Abstract:
A highly versatile interface that is capable of digital and audio signal coupling is provided. The interface comprises contacts ( 122, 124, 216, 218 ) that are used to couple both audio and digital signals, and separate contacts ( 126, 220 ) that are used initiate and negotiate signaling mode transitions. Transitions can be effected without creating glitches, e.g., audible noise, in audio signals that are being coupled through the interface.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to peripheral interfaces. More particularly, the present invention relates to a multi-purpose peripheral interface. 
     BACKGROUND OF THE INVENTION 
     The adaptation of handheld communication devices, such as cellular telephones, text messaging devices and devices that support multiple different communication modes, has had a transformative effect on personal communications over the last decade. Such handheld devices have untethered their users from the fixed Plain Old Telephone System (POTS) land lines and desktop computers networked through the POTS and have provided ubiquitous communications and instant reachability. 
     In the future, it is expected that handheld communication devices (in particular cellular telephones) will carry a variety of personal and/or financial information, and be able to interface with a variety of disparate systems. Such enhanced cellular telephones are expected to be used for, among other things, file storage and transfer, identification, access control, and making and receiving payments—in addition to communication. 
     Given the need to interface with a variety of systems, such as systems in cars, home entertainment systems, public and private infrastructure, personal computers, etc, and the limited size of handheld communication devices, it is desirable to provide a limited number of interfaces or one very versatile interface. One form of interface is wireless. An example of a wireless interface that might be used to provide local connectivity is known as Bluetooth. However, in certain circumstances, wireless security concerns, interference issues, and power dissipation issues weigh in favor of using a wired interface. Thus, it is desirable to provide a very versatile wired interface for handheld communication devices. 
     It is desirable to be able to use such versatile wired interface to couple analog signals, such as audio signals, and a variety of types of digital signals. It is furthermore desirable to be able to transition between different types of signaling without causing any glitches. In particular, it is desirable to be able to transition to and from audio signaling without causing audible noise. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention will be described by way of exemplary embodiments, but not limitations, illustrated in the accompanying drawings in which like references denote similar elements, and in which: 
         FIG. 1  is a block diagram of a wireless communication device according to an embodiment; 
         FIG. 2  is a block diagram of an accessory that is capable of interfacing with the wireless communication device shown in  FIG. 1  in several different modes according to an embodiment; 
         FIG. 3  is a first flowchart showing actions performed by a hosting device such as the wireless communication device shown in  FIG. 1  in connecting to an accessory such as the accessory shown in  FIG. 2 ; 
         FIG. 4  is a second flowchart showing actions performed by an accessory such as the accessory shown in  FIG. 2  in coordination with the actions shown in  FIG. 3  performed by a hosting device such as the wireless communication device shown in  FIG. 1 ; 
         FIG. 5  is a first signal chart showing signals exchanged between a hosting device such as the wireless communication device shown in  FIG. 1  and an accessory such as the accessory shown in  FIG. 2  in the process of connecting; 
         FIG. 6  is a third flowchart showing actions performed by a hosting device such as the wireless communication device shown in  FIG. 1  in the course of initiating analog mono audio mode signaling between the hosting device and an accessory such as the accessory shown in  FIG. 2 ; 
         FIG. 7  is a fourth flowchart showing actions performed by an accessory such as the accessory shown in  FIG. 2  in coordination with the actions shown in  FIG. 6  which are performed by a hosting device such as the wireless communication device shown in  FIG. 1 ; 
         FIG. 8  is a second signal chart showing signals exchanged between a hosting device such as the wireless communication device shown in  FIG. 1  and an accessory such as the accessory shown in  FIG. 2  in the process of initiating analog mono audio mode signaling; 
         FIG. 9  is a fifth flowchart showing actions performed by a hosting device such as the wireless communication device shown in  FIG. 1  in the course of initiating analog stereo audio mode signaling to an accessory such as the accessory shown in  FIG. 2 ; 
         FIG. 10  is a sixth flowchart showing actions performed by an accessory such as the accessory shown in  FIG. 2  in the coordination with the actions shown in  FIG. 9 ; 
         FIG. 11  is a third signal chart showing signals exchanged between a hosting device such as the wireless communication device shown in  FIG. 1  and an accessory such as the accessory shown in  FIG. 2  in the course of performing the actions shown in  FIGS. 9–10 ; 
         FIG. 12  is a seventh flowchart showing actions performed by a hosting device such as the wireless communication device shown in  FIG. 1  in order to transition from analog mono audio signaling mode to UART signaling mode; 
         FIG. 13  is a eighth flowchart showing actions performed by an accessory such as the accessory shown in  FIG. 2  in response to the actions shown in  FIG. 12  which are performed by a hosting device such as the wireless communication device shown in  FIG. 1  in order to transition from analog mono audio signaling mode to UART signaling mode; 
         FIG. 14  is a fourth signal chart showing signals exchanged between a hosting device such as the wireless communication device shown in  FIG. 1  and an accessory such as the accessory shown in  FIG. 2  in the course of performing the actions shown in  FIGS. 12–13  in order to transition from analog mono audio signaling mode to UART signaling mode; 
         FIG. 15  is a ninth flowchart showing actions performed by a hosting device such as the wireless communication device shown in  FIG. 1  in order to transition from analog stereo audio signaling mode to UART signaling mode; 
         FIG. 16  is a tenth flowchart showing actions performed by an accessory such as the accessory shown in  FIG. 2  in response to the actions shown in  FIG. 15  which are performed by a hosting device such as the wireless communication device shown in  FIG. 1  in order to transition from analog stereo audio signaling mode to UART signaling mode; 
         FIG. 17  is a fifth signal chart showing signals exchanged between a hosting device such as the wireless communication device shown in  FIG. 1  and an accessory such as the accessory shown in  FIG. 2  in the course of performing the actions shown in  FIGS. 15–16  in order to transition from analog stereo audio signaling mode to UART signaling mode; 
         FIG. 18  is a eleventh flowchart showing actions performed by an accessory such as the accessory shown in  FIG. 2  in order to transition from analog mono audio signaling mode to UART signaling mode; 
         FIG. 19  is a twelfth flowchart showing actions performed by a hosting device such as the wireless communication device shown in  FIG. 1  in response to the actions shown in  FIG. 18  which are performed by an accessory such as the accessory shown in  FIG. 2  in order to transition from analog mono audio signaling mode to UART signaling mode; 
         FIG. 20  is a sixth signal chart showing signals exchanged between a hosting device such as the wireless communication device shown in  FIG. 1  and an accessory such as the accessory shown in  FIG. 2  in the course of performing the actions shown in  FIGS. 18–19  in order to transition from analog mono audio signaling mode to UART signaling mode; 
         FIG. 21  is a thirteenth flowchart showing actions performed by an accessory such as the accessory shown in  FIG. 2  to transition from analog stereo audio signaling mode to UART signaling mode; 
         FIG. 22  is a fourteenth flowchart showing actions performed by a hosting device such as the wireless communication device shown in  FIG. 1  in response to the actions shown in  FIG. 21  which are performed by an accessory such as the accessory shown in  FIG. 2  in order to transition from analog stereo mode signaling to UART signaling; 
         FIG. 23  is a seventh signal chart showing signals exchanged between a hosting device such as the wireless communication device shown in  FIG. 1  and an accessory such as the accessory shown in  FIG. 2  in the course of performing the actions shown in  FIGS. 21–22  in a case in which there is no interrupt collision; 
         FIG. 24  is a eighth signal chart showing signals exchanged between a hosting device such as the wireless communication device shown in  FIG. 1  and an accessory such as the accessory shown in  FIG. 2  in the course of performing the actions shown in  FIGS. 21–22  in a case in which there is an interrupt collision. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. 
