AN2148 - APPLICATION NOTE

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2 - CARKIT INTERFACE ARCHITECTURE

The phone‘s Mini-AB USB receptacle consists of four signal lines (VBUS, D+, D-, and ID), ground, and a

shield. The carkit interface allows the use of these wires in three different signaling modes:

– UART mode

– mono mode

– stereo mode

Alternate functions of the USB signal lines are described in Table 1 for each particular carkit signaling

mode.

Note

(*) The ID line is not required for 4-wire carkit interface

In all three modes, the VBUS, GND, SHLD, and ID lines perform the same function. The VBUS line

carries a 5V supply voltage from the carkit to the phone. The GND line is a common ground carrying

return current from the phone to the carkit. The SHLD acts as an analog reference between the phone

and the carkit. The ID line is used to signal interrupt and control information for the five-wire protocol. If a

phone is connected to a carkit using standard Mini-USB cable, it is not possible to use the ID line for

signaling. In this case, it is necessary to use the four-wire communication protocol between the phone

and carkit. When the carkit contains captive cable with an ID wire, then it is possible to use either the

four-wire or five wire protocol. Both these protocols are defined in the specification [3].

In UART mode, the D- line carries the TXD signal from the phone to the carkit, and the D+ line carries the

RXD signal from the carkit to the phone.

In mono mode, the D- line carries the mono speaker signal from the phone to the carkit and the D+ line

carries the microphone signal from the carkit to the phone.

In stereo mode, the D- line carries the left speaker signal from the phone to the carkit and the D+ line

carries the right speaker signal from the phone to the carkit.

Table 1 : Alternate Functions of a USB Signal Lines

Line

UART Mode

Mono Mode

Stereo Mode

VBUS

VBUS

VBUS

VBUS

D+

RXD

MIC

SPKR_R

D-

TXD

SPKR

SPKR_L

ID

CTL*

CTL*

CTL*

GND

GND

GND

GND

SHLD

SHLD

CMR

CMR