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May 2005

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.

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

Figure 1 : Car Stereo Carkit Implementation

Mini-A Plug

Mini-B Plug

Car Stereo with Integrated Carkit

Cellular Phone

Car HiFi with Integrated Carkit

Car HiFi with Integrated Carkit

AN2148

APPLICATION NOTE

STOTG04 IN AUDIO CARKIT ENVIRONMENT

Rev. 1