     The terms a or an, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. 
       FIG. 1  is a block diagram of a wireless communication device  100  according to an embodiment. The wireless communication device  100  is one example of an electronic apparatus that can serve as a hosting device according to the teachings described herein, and interface with an external device. The wireless communication device  100  has a first controller  102  with a first microprocessor  104 , a first memory  106 , a first Universal Serial Bus (USB) module  108 , a first Universal Asynchronous Receiver/Transmitter (UART) module  110 , a Digital-to-Analog converter (D/A)  112 , and an Analog-to-Digital converter (A/D)  114 . Alternatively, rather than being integrated in the first controller  102 , the foregoing components can be implemented separately. The memory  106  is used to store programs that are executed by the first microprocessor  104  to operate the device  100 . Aspects of the operation of the device  100  are described below with reference flowcharts. The first USB module  108  and the first UART module  110  are used to communicate digital message signals with external devices. The D/A  112  is used, in certain cases, to generate analog signals that are coupled out to external devices. The A/D  114  is used, in certain cases, to digitize analog signals that are received from external devices. 
     A transceiver  116  is coupled to the first controller  102 . The transceiver  116  is used to send and receive wireless communications through, for example, a cellular network, a satellite network, or a wireless Local Area Network (LAN). 
     A first connector  118  is used to connect the device  100  with other, external devices. The first connector  118  has a first bus voltage connection contact  120 , a first signaling line contact (D+)  122 , a second signaling line contact (D−)  124 , a first separate interrupt line contact (ID)  126  and a first ground reference connection contact  128 . Within, the device  100 , the first bus voltage connection contact  120  is coupled to a power regulator  130  and a bus voltage level detector  131 , the first signaling line contact  122  is coupled to a first switch network, in particular a first multiplexer/demultiplexer (MUX/DEMUX)  132 , the second signaling line contact  124  is coupled to a second switch network, in particular a second MUX/DEMUX  133 , and the first interrupt line contact  126  is coupled to a first ID line driver  134  and to a first ID level detector  136 . The first ground reference contact  128  is coupled to a ground plane (not shown) of the device  100 . 
     The first MUX/DEMUX  132  is also coupled to the first USB module  108 , the first UART module  110 , the D/A  112  and the A/D  114 . A first terminal  138  of the first MUX/DEMUX terminal  132  is coupled to an input contact  140  of the first UART module  110 , a second terminal  142  of the first MUX/DEMUX  132  is coupled to a first channel output  144  of the D/A  112 , a third terminal  146  of the first MUX/DEMUX  132  is coupled to an input  148  of the A/D  114 , and a fourth terminal  150  of the first MUX/DEMUX  132  is coupled to the first signaling line contact (D+)  122 . The first MUX/DEMUX  132  serves to selectively couple the first signaling line contact (D+)  122  to either the input  140  of the first UART module  110 , the first channel output  144  of the D/A  112 , the input  148  of the A/D  114  or the USB module  108 . With respect to the functioning of the terminals of the first MUX/DEMUX  132 , the first terminal  138  serves as an output, the second terminal  142  serves as an input, the third terminal  146  serves as an output, and the fourth terminal  150  serves as both an input and an output. 
     The second MUX/DEMUX  133  is also coupled to the first USB module  108 , the first UART module  110 , and the D/A  112 . In particular a first terminal  152  of the second MUX/DEMUX  133  is coupled to a second channel output  154  of the D/A  112 , a second terminal  156  of the second MUX/DEMUX  133  is coupled to an output terminal  158  of the first UART module  110 , and a third terminal  160  of the second MUX/DEMUX  133  is coupled to the second signaling line contact (D−)  124 . The second MUX/DEMUX  133  serves to selectively couple the second signaling line contact (D−)  124  to either the second channel output  154  of the D/A  112 , the output terminal  158  of the first UART module  110 , or the first USB module  108 . Note that the first USB module uses differential signaling for receiving and sending signals. Regarding the terminals of the second MUX/DEMUX  133 , the first terminal  152  serves as an input, the second terminal  156  serves as an input and the third terminal  160  serves as both an input and an output. 
     The first signaling line contact (D+)  122  and the second signaling line contact (D−)  124  are coupled to the first UART module  110  or the USB module  108  when the device  100  is configured for digital message signaling. In UART mode, digital signal messages will be input through the first signaling line contact (D+)  122  and output through the second signaling line contact (D−)  124 . When the device  100  is to be configured to output stereo (two channel) audio analog signals through the connector  118 , the first channel output  144  of the D/A  112  is coupled to the first signaling line contact (D+)  122  and the second channel output  154  of the D/A  112  is coupled to the second signaling line contact (D−)  124 . When the device  100  is to be configured for duplex mono audio analog signaling the first signaling line contact (D+)  122  is coupled to the input  148  of the A/D  114 , and the second channel output  154  of the D/A  112  is coupled to the second signaling line contact (D−)  124 . 
     The first microprocessor  104  is also coupled to the first MUX/DEMUX  132  and the second MUX/DEMUX  133  and controls the routing of signals by the first MUX/DEMUX  132  and the second MUX/DEMUX  133 . 
