P19: I²C1 Clock

P19 is the Clock line of the micro:bit's I²C bus. I²C is a communication protocol that allows integrated circuits to talk to each other.

Two devices are connected to the micro:bit's I²C bus internally; the Accelerometer and Magnetometer. The I²C address used is dependent on the variant of the device you have. You cannot connect an external device that uses either of these addresses.

The I²C connections are pulled up to 3V via 4k7 Ohm resistors onboard the micro:bit.