1/8 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
AN2148 - application note 2/8 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
AN2148 - application note 3/8 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 3 shows the interface between a phone and a carkit using captive cable with five wires. figure 2 : carkit four-wire interface figure 3 : carkit five-wire interface v bus d- d+ id gnd shld v bus d+ d- id sda scl rcv oe_tp_int/ se0_vm dat_vp sda scl rcv oe_n vp vm rxd txd 5v_in power management usb-otg controller phone processor usb-otg transceiver mini-ab receptable spkr_mic spkr_l mini-a receptable v bu s d- d+ id gnd shld v bus d- d+ id gnd shld v bus d- d+ id gnd shld spkr_mic spkr_l standard mini-usb cable carkit amplifier cmr rxd_sda txd_scl carkit control voltage regulator 5v_out v bus d- d+ id gnd shld v bus d+ d- id sda scl rcv oe_tp_int/ se0_vm dat_vp sda scl rcv oe_n vp vm rxd txd 5v_in power management us b-otg controller phone processor usb-otg transceiver mini-ab receptable spkr_mic spkr_l v bus d- d+ id gnd shld spkr_mic spkr_l carkit amplifier cmr rxd_sda txd_scl carkit control voltage regulator 5v_out
AN2148 - application note 4/8 3 - stotg04 application the stotg04 transceiver supports carkit signaling on both d+ and id lines. therefore, it is able to operate with both four-wire and five-wire protocol. it is up to the carkit accessory which protocol will be used. figure 4 shows connection of the stotg04 full-speed usb-otg transceiver in an application requiring even audio mode operation. the transceiver in figure 4 can operate in all four operating modes: usb, i 2 c, uart, and audio. for operation in audio mode it is necessary to release internal drivers to force the d+ and d- lines into high impedance state. the uart mode is also needed for carkit. to use the transceiver in the uart mode it is necessary to properly set the direction of uart drivers. the correct setup of control registers of the transceiver is described in table 2 for both audio and uart modes. in the uart mode after a proper setup it is possible to transmit data from the se0_vm pin to the d- line and to receive data from the d+ line on the dat_vp pin. figure 4 : carkit five-wire interface table 2 : alternate functions of a usb signal lines register uart mode audio mode control 1 x 1 x x 0 x x x (transp_en = 0, uart_en = 1) x 0 x x 0 x x x (transp_en = 0, uart_en = 0) control 2 0 0 0 0 0 0 0 0 (release all pu/pd resistors) 0 0 0 0 0 0 0 0 (release all pu/pd resistors) control 3 x x x 1 0 x x x (bdir[0] = 0, bdir[1] = 1) x x 1 x x x x x (audio_en = 1) vif sda scl adr_psw reset/ int/ speed suspend vm vp rcv oe_tp_int/ se0_vm dat_vp gnd vbat cap1 cap2 id vbus d+ d- vtrm cgnd id d+ d- vbus gnd 2.7?5.0 v 1.6?3.6 v stotg04e mini-ab 20 ? 5% 220nf 4.7 f 1 f txd rxd spkr_mic spkr_l vcc vcc phone processor usb-otg controller sda scl reset/ int/ vm vp rcv oe_tp_int/ se0_vm dat_vp
AN2148 - application note 5/8 in audio mode it is necessary to tie the oe_tp_int/ pin to a high level to deactivate the usb driver. when all the pull-up and pull-down resistors are disc onnected, the d+ and d- u sb lines will be in high impedance state and it is possible to send audio signals over the lines. to be able to detect carkit interrupts, it is necessary to set the audio_en bit of the control register 3. then, depending on the protocol used (either four-wire or five-wire) it is necessary to set the proper bits of the interrupt mask false and interrupt mask true registers. if a four-wire protocol is used then d+ carkit interrupt should be utilized and therefore the cr_int bit of interrupt mask registers should be set. if a five-wire protocol is used then id line carkit interrupt can be utilized and therefore the id_gnd bit of interrupt registers should be set.
AN2148 - application note 6/8 4 - references the following publications contain detailed information regarding the topic of this application note: 1 stotg04 usb-otg full-speed transceiver datasheet 2 cea-2011, otg transceiver specification 3 cea-936-a, mini-usb analog carkit interface 4 otg carkit transceiver, revision 0.63
AN2148 - application note 7/8 5 - revison history table 3 : revision history date revision description of changes 12-may-2005 1 first release.
AN2148 - application note 8/8 information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the co nsequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this publicati on are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics prod ucts are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectro nics. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners ? 2005 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - united states of america www.st.com
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