     A first variable bias network  162  is coupled to the first signaling line contact (D+)  122 , and a second variable bias network  164  is coupled to the second signaling line contact (D−)  124 . The biasing networks  162 ,  164  (and  256 ,  258 ,  FIG. 2 ) include, for example, one or more voltage sources, and one or more biasing resistors. The biasing networks  162 ,  164  are used to bias the signaling line contacts  122 ,  124  to a plurality of different voltage levels that are appropriate for different signaling modes, i.e. UART signaling, USB signaling, and analog audio signaling. 
     The ID line driver  134  is used to drive the first interrupt line contact  126  to different levels in the course of negotiating transitions between different signaling modes with an external device. The level detector  136  is used to detect changes in voltage levels on the first interrupt line contact  126  which are caused by a line driver in an external device in the course of transitions between signaling modes. The use of the interrupt line contact is described in more detail below. The bus voltage level detector  131  is used to detect the connection of the wireless communication device  100  to an external device. 
     The device  100  optionally includes an internal audio system  166  that includes, for example, an internal microphone, an internal speaker, and amplifiers. 
       FIG. 2  is a block diagram of an accessory  200  that is capable of interfacing with the wireless communication device  100  shown in  FIG. 1  in several different modes according to an embodiment. The accessory  200  is one example of a device that can serve as the external device referred to above that can interface with the wireless communication device  100 . From the perspective of the accessory  200 , the wireless communication device  100  is an external device. 
     The accessory  200  includes a second controller  202  with a second microprocessor  204 , a second memory  206 , a second USB module  208 , and a second UART module  210 . The accessory  200  interfaces to an external device, e.g., the wireless communication device  100 , through a second connector  212 . The second connector  212  has contacts for coupling, e.g., directly or through a cable, to the contacts  120 ,  122 ,  124 ,  126 ,  128  of the first connector  118 . In particular, the second connector  212  includes a second bus voltage connection contact  214 , a third signaling line contact (D+)  216 , a fourth signaling line contact (D−)  218 , a second separate interrupt line contact  220 , and a second ground reference contact  222 . The third signaling line contact (D+)  216  couples to the first signaling line contact (D+)  122  of the first connector  118 , and the fourth signaling line contact (D−)  218  couples to the second signaling line contact (D−)  124  of the first connector  118 , directly or through a cable. The coupling of the third signaling line contact (D+)  216  and the first signaling line contact (D+)  122  is referred to as a D+ line, and the coupling of the fourth signaling line contact (D−)  218  to the second signaling line contact (D−)  124  is referred to as a D− line. The first separate interrupt line contact (ID)  126  and the second separate interrupt line contact  220  are coupled together (e.g., through a cable, or directly) forming what is referred to hereinbelow as an ID line. The second bus voltage connection contact  214  couples to the first bus voltage connection contact  120 . The second ground reference contact  222  couples to the first ground reference connection contact  128 . 
     The third signaling line contact (D+)  216 , a microphone amplifier  224 , a first speaker amplifier  226 , the second USB module  208 , and the second UART module  210  are coupled to a third switch network in particular a third MUX/DEMUX  228 . In particular, an output  230  of the microphone amplifier  224  is coupled to a first terminal  232  of the third MUX/DEMUX  228 , a second terminal  234  of the third MUX/DEMUX  228  is coupled to an input  236  of the first speaker amplifier  226 , a third terminal  238  of the third MUX/DEMUX  228  is coupled to an output  240  of the second UART module  210 , and a fourth terminal  242  of the third MUX/DEMUX is coupled to the third signaling line contact (D+)  216 . The third MUX/DEMUX  228  serves to selectively couple the third signaling line contact (D+)  216  to either the output  230  of the microphone amplifier  224 , the input  236  of the first speaker amplifier  226 , or the output  240  of the UART module  210  or the second USB module  208 . Regarding the terminals of the third MUX/DEMUX  228 , the first terminal  232  serves as an input, the second terminal  234  serves as an output, the third terminal  238  serves as an input and the fourth terminal  242  serves as both an input and an output. 
     The fourth signaling line contact (D−)  218 , a second speaker amplifier  244 , the second UART module  210  and the second USB module  208  are coupled to a fourth switch network in particular a fourth MUX/DEMUX  246 . In particular, a first terminal  248  of the fourth MUX/DEMUX  246  is coupled to an input  250  of the second speaker amplifier  244 , a second terminal  252  of the fourth MUX/DEMUX  246  is coupled to an input  254  of the second UART module  210 , and a third terminal  255  of the fourth MUX/DEMUX  246  is coupled to the fourth signaling line contact (D−)  218 . The fourth MUX/DEMUX  246  serves to selectively couple either the input  250  of the second speaker amplifier  244 , the input  254  of the second UART module  210 , or the second USB module  208  to the fourth signaling line contact (D−)  218 . Regarding the terminals of the fourth MUX/DEMUX  246 , the first terminal  248  serves as an output, the second terminal  252  serves as an output and the third terminal  255  serves as both an input and an output. 
     A microphone  225  is coupled to an input  227  of the microphone amplifier  224 , a first loudspeaker  229  is coupled to the first speaker amplifier  226 , and a second loudspeaker  245  is coupled to the second speaker amplifier  244 . 
     For duplex mono analog audio signaling, the third MUX/DEMUX  228  is configured to couple the output  230  of the microphone amplifier  224  to the third signaling line contact (D+)  216 , and the fourth MUX/DEMUX  246  is configured to couple the fourth signaling line contact (D−)  218  to the input  250  of the second speaker amplifier  244 . For stereo audio signaling, the third MUX/DEMUX  228  is configured to couple the third signaling line contact (D+)  216  to the input  236  of the first speaker amplifier  226 , and the fourth MUX/DEMUX  246  is configured to couple the fourth signaling line contact (D−)  218  to the input  250  of the second speaker amplifier  244 . For sending and receiving UART digital signal messages, the third MUX/DEMUX  228  is configured to couple the third signaling line contact (D+)  216  to the output  240  of the second UART module  210  and the fourth MUX/DEMUX  246  is configured to couple the fourth signaling line contact (D−)  218  to the input  254  of the second UART module  210 . 
     The second microprocessor  204  is also coupled to the third MUX/DEMUX  228  and the fourth MUX/DEMUX  246  and controls the routing of signals by the third MUX/DEMUX  228  and the fourth MUX/DEMUX  246 . The USB module uses differential signaling, using the third signaling line contact (D+)  216  and the fourth signaling line contact (D−)  218  in both receive and transmit mode. 
