ST AN2148 APPLICATION NOTE

AN2148

APPLICATION NOTE

STOTG04 IN AUDIO CARKIT ENVIRONMENT

1 - INTRODUCTION

This application note describes the application of the STOTG04 full-speed USB-OTG transceiver in an audio carkit environment. The paper also illustrates the carkit use and operation, connection between cellular phone and the carkit, physical connection and external circuitry of the STOTG04 required for a correct operation. Proper setup of the internal registers of the STOTG04 needed for audio mode is also described. The information in this application note is intended for system design engineers who plan to use the STOTG04 transceiver as a USB-OTG physical layer device in equipment using the universal serial bus even for audio signals.

Specification [3] defines a standard method for routing audio and UART signals to an analog carkit and other accessories (chargers and RS232 devices) through a phone’s Mini-AB USB receptacle. A carkit consists of a microphone, speaker, and Mini-B plug to connect a phone. It may also contain a button, LED, and a cradle for the phone. The carkit may be integrated into a car stereo as shown in Figure 1. The connection between a phone and a carkit can be provided using either a standard Mini-USB cable with four wires plus shield or a captive cable with five wires plus shield.

Figure 1 : Car Stereo Carkit Implementation

Cellular Phone

Mini-A Plug	Car HiFi with Integrated Carkit
Mini-B Plug

Car Stereo with Integrated Carkit

The carkit interface is intended to enable the following features between a phone and a car stereo:

–phone charging

–hands free speakerphone

–push-to-talk

–stereo playback

–others (like telematics, navigation system, etc.)

As most phones currently support USB signaling, it is planned that car stereos will support USB too. It will allow the transfer of following data using the universal serial bus:

–audio files

–video files

–images

–data

Rev. 1

May 2005

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AN2148 - APPLICATION NOTE

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.

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

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.

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AN2148 - APPLICATION NOTE

The carkit connection architecture is shown in the two figures below. Figure 2 shows the interface between a phone and a carkit using standard Mini-USB cable.

Figure 2 : Carkit Four-Wire Interface

Power Management

5V_IN
USB-OTG Controller	USB-OTG Transceiver
SDA	SDA	VBUS
SCL	SCL	D+
RCV	RCV	D-

ID

OE_N OE_TP_INT/

VP DAT_VP

VM SE0_VM

Phone Processor

RXD

TXD

SPKR_MIC

SPKR_L

Mini-AB	Mini-A
Receptable	Receptable

VBUS	VBUS	VBUS
D+	D+	D+
D-	D-	D-
ID	ID	ID
GND	GND	GND
SHLD	SHLD	SHLD

Standard Mini-USB Cable

VBUS

D+

D-

ID

GND

SHLD

Voltage Regulator

5V_OUT

Carkit Amplifier

SPKR_MIC

SPKR_L

CMR

RXD_SDA

TXD_SCL

Carkit Control

Figure 3 shows the interface between a phone and a carkit using captive cable with five wires.

Figure 3 : Carkit Five-Wire Interface
Power Management
					Voltage Regulator
5V_IN
					5V_OUT
USB-OTG Controller	USB-OTG Transceiver		Mini-AB
			Receptable		Carkit Amplifier
SDA	SDA	VBUS	VBUS	VBUS
SCL	SCL	D+	D+	D+	SPKR_MIC
RCV	RCV	D-	D-	D-	SPKR_L
OE_N	OE_TP_INT/	ID	ID	ID	CMR
VP	DAT_VP		GND	GND
VM	SE0_VM		SHLD	SHLD
Phone Processor
					RXD_SDA
RXD					TXD_SCL
TXD
SPKR_MIC
SPKR_L					Carkit Control

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