Trigger an optocoupler with NRST pin

[VPatr.2]

Hello everybody. I need to share external and software RESET between two different electronic boards. I designed a circuit based on transistors and optocouplers. The soft reset ground the NRST pin, this way I would like to trigger an optocoupler, but I couldn't find information about the current that the CMOS supports. Can I consider it as a common IO? Here are images of the circuit I drew and the information I found. Thanks.

[Peter BENSCH · Accepted solution]

Well, NRST can also output an internally generated reset as a ≥20µs pulse to the outside electronics, but it is still not comparable with a GPIO, which is why no maximum current is specified for it. Typically, you would connect external digital inputs to NRST, for which the current requirement is usually negligible.

However, you can still drive an optocoupler as you have drawn if its LED current is not too large. Since NRST is driven internally by a CMOS transistor, the resulting voltage at NRST depends on the flowing current. This voltage must remain clearly below VIL of internal and external logic so that both can detect the reset. With low current optocouplers, however, this should be easily achievable.

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Good luck!

Regards

/Peter