     The accessory  200  has a third variable bias network  256  that is coupled to the third signaling line contact (D+)  216  and a fourth variable bias network  258  that is coupled to the fourth signaling line contact (D−)  218 . The third  256  and fourth  258  variable biasing networks serve to bias the third  216  and fourth  218  signaling line contacts to levels that are appropriate for different types of signals, e.g. USB signals, UART signals and analog audio signals. 
     The accessory  200  includes a second ID line driver  260 , and a second ID level detector  262  which are coupled to the second separate interrupt line contact  220 . The second ID line driver  260  serves to drive a voltage on the second separate interrupt line contact  220  to different levels in the course of negotiating transitions between different signaling modes with the wireless communication device  100  or another external device. The second ID level detector  262  is used to detect changes in voltage levels on the second interrupt line contact  220  which are caused by the first line driver  134  in the wireless communication device  100  or by a line driver in another device with which the accessory  200  is interfaced in the course of transitions between signaling modes. The first ID level detector  136  and the second ID level detector  262  detect high and low signal states by comparing the voltage on the ID line to one or more voltage thresholds that are intermediate a voltage corresponding to the high signal state and the a voltage corresponding to the low signal state. The use of the interrupt line contact is described in more detail below. A bus voltage regulator  264  is coupled to the second bus voltage connection contact  214 . When the accessory  200  is connected to the device  100 , the bus voltage level detector  131  will sense that the device  100  is connected to the accessory  200  by sensing that the voltage on the first bus voltage connection contact  120  has been regulated to a predetermined voltage level. 
       FIG. 3  is a first flowchart  300  showing actions performed by a hosting device in connecting to the accessory shown in  FIG. 2 ,  FIG. 4  is a second flowchart  400  showing actions performed by an accessory in coordination with the actions shown in  FIG. 3 , and  FIG. 5  is a first signal chart  500  showing signals exchanged between the hosting device and the accessory in the process of connecting. The wireless communication device  100  or another device with the capability to perform the actions shown in  FIG. 3  can serve as the hosting device. The accessory  200  shown in  FIG. 2  or another accessory with the capability to perform the steps shown in  FIG. 4  can serve as the accessory that connects to the hosting device. 
     As indicated in block  302  of the first flowchart  300  and block  402  of the second flowchart  400 , the process of connecting starts with the hosting device (e.g.,  100 ) and the accessory (e.g.,  200 ) in a disconnected state. Following block  302 , block  304  of the first flowchart is a decision block, the outcome of which depends on whether the voltage on the first bus voltage connection contact  120  is at a predetermined level that indicates that the hosting device is connected to the accessory (e.g.,  200 ). If the outcome of block  304  is negative then the hosting device continues in the disconnected state. If, on the other hand, the outcome of decision block  304  is affirmative, then the hosting device proceeds to block  306  and enters an initial (pre-connection) state. 
     Referring momentarily to  FIG. 5 , a brief explanation of the first signal chart  500  will be given. The first line  502  indicates the state of the hosting device (e.g.,  100 ). The states of the hosting device are labeled ph_disc which is the disconnected state, ph_init which is the initial state entered in block  306 , and ph_uart which is a UART signaling state. The second line  504  indicates the state of the accessory (e.g.,  200 ). The states shown in the first signal chart  500  are labeled cr_disc which stands for the accessory&#39;s disconnected state, cr_init which stands for an initial (pre-connection) state for accessory, and cr_uart which is a UART signaling state. The third line  506  indicates the voltage level on the first  120  and second  214  bus voltage connection contacts which are now coupled (e.g. via a cable, or directly). The fourth line  508  shows the signal on the second signaling line contact (D−)  124  and the fourth signaling line contact (D−)  218  which are now coupled. The fifth line  510  shows the signal on the first signaling line contact (D+)  122  and the third signaling line contact (D+)  216  which are now coupled. The sixth line  512  shows time intervals between host and accessory initiated events. In the seventh line  514  the initiator of each particular signal event occurring at a particular time is identified. In the seventh line  514 , the hosting device is identified by the letter P, the accessory is identified by the letter C, and events initiated by both devices are identified with the letter B. The signal charts shown in  FIGS. 8 ,  11 ,  14 ,  17 ,  20 ,  23 ,  24  do not include line  506 , but do include the remaining lines shown in  FIG. 5  and also include an eighth line  802  that shows signals on the first separate interrupt line contact (ID)  126  and the second separate interrupt line contact  220 . 
     Referring now as well to  FIG. 3 , the state transition that occurs in block  306  is between the ph_disc and the ph_init state. The latter transition is shown in the first line  502  of the first signal chart. In block  308  the hosting device pulls the first signaling line contact (D+)  122  high. In block  404  the accessory checks the voltage level on the third signaling line contact (D+)  216  (now coupled to the first signaling line contact (D+)  122 ). If the accessory (e.g.,  200 ) were to find that the third signaling line contact (D+) was low, then the accessory (e.g.,  200 ) would continue in the disconnected state. When the accessory detects that the third signaling line contact (D+)  216  has been pulled high, the accessory enters the accessory initial state cr_init as indicated in block  406 , and shown in the second line  504  of the first signal chart  500 . By pulling the first signaling line contact (D+)  122  high the hosting device initiates a handshake. 
     After entering the initial state cr_init, as shown in block  406 , the accessory will pull the fourth signaling line contact (D−)  218  high, as shown in block  408 , to send an acknowledgment to the hosting device. In block  310 , a predetermined time after executing block  308 , the hosting device will check the voltage level on the second signaling line contact (D−)  124  (now connected to the fourth signaling line contact (D−)  218 ). If it is determined that the voltage level on the second signaling line contact (D−)  124  is low, the hosting device will assume that the accessory is operating in USB mode and will proceed in block  312  with USB handshaking. The details of the USB handshaking are outside the focus of this description. If on the other hand the hosting device detects that the second signaling line contact (D−)  124  has been pulled high then the hosting device will transition to UART state as indicated in block  314 . After entering the UART state, the hosting device will stop pulling the first signaling line contact (D+)  122  high as indicated in block  316  and, as indicate in block  318 , pull the second signaling line contact (D−)  124  high. After pulling the fourth signaling line contact (D−)  218  high, the accessory (e.g.,  200 ) will, in block  410 , check the level of the third signaling line contact (D+)  216  to ascertain if the hosting device has transitioned to the UART state. When the accessory determines that the signal level on third signaling line contact (D+)  216  is low indicating that the hosting device has entered the UART state, the accessory transitions to UART state as shown in block  412 , and thereafter, as indicated in block  414 , stop pulling the fourth signaling line contact (D−)  218  high, and as indicated in block  416  pulls the third signaling line contact (D+)  216  high. Thereafter as indicated in blocks  320 ,  322  as long as the hosting device continues to detect that the first signaling line contact (D+)  122  remains biased high the hosting device will continue in the UART state conducting UART signaling, including for example issuing commands to the accessory as shown in block  324 . When the hosting device detects that the first signaling line contact (D+)  122  is no longer biased high (not shown in  FIG. 5 ) the hosting device will transition to the disconnected state as indicated in block  322 . 
