Raspberry Pi Compute Module 1 mini-UART

[edrose <edrose@outramresearch.co.uk>]

I've done a little more work on this. I've been able to swap the UART pins
around to a certain extent, but it's still being stubborn. I've been able to
move the console output from the main UART from GPIO14 to GPIO32. I've
somehow also managed to get the main UART to output to both GPIO32 and
GPIO14 simultaneously (although it wouldn't RX anything when that was
happening)! However I still can't get it to simply move the console over to
using the mini-UART.

I've worked out that the serial port seems to be selected by the status of
the pin multiplexers. It checks to see whether GPIO15 is multiplexed to the
main port. If not, it checks to see whether GPIO15 is multiplexed to the
alternate port. I tried forcing it to think that the secondary UART was
multiplexed by forcing a return value in the is_serial_muxed() functions to
be false for the main UART and true for the mini-UART, however it just gives
me a rainbow splash screen of death.

Can I have some pointers as to how I debug what's causing the Pi to fail to
boot when I try forcing the mini-UART? Clearly something is wrong and
causing it to fail. Will it output anything over the early debug UART and,
if so, what values do I need to put into the config for that to work? I
tried adding the register addresses I found in the datasheet, however I
couldn't get anything out of it.

Kindest regards,
Ed



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