SPI

SPI is a protocol which allows one master chip to communicate with one or more slaves. It is commonly used in sensors and SD cards and requires 3 or 4 lines.

SPI uses a data and a clock line to synchronize senders and recivers. The clock signal is controlled by the master device. Whie the master is sending, data is sent on the MOSI line (master-out/slave-in), or COPI (controller-out/peripherial-in). If the master requires a response, it keeps the clock running for some cycles to allow the slave to answer on the MISO/CIPO line.

The last data line is the CS (chip select) line. It wakes up the slave or optionally selectes which of multiple slaves is addressed. The CS line is active low (held high during idling). Before transmission, the line is pulled low to activate the device.

Multiple devices can be accessed by having multiple CS lines on the master side and connecting them to the respective slave.

Advantages

• Faster than asynchronous serial (about 10MHz)
• Simple hardware
• Multiple peripherials

Disadvantages

• Many lines
• Fixed amount of data (since master keeps clock running for predefined amount of cycles)
• Multiple peripherials require multiple CS lines