     Similarly, as indicated in block  418 ,  420  as long as the accessory continues to detect that the fourth signaling line contact (D−)  218  is biased high the accessory continues in the UART state conducting UART signaling. When the accessory detects that the fourth signaling line contact (D−)  218  is no longer biased high, the accessory will transition to the disconnected state as indicated in block  422   
       FIG. 6  is a third flowchart  600  showing actions performed by a hosting device, such as the wireless communication device  100 , in the course of transitioning from UART signaling to analog mono audio mode signaling between the hosting device and an accessory such as the accessory shown in  FIG. 2 .  FIG. 7  is a fourth flowchart  700  showing actions performed by an accessory such as the accessory  200  shown in  FIG. 2  in coordination with the actions shown in  FIG. 6  performed by a hosting device.  FIG. 8  is a second signal chart  800  showing signals exchanged between a hosting device such as the wireless communication device and an accessory such as the accessory  200  shown in  FIG. 2  in the process of transitioning from UART signaling to analog mono audio mode signaling. As indicated in blocks  602  and  702  of  FIG. 6  and  FIG. 7 , the hosting device and the accessory start in the UART state which is the state reached upon executing programs embodying the first flowchart  300  and the second flowchart  400 . In block  604  the hosting device drives the ID line low as indicated by reference numeral  804  in  FIG. 8 . The low state and a high state of the ID line correspond to predetermined voltage levels. In block  606  the hosting device transmits a SET_AUDIO UART command to the accessory through the D− line as shown by reference numeral  806  in  FIG. 8 . The SET_AUDIO UART command is an instruction to the accessory to configure itself to receive a mono analog audio signal from the hosting device through the D− line, and to send a mono analog audio signal to the hosting device through the D+ line. 
     Block  704  is a decision block, the outcome of which depends on whether the SET_AUDIO UART command has been received by the accessory. If not, the accessory remains in the UART state. If, on the other hand, the SET_AUDIO UART command is received then, in block  708 , the accessory also starts driving the ID line low. The time at which the accessory starts driving the ID line is indicated by a vertical tick mark  808  on the eighth line signal  802  ( FIG. 8 ). This vertical tick mark  808  is not an actual signal event. The ID line is already being driven low by the hosting device. After starting to drive the ID line low, in block  710 , the accessory will send a UART message signal  810  acknowledging receipt of the SET_AUDIO command. Block  608  depends on whether the acknowledgement  810  is received by the hosting device. If not, the hosting device will retry initiating contact with the accessory a predetermined number of times as indicated in block  610 . If acknowledgment is received, then, in block  612  the hosting device will transition to a state labeled ph_bias in  FIG. 8  and drive the D− line to a bias level appropriate for a speaker or other audio device included in the accessory. In the case of wireless communication device  100  shown in  FIG. 1  the second variable bias network  164  is used to bias the D− line. After sending the acknowledgment  810 , in block  712  the accessory will transition to a state labeled acc_bias in  FIG. 8  and drive the D+ line to a bias appropriate for a microphone or other audio device included in the accessory. In the case of the accessory shown in  FIG. 2  the third variable bias network  256  is used to bias the D+ line. After the hosting device has biased the D− line, in block  614  the hosting device will release the ID line. After the accessory has biased the D+ line, in block  714  the accessory will release the ID line. As indicated in block  616  the hosting device will wait until the ID line rises as indicated at  812 ,  FIG. 8  above a predetermined threshold before entering a state labeled ph_aud on the first line  502  of  FIG. 8  and enabling audio signaling in block  618 . As indicated in block  716 , the accessory will also wait until the ID line rises as indicated at  812 ,  FIG. 8  above a predetermined threshold before entering a state labeled acc_aud in the second line  504  of  FIG. 8  and enabling audio signaling in block  718 . In the case of the wireless communication device shown in  FIG. 1 , enabling the audio signaling includes configuring the first MUX/DEMUX  132  to couple the A/D  114  to the first signaling line contact (D+)  122 , and configuring the second MUX/DEMUX  133  to couple the D/A  112  to the second signaling line contact (D−)  124 . In the case of the accessory  200  having the design shown in  FIG. 2 , enabling audio signaling includes configuring the third MUX/DEMUX  228  to couple the output  230  of the microphone amplifier  224  to the third signaling line contact (D+)  216  and configuring the fourth MUX/DEMUX  246  to couple the input  250  of the second speaker amplifier  244  to the fourth signaling line contact (D−)  218 . 
       FIG. 9  is a fifth flowchart  900  showing actions performed by a hosting device such as the wireless communication device  100  shown in  FIG. 1  in the course of initiating analog stereo audio mode signaling to an accessory such as the accessory  200  shown in  FIG. 2 .  FIG. 10  is a sixth flowchart  1000  showing actions performed by an accessory such as the accessory  200  shown in  FIG. 2  in coordination with the actions shown in  FIG. 9 .  FIG. 11  is a third signal chart  1100  showing signals exchanged between a hosting device such as the wireless communication device  100  shown in  FIG. 1  and an accessory such as the accessory  200  shown in  FIG. 2  in the course of performing the actions shown in  FIGS. 9–10 . As indicated in blocks  902 ,  1002  the operations shown in the fifth  900  and sixth  1000  flowcharts commence with the hosting device and the accessory in the UART state which is entered upon executing programs embodying the first flowchart  300  and the second flowchart  400 . This is reflected in the first  502  and second  504  lines of the third signal chart. In block  904  the hosting device drives the ID line low, as shown at  1102  in  FIG. 11 , and in block  906  the hosting device transmits a SET_AUDIO UART command  1104  on the D− line to configure the accessory to receive stereo analog audio signals. Block  1004  is a decision block the outcome of which depends on whether the accessory receives the SET_AUDIO command. When the SET_AUDIO command is received, in block  1006 , the accessory sends an acknowledgement  1106  of the SET_AUDIO command in the form of a UART message signal sent back to the hosting device through the D+ line. As indicated in block  908 , after having sent the SET_AUDIO UART command, the hosting device waits for acknowledgment. As indicated in block  910 , when the acknowledgement is received, the hosting device will drive the D− and D+ lines to a speaker bias voltage as shown at  1108  in  FIG. 11 . In addition, after the acknowledgement is received, the hosting device releases the ID line in block  912 . The hosting device then waits for a predetermined period in block  914  and then commences to output audio on the D+ and D− lines in block  916 . After the ID line is released, the voltage level on the ID line will then rise as indicated at  1110 . When, in block  1010 , the accessory detects that the voltage on the ID line has risen above a predetermined threshold in block  1012  the accessory enables speaker audio. In the case of the accessory  200  shown in  FIG. 2  enabling speaker audio includes operating the third MUX/DEMUX  228  to couple the D+ line to the first speaker amplifier  226  and operating the fourth MUX/DEMUX  246  to couple the D− line to the second speaker amplifier  244 . 
