Im using several of the SYSBOOT pins as outputs in my project. This means they are connected to the inputs of other ICs.
My problem is that the Beaglebone no longer boots from uSD when connected to these peripherals.(sporadically, very rarely it does work)
My assumption was that connecting SYSBOOT pins to INPUTS of other devices shouldnt affect booting, as they are not actively being driven!
Is this wrong?
I’ve also heard that the pullup/down resistors for the SYSBOOT pins on the Beaglebone are “weak” and additional resistors should be provided externally
to make them more reliable. If so, which pins require which resistor value and which type(pull up or down)?
It would be nice if the solution was a simply as providing some additional pullup/down resistors!
Do not apply any load to those pins until after the SYS_RESET line goes high. Another thing you can try is to apply lower resistance pullups and pulldowns that match the ones on the board, except with a lower value. That will affect the video, but as you want to use them for outputs, the lower values should be OK.
I had problems with those pins and found that the resistive “loading” at boot needs to be quite high.
I did not get this at first, and I had a bipolar transistor switch connected which was something on the order of 50kohms input resistance.
Well, that was too low for reliable boot process! Sometimes it would work, sometimes not. Very confusing until I found the special function of those pins.
Another example I had is the PRU cape. It uses those pins, but the circuit attached is MOSFET input gates which are extremely high input resistance.
With this sort of circuit, the boot process is unaffected. So it can be done, just be aware of the circuit you are attaching.
Since none of my projects require more than a few GPIOs, I simply avoid using the pins which can impact boot.
That’s one nice thing about Beaglebone, it has a lot of GPIOs!