Configure Pull Ups at Power On

[dzar]

I have a need to configure some of the GPIOs on the NanoPI M4 to use their internal pullups at power up. Is there a way to do that? Or am I stuck based on the specific GPIOs brought out to the header on this board? I'm trying to sue an existing HAT board which is why I ask.

I know I can add the resistor externally, but I'm trying to not modify the board.

Thanks

[v8dave]

The best place to do this is in u-boot as it will be the first thing to run. You can do this in the kernel but that runs a little later. To guarantee a GPIO is low or high at power up, the only real solution is a pullup or down resistor.

[dzar]

Thanks. I get this, but even Uboot is likely too late. I have some HAT boards that require a pin to be high to enable a power supply. Hence, if that pin is low on power-up, the supply will turn off before the CPU boots far enough to keep the supply on. I can modify my boards, but I want them to work on multiple hardware setups and 99% of them (all until this board, at least) have pull-ups for unused I/Os.

Oh well, I was hoping for a flash register or some way to configure this in the CPU but it could well be at the silicon level with no way to affect it before the CPU comes up far enough to run code. Just too late for me, likely.