BPI-R4 UART Dead

Banana Pi Router design BPI-R4/BPI-R4 Pro(MT7988)
1

Hi,

I had my R4 powered off for a while. I’m about to convert it to a PoE model, but am still waiting on the BPI-5400B to come in, and so I am still powering it with a standard barrel jack. Today, I installed OpenWrt 24.10.4 for the PoE model onto an SD card and upgraded the bootloader on NAND. After a reboot, UART is dead:

  • GND pin is not showing connected to ground, tested with a multimeter between the pin and usb shield/switch shield/SFP cages.
  • TX is measuring 1.3V to 1.6V, nowhere near 3.3V.

Any ideas how to diagnose? FWIW, it does still boot successfully from eMMC.

2

Try to measure between the GND on the UART and the GND on the GPIO header.

3

What if you reverted back to non PoE / old working firmware and boot loader? Just to make sure first it’s a “soft error”, not permanent damage to the UART.

4

Personal thoughts: Perhaps we can try re-creating the official image via SSH to determine if it’s a problem with OpenWrt 24.10.4. If none of the other images work after several attempts, it might indicate a fault in the TTL circuit.

5

Strangest thing is happening. The board has been powered on all night and UART GND seems normal now. TX is still sitting at ~1.6V though, and I get no output, not even from early stages.

The version of OpenWrt I left on the eMMC has the standard BPi R4 (non-PoE) bootloader on it.