I have a sensor controlled by a Pico and a much larger embedded application controlled by a Raspberry Pi 4. When the Pico senses a condition, it raises a signal on a GPIO pin which I would like to detect on the Pi 4. The Pico and the PI4 are electrically connected by 5V power and ground -- the PI4 is powering the Pico -- but this may not always be the case. Is it safe in this case to simply connect a Pico GPIO output pin to a PI4 input pin? Would a resistor be needed? Should I use an opto-isolator instead? There are other situations in which the Pico would be separately powered because of distance from the pi; in this case, I suspect I'll have to build an opto-isolator module anyway.

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The Pi and the Pico both have 3V3 GPIO. It is safe to connect a Pi GPIO to a Pico GPIO.

Make sure a Pi ground pin is connected to a Pico ground pin (so that they have a common voltage reference).

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