Digital I/O
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Board-Specific Pins
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The Raspberry Pi Pico RP2040 chip supports up to 30 digital I/O pins,
however not all boards provide access to all pins.

Tone/noTone
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Simple square wave tone generation is possible for up to 8 channels using
Arduino standard ``tone`` calls.  Because these use the PIO to generate the
waveform, they must share resources with other calls such as ``I2S`` or
``Servo`` objects.