I have a cape-like thingy that needs a few output-only GPIOs for select lines. I’m not using HDMI, so I chose GPIO2_7 through GPIO2_11 because they were physically convenient. I have HDMI turned off in u-boot, and GPIOs enabled with a device tree overlay, but I never get there. The SRM says they are also “sysboot” pins, and they should not be driven before SYS_RESET_N; but these are outputs only, so I don’t see how that can be the problem. At any rate, the bone never makes a peep; I can’t see any boot attempt at all when it’s plugged into my board. I’d hate to have to find 6 other GPIOs if I don’t have to, so is there some simpler way of making these not interfere with the boot process?
Are you sure that your ‘cape-like thingy’ doesn’t interfere with the boot sequence? (HDMI pins control the boot process!) I’d change to other pins (ie. P8_07 to P8_11).
You can do pin-muxing from user space (without any device tree overlays) using libpruio. By default it locks system pins (like HDMI), but you can unlock them for customization.
I use a 3-state buffer driven by SYS_RESETn. The buffer is open during reset.
- Mike