Introduction to the Serial Peripheral Interface (SPI)

Serial Peripheral Interface (SPI) is a synchronous serial data protocol used by microcontrollers for communicating with one or more peripheral devices quickly over short distances. It can also be used for communication between two microcontrollers.

With an SPI connection there is always one master device (usually a microcontroller) which controls the peripheral devices. Typically there are three lines common to all the devices,

  • Master In Slave Out (MISO) - The Slave line for sending data to the master,
  • Master Out Slave In (MOSI) - The Master line for sending data to the peripherals,
  • Serial Clock (SCK) - The clock pulses which synchronize data transmission generated by the master, and
  • Slave Select pin - the pin on each device that the master can use to enable and disable specific devices. When a device's Slave Select pin is low, it communicates with the master. When it's high, it ignores the master. This allows you to have multiple SPI devices sharing the same MISO, MOSI, and CLK lines.

The SPI standard is loose and each device implements it a little differently. This means you have to pay special attention to the device's datasheet when writing your code. Generally speaking, there are three modes of transmission. These modes control whether data is shifted in and out on the rising or falling edge of the data clock signal, and whether the clock is idle when high or low.

To write code for a new SPI device you need to note a few things:

  • Is data shifted in Most Significant Bit (MSB) or Least Significant Bit (LSB) first?
  • Is the data clock idle when high or low?
  • Are samples on the rising or falling edge of clock pulses?
  • What speed is the SPI running at?

Once you have your SPI Control Register set correctly you just need to figure out how long you need to pause between instructions and you are ready to go. Now that you have a feel for how SPI works, let's take a look at the details of the EEPROM chip.