SparkFun Pro Micro pinout: every pin, its 32U4 port, and no pin 13

With the USB socket at the top, the left row reads TXO, RXI, GND, GND, then 2 to 9. The right row reads RAW, GND, RST, VCC, A3 to A0, then 15, 14, 16 and 10. TXO and RXI are pins 1 and 0. The chip pin column is the port and bit, PD3 for TXO, and that is how QMK names a pin: a keyboard config that says D3 means TXO, not Arduino pin 3.

Pins 14, 15 and 16 sit out of order at the bottom of the right row because they are the chip’s SPI pins, and pin 13 is missing altogether: PC7, which is pin 13 and the L LED on a Leonardo, has no pad on this board.

LED_BUILTIN is still 13, so Blink compiles and lights nothing, on a desk and in Mokxi. The two lamps the board does have are RX, on PB0, which is pin 17 and has no pad, and TX, on PD5, which has no pin number at all. Both are wired from VCC through a resistor to the pin, so they light when the pin is LOW.

RXLED1 and TXLED1 light them, RXLED0 and TXLED0 put them out, and TX_RX_LED_INIT sets both pins up; the USB core also flashes them when data moves. The red lamp is the power light and is on no pin.

Serial is the chip’s own USB port, which is the serial monitor, and it reaches no pin. Serial1 is the UART on TXO and RXI, a port of its own. On a real board while (!Serial) {} waits for a computer to open the port; in Mokxi it returns at once. I2C is pin 2 for SDA and 3 for SCL, the Leonardo’s pins, not A4 and A5 as on an Uno.

SPI is where it is on every 32U4 board: SCK on 15, MISO on 14 and MOSI on 16, here and on a desk. SS is 17, which is PB0, the RX lamp’s pin, and has no pad, so give a module’s chip select a pin of its own, such as 10, and drive it yourself. SPI.begin() sets 17 as an output and high, which puts the RX lamp out, as it does on the board.

The USB socket is the upload port too. After a reset the bootloader waits eight seconds for a new program, and the board has no reset button, so touching RST to GND twice quickly is how you recover one whose sketch broke its USB.