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1/8May 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 CarkitCar HiFi with Integrated Carkit
AN2148
APPLICATION NOTE
STOTG04 IN AUDIO CARKIT ENVIRONMENT
Rev. 1
