Transmitting ASCII 'A' over SPI Protocol
SPI devices also typically have a chip-select signal (sometimes referred to as a frame sync) that is used to multiplex devices on a single SPI bus. Thus, SPI is often referred to as a 3-wire or 4-wire interface (MISO, MOSI, CS, and CLK; for unidirectional devices, either MISO or MOSI can be omitted). A frame is typically assumed to start at the first bit after the chip-select signal is asserted, but every device has its own protocol. You'll need to adhere to whatever the other side of the link expects.