       FIG. 12  is a seventh flowchart  1200  showing actions performed by a hosting device such as the wireless communication device  100  shown in  FIG. 1  in order to transition from analog mono audio signaling mode to UART signaling mode.  FIG. 13  is an eighth flowchart  1300  showing actions performed by an accessory such as the accessory  200  shown in  FIG. 2  in response to the actions shown in  FIG. 12 .  FIG. 14  is a fourth signal chart  1400  showing signals exchanged in the course of performing the actions shown in  FIGS. 12–13  in order to transition from analog mono audio signaling mode to UART signaling mode. As indicated in blocks  1202 ,  1302  the seventh flowchart  1200  and the eighth flowchart  1300  commence with the hosting device and the accessory in mono audio signaling mode. This also shown in the first  502  and second  504  lines of fourth signal chart  1400 . The latter mode is reached upon executing programs embodying the third flowchart  600  and the fourth flowchart  700  shown in  FIGS. 6 and 7 . 
     In block  1204  the hosting device enters a mute state labeled ph_mute in  FIG. 14 . The hosting device enters the mute state in response to a call from higher level software, e.g., application software that is beyond the scope of the present description. In block  1206  the hosting device mutes the audio going out on the D− line to a speaker in the accessory. In the case of the wireless communication device  100 , muting the audio going out on the D− line includes ceasing to output audio through the D/A  112 , and reconfiguring the second MUX/DEMUX  132  to decouple the D/A  112  from the second signaling line contact (D−)  124 . The cessation of audio signaling on the D− line is shown at  1404  in  FIG. 14 . In block  1208  the hosting device mutes audio on the D+ line that is coming from a microphone (e.g.,  225 ) in the accessory (e.g,  200 ). In the case of the wireless communication device  100 , muting the audio coming in on the D+ line includes operating the first MUX/DEMUX  132  to decouple the first signaling line contact (D+)  122  from the A/D  114 . In block  1210  the hosting device enters the interrupt state (labeled ph_int in  FIG. 14 ), and in block  1212  the hosting device drives the ID line low for a time Tph_id_int as indicated at  1406  in  FIG. 14 . In the case that the hosting device is the wireless communication device  100  shown in  FIG. 1 , the first ID line driver  134  is used in block  1212  to drive the ID line low. Thereafter, in block  1214  the hosting device goes into a wait for acknowledgement state which is labeled ph_wfa in  FIG. 14 . 
     Referring now to  FIG. 13 , block  1304  is a conditional block the outcome of which depends on the ID line being sensed by the accessory to be low for the period Tph_id_int or fraction thereof. If the ID is sensed to be low for the period Tph_id_int or the fraction thereof, in block  1306  the accessory enters an accessory mute state which is labeled acc_mute in  FIG. 14 . In the case of the accessory  200  shown in  FIG. 2  the second ID level detector  262  is used to sense the signal level of the ID line. In block  1308  the accessory mutes the audio coming in on the D− line and in block  1310  the accessory mutes the audio going out on the D+ line. The latter event is shown at  1408  in  FIG. 14 . In the case of the accessory  200  shown in  FIG. 2  muting the audio coming in on the D− line includes reconfiguring fourth MUX/DEMUX  246  to decouple the fourth signaling line contact (D−)  218  from the second speaker amplifier  244  and muting audio going out on the D+ line includes reconfiguring the third MUX/DEMUX  228  to decouple the third signaling line contact (D+)  216  from the microphone amplifier  224 . In block  1312  the accessory drives the D+ line to a UART idle state, and then after waiting for a time Tacc_ack_wait in block  1314 , in block  1316  the accessory enters an accessory acknowledge state, which is labeled acc_ack in  FIG. 14 . In the case of the accessory  200  shown in  FIG. 2  the third variable bias network  256  is used to drive the D+ line to the UART idle state. Next, in block  1318  the accessory drives the ID line low for Tacc_id_int, in order to acknowledge the driving of the ID line low by the hosting device in block  1212 . In the case of the accessory  200  shown in  FIG. 2  the second ID line driver  260  is used to execute block  1318 . In block  1320  the accessory waits for acknowledgement state for a time Tacc_cmd_wait, after which in block  1322  the accessory enters a UART state, labeled acc_uart in  FIG. 14 , to await receipt of a UART message signal in block  1324 . 
     When it is determined by the hosting device in block  1216  that the accessory, in block  1318 , has driven the ID line low, in block  1218  the hosting device enters a UART state, labeled ph_uart in  FIG. 14 , after which in block  1220  the hosting device drives the D− line to the UART idle state (high). The hosting device then waits for a time Tph_cmd_wait in block  1222  and then sends a UART digital message signal  1410  to the accessory on the D− line in block  1224 . The UART digital message signal can include queries, commands and/or data. For example, the UART digital message signal can include a command to the accessory to raise or lower audio volume. Such a command could be issued in response to a user input using an optional control button (not shown) of the wireless communication device  100 . As another example, the UART digital signal message can include caller ID information which would be displayed on a display (not shown) that is optionally included in the accessory  200 . 
     In the case of the wireless communication device  100  shown in  FIG. 1 , the first ID level detector  136  is used to detect the ID line being driven low by the accessory, and the first UART module  110  is used to send the UART digital message signal. In carrying out block  1216 , the ID level detector  136  compares the voltage on the ID line to an upper bound voltage level. 
