UART / RS485 Development and Testing

Confirm the active driver

grep -Rni 'CONFIG_AW_UART_NG\|CONFIG_AW_UART=' device bsp
grep -Rni 'compatible = "allwinner,.*uart' bsp/configs device/config
dmesg | grep -i uart

The two implementations do not use exactly the same DTS properties or debug nodes; do not mix their configuration.

Port setup and loopback

stty -F /dev/ttyS7 115200 cs8 -cstopb -parenb -ixon -ixoff raw -echo
cat /dev/ttyS7 &
echo 'uart-test' > /dev/ttyS7

Short TX to RX for a hardware loopback. RS485 requires an external transceiver and validation of DE/RE direction timing.

Special baud rates

Let the serial core calculate the divisor for normal baud rates. Change the UART APB source/frequency only when the resulting error is outside the permitted range.

Actual baud = UART input clock / (oversampling factor × divisor)
Error = |actual - target| / target

For a console, keep U-Boot baudrate, the kernel console=ttySx,baud argument, and userspace terminal settings consistent, otherwise one boot stage will be garbled.

Common build and flash workflow

cd <ANDROID_TOP>/longan
./build.sh kernel
./build.sh pack

After flashing, confirm the kernel/DTB build timestamp and version so that new changes are not evaluated on an old image.