UART not pulling down properly on Rock5B

[Zipdox2]

I'm trying to use UART but the Rock5B is not pulling down the line properly, see the scope screenshot. This is with nothing else connected. Just the scope connected directly to the GND and TX pins.

I tried it with UART7_M2 and UART4_M2. Both had the same issue. Someone in the Radxa discord said it could be that the drive strength is too low.

[Zipdox2]

It appears UART2 M2 works, but /dev/ttyS2 is owned by the tty group, not the dialout group.

[RadxaYuntian]

Please tell us the system image you are using.

[Zipdox2]

rock-5b_debian_bullseye_cli_b36

[Zipdox2]

UART3 M1 also works fine.

[Zipdox2]

UART7 M1 also works. Very weird. At least I have 2 working UARTs now, but this still needs to get fixed.

[RadxaYuntian]

So which UART is not working correctly now?

[Zipdox2]

Works:

• UART2 M2
• UART3 M1
• UART7 M1

Does not work:

• UART4_M2
• UART7_M2

Not tested:

• UART2 M0
• UART1 M1 (RX only)

[RadxaYuntian]

@nascs please check this when you are back.

[nascs]

I downloaded the same image(rock-5b_debian_bullseye_cli_b36) but did not reproduce the problem, after restarting with UART7_M2 enabled and connecting pin 26 to GND, I measured 0V on pin 26, and after connecting pin 26 to 5V, I measured 5V on pin 26.

[RadxaYuntian]

Please check if you can get it actually working as an UART. Not connecting to external pull up and pull down.

[nascs]

It can be sent and received normally as a uart.