     When in block  1324  it is determined that UART digital message signal has been received, in block  1326  the accessory sends a UART response  1420  to the UART digital message signal  1410 . In the case of the accessory  200  shown in  FIG. 2  the second UART module  210  is used to send the response to the UART message signal. 
       FIG. 15  is a ninth flowchart  1500  showing actions performed by a hosting device such as the wireless communication device  100  shown in  FIG. 1  in order to transition from analog stereo audio signaling mode to UART signaling mode.  FIG. 16  is a tenth flowchart  1600  showing actions performed by an accessory such the accessory  200  shown in  FIG. 2  in response to the actions shown in  FIG. 15 .  FIG. 17  is a fifth signal chart  1700  showing signals exchanged between a hosting device such as the wireless communication device  100  shown in  FIG. 1  and an accessory such as the accessory  200  shown in  FIG. 2  in the course of performing the actions shown in  FIGS. 15–16  in order to transition from analog stereo audio signaling mode to UART signaling mode. 
       FIGS. 15–17  are analogous to  FIGS. 12–14 ; however  FIGS. 15–17  describe a process for transitioning from stereo analog audio signaling mode, as opposed to the process of transitioning from mono audio signaling mode shown in  FIGS. 12–14 . However, in both instances it is the hosting device that initiates the transitions. The following description addresses the aspects of transitioning from stereo analog signaling mode to UART signaling mode that differ from the process of transitioning from to mono analog signaling which is described above. As indicated in blocks  1502 ,  1602  the process shown in  FIGS. 15–16  commence with the hosting device and the accessory operating in stereo audio signaling mode. Stereo signaling mode is entered by executing the processes described above with reference to the fifth  900  and sixth  1000  flowcharts and the third  1100  signal chart. The principle difference in the actions performed by the hosting device and the accessory is that both the hosting device and the accessory mute both speakers. This is shown in block  1504  in the case of the hosting device, and in block  1604  in the case of the accessory. In the case of the wireless communication device shown in  FIG. 1  acting as the hosting device, muting both speakers includes ceasing to operate the D/A  148  to output audio, and reconfiguring the first MUX/DEMUX  132  and the second MUX/DEMUX  133  to decouple the D/A  148  from the first signaling line contact (D+)  122  and the second signaling line contact (D−)  124 . In the case of the accessory  200  shown in  FIG. 2  muting both speakers includes reconfiguring the third MUX/DEMUX  228  and the fourth MUX/DEMUX  246  to decouple the fourth signaling line contact (D−)  218  from the second speaker amplifier  244  and to decouple the third signaling line contact (D+)  216  from the first speaker amplifier  226 . Initial muting of both speakers by the hosting device is indicated by reference numeral  1702  in  FIG. 17 . 
       FIG. 18  is an eleventh flowchart  1800  showing actions performed by an accessory such as the accessory  200  shown in  FIG. 2  in order to transition from analog mono audio signaling mode to UART signaling mode.  FIG. 19  is a twelfth flowchart  1900  showing actions performed by a hosting device such as the wireless communication device  100  shown in  FIG. 1  in response to the actions shown in  FIG. 18 .  FIG. 20  is a sixth signal chart  2000  showing signals exchanged between the hosting device and the accessory in the course of performing the actions shown in  FIGS. 18–19  in a case in which no interrupt collision occurs. In contrast to the process described above with reference to  FIGS. 12–14 , in the process to be described with reference to  FIGS. 18–20  it is the accessory, not the hosting device, that initiates the signaling mode transition. 
     As indicated in blocks  1802 ,  1902  the actions shown in the eleventh flowchart  1800  and the twelfth flowchart  1900  commence with accessory and the hosting device operating in mono audio signaling modes labeled ph_aud, acc_aud in  FIG. 20 . The latter modes are reached upon completion of the processes shown in  FIGS. 6–7 . In block  1804  the accessory transitions to the audio mute state, labeled acc_mute in  FIG. 20 . In block  1806 , the accessory mutes audio coming in to the accessory&#39;s speaker or other device that receives analog audio signals. In block  1808  the accessory mutes audio going out from the accessory&#39;s microphone (e.g.,  225 ) or other device that generates analog audio signals. In block  1810  the accessory enters an interrupt state, labeled acc_int in  FIG. 20 , and in block  1812 , as indicated by reference numeral  2002  in  FIG. 20 , the accessory drives the ID line from high state to low state for a time period Tacc_id_int. The high state is characterized by one voltage level and the low state is characterized by another voltage level. It is noteworthy the time period Tacc_id_int for which the accessory drives the ID line low in order to initially signal the hosting device that a signaling mode transition is to be made is shorter than the time Tph_id_int that was mentioned above, for which the hosting device drives the ID line low in order to initially signal the accessory that a signaling mode transition is to be made. The significance of this difference is discussed further below. In block  1814  the accessory enters an Accessory Collision Check state, labeled acc_col_ck in  FIG. 20 , and after a time period Tacc_coll_det that is measured from when the ID line was first driven low elapses in block  1816  the accessory proceeds to block  1818 . Tacc_coll_det is suitably about equal to the time for which the ID line is driven low Tacc_id_int by the accessory plus a time required for the ID line to charge back up to a high state after being driven low in block  1812 . Tacc_coll_det is also less than the time Tph_id_int for which the hosting device drives the ID low in order to initially signal a mode transition. 
     Block  1818  is a decision block that is used in detecting interrupt collisions. An interrupt collision occurs when both the hosting device and the accessory attempt to interrupt each other at about the same time. Block  1818  tests if after Tacc_coll_det the ID line is still low. If after Tacc_coll_det the ID line is still low, despite the fact that Tacc_coll_det is long enough to allow the ID line to charge back up to the high state after having been pulled low by the accessory in block  1812 , it means that another device, i.e. the hosting device, is pulling the ID line to signal an interrupt. Tacc_coll_det is not long enough to miss the ID line having been pulled low for Tph_id_int by the hosting device, if the hosting device pulled the ID line low starting at about the same time as the accessory in order to initiate a mode transition. Note that Tacc_coll_det is shorter than the interval between when the ID is driven low to initiate a mode transition and a time at which the ID line is driven low to acknowledge the initiation of the mode transition. Consequently, if the ID line is found to be low in block  1818  its state is not attributed to the ID being set low in acknowledgement of the ID line having been set low in block  1812 . In block  1818  the accessory determines if the voltage on the ID line is below a predetermined voltage level. 
