I am having a boot issue with the CM3 (4GIG) at startup. I have ubuntu core installed and under normal operation the system boots up fine.
I have enable the UART 0 overlay and updated a gadget snap to enable a GPS to operate on UART 0.
The issue I am having is that when the GPS is connected to UART 0 (GPIO 14 and 15) and the system powers up the boot halts (ie PI boot process freezes). If however I connect the GPS RX and TX to the PI after full boot, then the UART serial comms works fine and the GPS sends data to the PI.
I have narrowed down the boot freeze problem to a signal/message that the PI communicates on the UART at startup. If the GPS is transmitting on the RX line while the PI is booting (this will occur under normal operation in our system as the PI and GPS are powered from the same power line) the boot process freezes. I have got the PI to successfully boot if the RX line is disconnected during boot.
Is there any way to stop the PI from communicating or reading the RX line on UART 0 during boot?
Thanks for any help on this matter.