Replicant

h1. Optimus Black serial

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The Optimus Black allows accessing an UART serial port through the USB connector. A MAX14526 MUIC is in charge of switching between the USB and UART lines. The UART line can be muxed either to the main processor's debug UART (*UART3*), to the modem's UART or to the modem's USB lines via a FSUSB63 DP3T switch. The modem's UART line can also be switched directly to the main processor via an analog switch.

Thus, a working configuration to access the serial port through the USB connector is to route the USB connector to the UART line with the MAX14526 MUIC and to route the UART line to the main processor's debug UART (UART3) with the FSUSB63 DP3T switch.

Block diagram of the USB and UART connection, according to the Optimus Black (P970) service manual:

!p970_usb_mux.jpg!

On the other hand, the main processor's debug UART (*UART3*) can be accessed directly on the board, via the pins of the debug connector or via the pins of the FSUSB63 DP3T switch.

h2. Hardware setup

h3. USB connector

According to the FSUSB63 DP3T switch datasheet and the Optimus Black (P970) service manual, *UART Rx* is routed to *USB DM (D-)* and *UART Tx* is routed to *USB DP (D+)*.

Hence, the *USB DM (D-)* has to be connected to the UART adapter's *Tx* pin and the *USB DP (D+)* has to be connected to the UART adapter's *Rx* pin.

*GND* has to be made common between the USB connector and the UART adapter. There is no need to connect the USB connector's VBUS to a positive voltage.

Note that there is no automatic software switching (yet) with [[OptimusBlackU-Boot|U-Boot]], so make sure that *the FSUSB63 DP3T switch is switched to the main processor's UART* and that *the MAX14526 MUIC is switched to UART* before connecting the UART receiver to the USB connector.

h4. FSUSB63 DP3T switch

The FSUSB63 DP3T switch can be switched to the main processor's UART by setting the OMAP_UART_SW GPIO (161) high and the IFX_UART_SW GPIO (162) low.

h4. MAX14526 MUIC

The MAX14526 MUIC can be switched to UART by writing *0x09* to register *0x03* on device address *0x44* on I2C bus *1*. It can be switched back to USB by writing *0x00* at the same address.

h3. UART to USB converters

It appears that UART to USB converters using the *PL2303* chip don't work properly with the device's UART. On the other hand, converters based on the *CP210x* chip were tested to work properly.

h2. Software setup

h3. Bootloader

Some development versions of [[OptimusBlackU-Boot|U-Boot]] allow switching the USB connector to UART by pressing the *G* key (on the side) at boot.

LG's downstream version of U-Boot, that ships with Android version prior to 4.0, allows switching the USB connector to UART by placing a *56 kOhm* resistor between the *ID* and *GND* pins of the connector and pressing the *volume down* key at boot.

LG's downstream version of LK, that ships with Android 4.0, may behave the same.

h3. Kernel

h3. References

* "Optimus Black (P970) Service Manual and Schematics":http://www.cpkb.org/wiki/LG_P970_Optimus_Black_service_manual

* "OMAP36xx Technical Reference Manual":http://www.ti.com/lit/pdf/swpu177

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