It is possible configure Raspberry Pi GPIOs very early on at boot time, by adding configuration to the /boot/config.txt file. The detailed instructions are here.

Here’s an example - which with the M-Bus Master Hat powers up the M-Bus at boot time:

’’’ gpio=26=op,dh ‘’’

Here:

  • 26 is the pin number (using BCM pin numbering not wiringPi pin numbering)
  • op means configure as an output
  • dh means drive high (i.e. a 1)

This appears to override any configuration on the Hat’s EEPROM, as the M-Bus Master Hat EEPROM configures the Pi to drive pin 26 low at boot - but with this configuration added to /boot/config.txt the pin goes high at boot and remains there.

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