     If the accessory detects an interrupt collision, i.e. if it is determined in block  1818  that the ID line is low, then the accessory proceeds to block  1820  and enters an interrupt servicing mode. In the interrupt service mode, the accessory function in the manner such as described above in reference to  FIG. 13 . In particular, as shown in  FIG. 18 , in block  1822  the accessory biases the D+ line to the UART idle state (i.e. high state), in block  1824  the accessory waits for a time period Tacc_ack_wait, in block  1826  the accessory enters an acknowledge state, in block  1828  the accessory drives the ID line low (a predetermined voltage level) for Tacc_id_int to communicate an acknowledgement of the hosting device&#39;s initiation a signaling mode transition, in block  1830  the accessory goes into a wait state for a period Tacc_cmd_wait, in block  1832  the accessory enters a UART state and awaits the UART digital message signal, and in block  1834  the accessory replies to the UART digital message signal. The UART digital message signal  2004  and the response  2006  thereto are shown in  FIG. 20 . 
     If on the other hand, it is determined in block  1818  that the ID line is not low, i.e. if an interrupt collision is not detected, then in block  1836  the accessory enters a wait for acknowledgment state, waits for a predetermined period of time Tph_int_wait, and then proceeds to block  1838 . Proceeding from block  1838  is conditioned on receipt of an acknowledgement from the hosting device in the form of the ID line being set low for the period Tph_id_int. In block  1838  the voltage on the ID line is compared to a predetermined voltage level, suitably the same predetermined voltage level used in block  1818 . When the acknowledgement is detected the accessory proceeds to block  1840  in which the D+ line is biased to the UART idle state, and thereafter the accessory proceeds to block  1832  and  1834  previously described. Note that the signal on the ID line shown in the eighth line  802  in  FIG. 20  is for the case that there is no interrupt collision. 
     According to the eleventh flowchart  1800  the accessory will defer to the hosting device in the case that both devices attempt to interrupt each other at about the same time. Alternatively, the hosting device defers to the accessory.  FIG. 19  shows the responses of the hosting device to the operation of the accessory depicted in FIG.  18  in the case that the hosting device is not trying to interrupt the accessory at the same time that the accessory is trying to interrupt the hosting device. Referring to  FIG. 19 , when the hosting device detects, in block  1904  that the accessory (in block  1812 ) has set the ID line low for Tacc_id_int, the hosting device transitions to a mute state (labeled ph_mute in  FIG. 20 ) in block  1906 , mutes audio going out on the D− line to a loudspeaker (e.g.,  229 ) or other device in the accessory in block  1908 , mutes audio coming in from a microphone (e.g.,  225 ) or other device in the accessory in block  1910 , biases the D− line to the UART idle state in block  1912 , waits a period Tph_ack_wait in block  1914 , transitions to an acknowledge state (labeled ph_ack in  FIG. 20 ) in block  1916 , drives the ID low for a time Tph_id_int to acknowledge the accessory, as shown at  2008  in  FIG. 20 , in block  1918 , waits a period Tph_cmd_wait in block  1920 , enters a UART signaling mode (labeled ph_uart in  FIG. 20 ) in block  1922  and sends the UART digital message signal  2004  in block  1924 , and awaits receipt of the response  2006  thereto in block  1926 . Further processing of the response to the contents of the UART digital message signal is handled by higher layer software and is beyond the focus of this description. 
       FIG. 21  is a thirteenth flowchart  2100  showing actions performed by an accessory such as the accessory  200  shown in  FIG. 2  to transition from analog stereo audio signaling mode to UART signaling mode.  FIG. 22  is a fourteenth flowchart  2200  showing actions performed by a hosting device such as the wireless communication device  100  shown in  FIG. 1  in response to the actions shown in  FIG. 21 .  FIG. 23  is a seventh signal chart  2300  showing signals exchanged between the hosting device and the accessory in the course of performing the actions shown in  FIGS. 21 ,  22  in a case in which there is no interrupt collision. As indicated in blocks  2102 ,  2202 , in contrast to the processes depicted in  FIGS. 18 ,  19  which start with the hosting device and the accessory operating in mono audio signaling mode, the processes depicted in  FIGS. 21 ,  22  commence with the hosting device and the accessory operating in stereo audio signaling mode. However, in as much as the processes are quite similar, only certain minor difference are described below. In particular, in block  2104  the hosting device mutes stereo audio going out on the D− and D+ lines to speakers or other stereo audio receiving devices in the accessory. Similarly, in block  2204  the accessory mutes audio coming in on the D− and D+ lines. A point at which audio going out on the D− and D+ lines is muted by the hosting device is shown at  2302  in  FIG. 23 . Note that  FIG. 23  depicts signals in the case that no interrupt collision is detected in block  1818 . 
       FIG. 24  is an eighth signal chart  2400  showing signals exchanged between a hosting device such as the wireless communication device  100  shown in  FIG. 1  and an accessory such as the accessory shown in  FIG. 2  in the course of performing the actions shown in  FIGS. 18 ,  19 ,  21 ,  22  in a case in which an interrupt collision occurs. The interrupt collision occurs when both the hosting device and the accessory attempt to pull the ID line low to initiate a signaling mode transition at the same time. In the eighth line  802  of  FIG. 24  a portion of graph  2402  depicting what the ID line signal would be if only the accessory had initiated the interrupt is shown with a dashed line. The true signal due to the hosting device also initiating a mode transition by pulling the ID line low is shown with a solid line. As discussed above when the accessory detects that the hosting device is also trying to initiate a signaling mode transition, the accessory will defer to the hosting device, and service the interrupt of the hosting device. In doing so, the accessory will pull the ID line low as indicated at  2404  to acknowledge the ID line having been pulled low by the hosting device at  2406 . Other aspects of the operation of the accessory in case an interrupt collision occurs are discussed above with reference to  FIGS. 18 and 21 . When a collision occurs, the hosting device need not alter its operation, because the accessory defers to the hosting device. The processes by which the hosting device transitions from audio signaling mode to UART signaling mode that are initiated by the hosting device are described above with reference to  FIGS. 12–17 . 
     By using the ID line as described above to initiate and negotiate transitions from audio signaling mode to digital signaling mode, generation of signal components on the D− and D+ lines that would be amplified and heard as audible noise or pops is avoided. 
     While the preferred and other embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions, and equivalents will occur to those of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the following claims.