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GPIO-driven feather reset?
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GPIO-driven feather reset?

by faelys on Sun Sep 19, 2021 4:19 am

Hello,

to try and work around another issue I have, I want to reset my feather STM32 board using a raspberry Pi GPIO. If I understand correctly, all feathers have the same RST pin, with the same behavior, so I'm asking the question more generally.

My first idea was to plug directly the rpi GPIO on the feather RST, and rpi ground to feather ground. But then I got worried about an electrical conflict, and instead of trying, I googled a lot but in vain, and decided to ask here.

As far as I can tell, the normal use of the RST pin is to put a button in there (maybe the embedded button is even directly on the same wires), so the pin is supposed to "float" in normal operation and be pulled to ground when resetting. I guess it doesn't really floats, but is pulled up by the feather, though I cannot find it documented anywhere.

So if I plug it directly on a rpi GPIO, both the rpi and the feather will try to pull it up. Am I understanding everything correctly so far?

Both board pulling the same pin up would be that the GPIO-to-RST wire would in effect connect rpi 3.3V with the feather 3.3V, with maybe some internal pull-up resistors along the way. However the rpi 3.3V and the feather 3.3V are not actually the same level, because the feather builds its own regulated 3.3V from the USB port, i.e. rpi's 5V. So that would mean a constant flow of current one way or the other, depending on the exact voltage output by both regulators.

I guess the correct solution would involve a rpi-controlled relay, but I have no idea how to do that.

On the other hand, on this guide https://learn.adafruit.com/programming-microcontrollers-using-openocd-on-raspberry-pi/wiring-and-test we see a direct wire from a rpi GPIO to a feather RST, and that seem to be officially endorsed.

So am I worrying for nothing? Is it OK because the 3.3V-regulators are close enough to make the current flow negligible? Or is it OK because the internal pull-up resistors can handle the load? Or is there something else I am missing?

Or is the guide dangerous, and should I really go for a transistor or a relay? What keyword should I google to find a suitable schematic and set of components?

Thanks in advance for your insights.

faelys
 
Posts: 12
Joined: Mon Sep 13, 2021 1:45 pm

Re: GPIO-driven feather reset?

by dastels on Sun Sep 19, 2021 11:19 am

I can't see anything to worry about with connecting a Pi output to the Feather's reset. If you really want to be cautious about it, use an optocoupler... you wouldn't even connect grounds and they'd be completely electrically isolated.

Dave

dastels
 
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Joined: Tue Oct 20, 2015 3:22 pm

Please be positive and constructive with your questions and comments.