GPIO Startup Characteristics
I'm using an Up Board Rev A 1.1 with a custom HAT PCB that has two AVRs onboard with the micros having control of powering up the UP Board. The UP 40-pin header is connected to each AVR over UART (TX and RX) with a GPIO for each to apply a reset (instead of UART_DTR), in order to be able to update firmware to both micros. However, the current revision doesn't account for the UP Board GPIO startup state and so when the UP boots, it pulls the reset line on both micros low and causes a reset that results in power never being supplied to the UP board for more than a few seconds.
A few solutions have been thrown around, but they all have some issues since the UP Board GPIOs seem to have varied startup characters from pin to pin. I've seen some stay low at boot-up, some start with a ~15s high pulse, and some with a ~45s high pulse. Is there any concrete documentation on the startup characteristics of the entire set of GPIO? The Pinout wiki page has some data, but nothing more in depth than mentioning the startup high pulse for an old revision of the UP Board.