25 Digital Pins on the Black Pro Micro
Since the Black Pro Micro uses the larger version of the AtMega32U4 it is possible to solder wires directly to the unused pins.B0
B7
C7
D5
D6
F0
F1
B0 and D5 are connected to LEDs. These may have problems used as an input. Should be fine as an output. These are easiest to connect to at the resistor near the LEDs.
E2 is the HWB pin. It must be pulled low so that the bootloader will run. On the Pro Micro it is connected directly to ground, so is useless as a data pin. If it was pulled low through a resistor you could possibly still use it as an output.
According to the datasheet the rest of the unused pins should be unconnected.
I have not tried using any of these except B0 and D5 on other Pro Micros. You would need to use 30AWG wire or thinner to make the connections.
An easier alternative to get more data pins is the new Adafruit ItsyBitsy with 23 accessible pins, or the Arduino Micro with 24 pins.
