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I have a board running at 5 V and I want to be able to ground the ATmega reset pin from a Pi so I can reset the ATmega on demand.

I need to make the reset voltage Pi safe.

My first thoughts were to use a voltage divider 1 kΩ/2 kΩ but that doesn't work as the 2 kΩ goes to ground and leaves the ATmega in a permanent reset position.

In short, I need to drop from 5 V to 3.3 V.

I'm looking for a minimal component cost solution, is there one?

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  • \$\begingroup\$ 3.3v Zener perhaps? \$\endgroup\$
    – Nikm
    Commented Jan 2, 2023 at 20:21

1 Answer 1

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An NPN bipolar transistor would be a good solution although perhaps not the very cheapest. You’ll want a pull-up resistor for the reset pin, 1 or 2k would do but I’d use 10k to save current. On the Pi side you’ll want a series resistor, again this could be 1 or 2k but I’d use 10 times the value of the pull-up resistor. Then get the Pi to output a high level to reset the Arduino.

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  • \$\begingroup\$ Could I not use an NPN with Emitter to ground, Collector to Reset pin and the base to the PI with a small resistor? Question, why would I need a pull-up on the reset pin as it's already high? \$\endgroup\$
    – Nikm
    Commented Jan 2, 2023 at 21:07
  • \$\begingroup\$ @Nikm is it allowed to be unconnected? If so then it's probably fine. Also the resistor can be quite big. If it's not allowed to be unconnected then you need the pullup because when the transistor is off it's basically unconnected. \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Jan 2, 2023 at 22:29
  • \$\begingroup\$ Yes seems fine when not connected, but I guess it won't do any harm to have a pull-up. Thinking of using an 2N3904, that looks suitable Thanks \$\endgroup\$
    – Nikm
    Commented Jan 2, 2023 at 22:42
  • \$\begingroup\$ @Nikm you can rely on the built-in pull-up or you may want an additional resistor to guard against unintentional resets. A 2N3904 would be a good choice. \$\endgroup\$
    – Frog
    Commented Jan 3, 2023 at 17:59

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