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What is SPI?

SPI is a simple interface that allows one chip to communicate with one or more other chips.
How does it look?

Let's start with a simple example where only two chips have to communicate together.

SPI requires 4 wires to be used in between the two chips.

As you can see, the wires are called SCK, MOSI, MISO and SSEL, and one of the chip is called the SPI master, while the other the SPI slave.

SPI fundamentals
  1. It is synchronous.
  2. It is serial.
  3. It is full-duplex.
  4. It is not plug-and-play.
  5. There is one (and only one) master, and one (or more) slaves.
In more details:
  1. A clock is generated by the master, and one bit of data is transferred each time the clock toggles.
  2. Data is serialized before being transmitted, so that it fits on a single wire.
  3. There are two wires for data, one for each direction.
  4. The master and slave know beforehand the details of the communication (bit order, length of data words exchanged, etc...)
  5. The master is always the one who initiates communication.

Because SPI is synchronous and full-duplex, every time the clock toggles, two bits are actually transmitted (one in each direction).

Simple transfer

Let's assume that the master and slave expect 8-bits data transfers, with MSB transmitted first.
Here's how would look a single 8-bits data transfer.

The line MOSI is the "master output" while MISO is the "slave output". Since SPI is full-duplex, both lines toggles simultaneously, with different data going from master-to-slave, and slave-to-master.

In more details:

  1. The master pulls SSEL down to indicate to the slave that communication is starting (SSEL is active low).
  2. The master toggles the clock eight times and sends eight data bits on its MOSI line. At the same time it receives eight data bits from the slave on the MISO line.
  3. The master pulls SSEL up to indicate that the transfer is over.

If the master had more than one 8-bits data to send/receive, it could keep sending/receiving and de-assert SSEL only when it is done.

Multiple slaves

An SPI master can communicate with multiples slaves by connecting most signals in parallel and adding SSEL lines, or by chaining the slaves.

With the multiple SSEL lines technique, only one SSEL line is activated at a time, and slaves that are not selected must not drive the MISO line.

How fast is it?

SPI can easily achieve a few Mbps (mega-bits-per-seconds). That means it can be used for uncompressed audio, or compressed video.