Replicant

h1. Nexus S (I902x) UART

The Nexus S (I902x) allows accessing an UART serial port through the USB connector. A FSA9480 switch is in charge of switching the USB connector between UART and USB (among other choices).

The UART signal can itself be switched between the S5PC110 SoC UART and the XMM6160 modem UART by the MAX8998 PMIC. It is switched to the S5PC110 SoC UART by default.

According to the Nexus S (I902x) Schematics:

!crespo_fsa9480.jpg!

Depending on the resistor value between the USB connector ID and GND pins, the FSA9480 will automatically switch the USB connector to the selected device, as mentioned on the FSA9480 datasheet. A resistor value of 150k Ohm will switch the USB connector to UART.

h2. Hardware setup

In order to switch the USB connector to UART, a *150k Ohm* resistor has to be placed between the *ID* and *GND* pins of the USB connector.

According to the FSA9480 datasheet, table 3 (Accessory auto configuration table), *UART Rx* is routed to *USB DP (D+)* and *UART Tx* is routed to *USB DM (D-)*.

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

The cable can be connected to the device at any time (the FSA9480 will automatically switch the USB connector to UART, even when the device is already on).

h2. Software setup

h3. USB connector switch

Alternatively, it is possible to switch the USB connector to UART at run-time. It is advised to follow the following procedure:

# /sys/devices/platform/i2c-gpio.7/i2c-7/7-0025/switch