P20: I²C1 Data
P20 is the Data 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 on address
0x1D and the Magnetometer on address
0x0E. 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.