Reply To: Pi 4 locks up with a kernel oops as soon as both spi4 and spi5 overlays are on

I ran into nearly the same thing on a lab Pi 4 with two thermocouple boards. I never got to the bottom of the kernel side, so I can’t tell you whether it’s a driver bug. But spi5 sits on GPIO12 to 15, and GPIO14/15 are also the UART pins your console is using, so with enable_uart=1 two overlays are fighting over the same pins. That alone is enough for me not to trust the setup.

What I did, and I’d do it again: you don’t need two buses for two MAX31865s. They are SPI devices with their own chip select, so both sit on SPI0 with CE0 and CE1:

# /boot/firmware/config.txt
dtparam=spi=on
# remove the spi4-1cs and spi5-1cs lines

Wiring: SCLK GPIO11, MOSI GPIO10, MISO GPIO9 shared by both boards. CS of board 1 to CE0 (GPIO8), board 2 to CE1 (GPIO7). In CircuitPython / Blinka that’s digitalio.DigitalInOut(board.CE0) and board.CE1 as the CS pins. The MAX31865 is happy at 1 MHz, the shared bus is nowhere near busy reading two temperatures every few seconds.

Keep the PT100 config (2/3/4 wire) the same as it was per board, otherwise your readings shift by a few degrees and someone will open a ticket about the boiler.