Using the debug usb uart serial on the Asus C201

The usb port nearest to the display on the C201 can be used as a debug uart/serial interface allowing you to open a terminal into the C201 on another computer and retrieve debug information and run terminal commands.

To do this you will need the following things:

1. a spare standard USB type A to type anything cable
2. a C201
3. another computer
4. a serial to usb adapter (some models of arduino work for this!)

The C201 needs to have the following console=ttyS2,19200n8 earlyprintk=ttyS2,19200n8 and rockchip.usb_uart=1 in the kernel command line

Baud rates higher than 19200 cause issues with usb reliability

NOTE: when rockchip.usb_uart=1 is set, the usb nearest the display will not work as a usb

By default the commandline file built into the PrawnOS linux kernel has rockchip.usb_uart=0 Modifying https://github.com/SolidHal/PrawnOS/blob/master/resources/BuildResources/cmdline and building a new image will get you PrawnOS with usb uart debug.

Cut the spare USB cable. We need the D- which is green, D+ which is white, and ground which is black or shielded. Make sure the pinout matches the diagram here with a multi-meters continuity test or resistance reading.

Be careful to not use the red power pin of the usb cable as this can damage the c201

Now connect D- to the rx of your serial to usb adapter, D+ to the tx, and ground to ground. To use an arduino uno, tx and rx can be found in the digital pin section. The uno also has to be held in reset to enable serial passthrough, so connect a jumper between the reset and ground pins.

Now use a terminal or program like coolterm to connect to the adapter as 115200 baud.

Connect your spliced usb cable to the usb closest to the display on the C201, and you're done!