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I have the following hardware/software setup:

  • Sferalabs StratoPi Base (Raspberry Pi 3B+ with a HAT for RS232/485)
  • Distro built with Yocto and meta-raspberrypi on thud branch
  • U-Boot
  • SysVinit

I want to be able to free the GPIO 14/15 (TX/RX pins for UART) because the HAT needs these pins for RS232/485 communication.

As soon as I stop using U-Boot, the StratoPi device works and I can communicate with the devices on the RS485 bus. But I can't free the UART from getting used by Linux/U-Boot if I use U-Boot.

Does someone know how to make the UART pins available to the HAT?

I've tried to:

  1. Remove all references to ttyAMA0 in cmdline.txt and in /etc/inittab
  2. Patch U-Boot like here : https://stackoverflow.com/a/64583919/9077458
  3. Use pi3-disable-bt overlay

None of them worked for me.

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