Datasheet

Adapting Sketches to M0

The ATSAMD21 is a very nice little chip but its fairly new as Arduino-compatible cores go. Most sketches & libraries will

work but here's a few things we noticed!

The below note are for all M0 boards, but not all may apply (e.g. Trinket and Gemma M0 do not have ARef so you can

skip the Analog References note!)

Analog References

If you'd like to use the ARef pin for a non-3.3V analog reference, the code to use is analogReference(AR_EXTERNAL)

(it's AR_EXTERNAL not EXTERNAL)

Pin Outputs & Pullups

The old-style way of turning on a pin as an input with a pullup is to use

pinMode(pin, INPUT)

digitalWrite(pin, HIGH)

This is because the pullup-selection register is the same as the output-selection register.

For the M0, you can't do this anymore! Instead, use

pinMode(pin, INPUT_PULLUP)

which has the benefit of being backwards compatible with AVR.

Serial vs SerialUSB

99.9% of your existing Arduino sketches use Serial.print to debug and give output. For the Official Arduino SAMD/M0

core, this goes to the Serial5 port, which isn't exposed on the Feather. The USB port for the Official Arduino M0 core, is

called SerialUSB instead.

In the Adafruit M0 Core, we fixed it so that Serial goes to USB when you use a Feather M0 so it will automatically work

just fine.

However, on the off chance you are using the official Arduino SAMD core not the Adafruit version (which really, we

recommend you use our version because as you can see it can vary) & you want your Serial prints and reads to use

the USB port, use SerialUSB instead of Serial in your sketch

If you have existing sketches and code and you want them to work with the M0 without a huge find-replace, put

#if defined(ARDUINO_SAMD_ZERO) && defined(SERIAL_PORT_USBVIRTUAL)

// Required for Serial on Zero based boards

#define Serial SERIAL_PORT_USBVIRTUAL

#endif

right above the first function definition in your code. For example:

https://learn.adafruit.com/adafruit-feather-m0-express-designed-for-circuit-python-

circuitpython

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