Music Memorabilia

Opening Pin

How to configure unused IO pin for a microcontroller?

Hi, I have an IO pin that would normally be an input (for an IR receiver) but sometimes I want to disable that functionality. I would just configure it as an output and set it high (3V3), but there might be an external pull-up to 5V in which case an open-drain output would be better. But when the pull-up isn’t there is it still ok if the pin is configured as open-drain?

In general, you don’t want to leav an I/O pin floating – make it an input and tie it high or low – or, make it an output and leave it open. (what I usually do). I usually set outputs low.

If you know a pull-up will be there, you can just set it as an input and leave it that way, even if you aren’t using the IR receiver.

Note that most microcontroller I/O pins can’t be programmed as open-drain. That is more a function of hardware. Occasionally, a uC will have a pin that doesn’t have the P-channel transistor pulling high, in which case you have no option but to use it as open-drain.

You can always emulate open drain this way, though (with caveat, see below): To drive low, set pin as an output, low. To “open”: re-configure as an input and let the external circuit (e.g., pull-up) pull the pin high. If you leave it as an output, then the uC will pull high with its internal P-channel transistor.

Caveat: BE CAREFUL! You suggest pulling a uC pin higher than it’s supply voltage (pulling a pin driven with 3.3V up to 5V). There are a FEW uCs that have a pin or two you can do this with, but it isn’t common. In general, you should never pull a pin higher than the supply voltage. Reason, internally, there are parasitic structures (diodes) that will forward bias when you try to pull the pin higher than supply, conducting potentially damaging current from the I/O pin to the 3.3V supply.

CHECK THE SPECS to make sure you can do this – you probably can’t.

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