For SPI, this is the Serial Clock (which says when to read/write data.) Typically Mode 0 (Idle LOW, Sample Rising Edge, Shift Falling Edge.)

Generic GPIO uses PB3.

CHECK: "ADC1.9/COMP1 IN- COMP2 IN-/SWO"


Devices:
- BME280[SPI] (SCK)
- BMP280[SPI] (SCK)
- NRF24L01[SPI] (SCK)
- NRF24L01[SPI] (SCK)
- CC1101[SPI] (SCLK)