Stub (only covers SPI bus for storage).
SPI data port used for communication with the SD card. Write to send and receive a byte; read to get the last received byte.
7 bit 0 ---- ---- b... .als | ||| | ||+- Chip select: 1 = high, 0 = low | |+-- 1 to use ~400kHz speed (for init), 0 to use full speed | +--- ? +--------- 1 if SPI byte transfer ongoing
Write 0xA5 to unlock registers; write 0x00 to lock registers.