscillator of a microcontroller based on a remote clock source. Those skilled in the art will appreciate that the invention can be practiced by other than the described embodiments, which are presented for the purpose of illustration rather than of limitation, and the invention is limited only by the claims which follow.

Metadata:
Filing Date: 20071219
Publication Date: 20131015
Grant Date: 20131015
Priority Date: 20071219
Inventors: ANANNY JOHN M.
KALAYJIAN NICHOLAS R.
RABU STANLEY
TIKALSKY TERRY
Assignee: APPLE INC
CPC Classifications: [{"code": "G06F13/4045", "inventive": true, "first": true, "tree": "[]"}, {"code": "G04G7/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "G04G7/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F13/4045", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 40788616