How to use UART for RK3399 RSB4710

Overview

The Android/Linux UART/serial port access from user is through the tty-devices. The tty-devices have different names depending on UART driver on different board.

RS-485

RS-485 uses half-duplex communication, which means that one medium is shared for transmitting and receiving data.

Therefore the system needs to control the RS-485 transceiver's transmit mode.

Usually the UART RTS signal is used to switch the transmitter on and off.

Boards

RSB4710 Borad

COM Name	Device Node	Remark
COM0	/dev/ttyS0	for BT Data
COM2	/dev/ttyS2	Debug Port
COM4	/dev/ttyS4	support RS232, no flow control
USB2COM0	/dev/ttyUSB0	support RS232
USB2COM1	/dev/ttyUSB1	support RS232
USB2COM2	/dev/ttyUSB2	support RS232 and RS485
USB2COM3	dev/ttyUSB3	support RS232 and RS485

Note : 

1. Only COM4、USB2COM0、USB2COM1、USB2COM2、USB2COM3 are designed for custom using.

2. Only USB2COM2 and USB2COM3 support RSB485.

3. COM2 is designed as debug port default. 

How to config RS232 or RS485 mode

Enter U-boot interrupt mode

Connect device to debug port, and open debug console. See more : How_to_debug.

Press "ctrl + c" key before power up the device, until get the following information on debug console : 

Hit any key to stop autoboot: 0
=> <INTERRUPT>
=>

Then you can input the following cammand to config UART mode.

Config Uart Mode

Only ttyUSB2 and ttyUSB3 support RS485 mode.

RS485 Mode: 

setenv uart_mode 0x0c
saveenv
reset

RS232 Mode: 

setenv uart_mode 0x00
saveenv
reset

Note : 

The default value is RS232. 

Cp210x support four port, but RSB4710 Only ttyUSB2 and ttyUSB3 support RS485 mode.

Above command "setenv uart_mode 0x0c", 0x0c means set bit2(ttyUSB2) and bit3(ttyUSB3) to 1 (0 means to RS232; 1 means to RS485).

If you just want ttyUSB2 (or ttyUSB3)to be RSB485 mode, please change 0x0c to 0x04 (or 0x08).