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SPI 1 - 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 chips is the "master" while the other is the "slave".

SPI fundamentals
Basically:
  1. It is synchronous.
  2. It is full-duplex serial.
  3. It is not plug-and-play.
  4. There is one (and only one) master, and one (or more) slaves.
In more detail:
  1. Synchronous: a clock is generated by the master.
  2. Full-duplex serial: data is serialized, one bit of data is transferred in each direction during each clock period, so two data wires are used (MOSI and MISO).
  3. Not plug-and-play: The master and slave know beforehand the details of the communication (bit order, length of data words exchanged, etc...).
  4. One master: slave(s) cannot initiate communication, only the master can. The slave(s) listen and respond.
Simple transfer

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

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

In more detail:

  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-bit 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 multiple slaves in two ways: 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 (megabits per second). That means it can be used for uncompressed audio, or compressed video.

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