SPI – The Serial Peripheral Interface

SPI means Serial Peripheral Interface. With SPI you can exchange data blazingly fast between two devices. And you only need four wires.

SPI Characteristics

The serial peripheral interface is a synchronous communication interface for short distances. Synchronous means that the data is sent in form of blocks or frames and the the two devices are synchronized using a clock. It is a serial interface so the devices send one bit at a time (unlike parallel communication). One of the devices is the “Master” and dictates the clock cycle. The other device is the “Slave”. Usually the communication is in full duplex mode. Hence both devices can send data, even at the same time.

Wiring

You only need four wires for the connection so it is very easy to set up.

There are four pins that you need for an SPI connection are the Serial Clock (SCL or SCLK), the Master Output Slave Input (MOSI), the Master Input Slave Output (MISO) and the Chip Select or Slave Select pin (CS or SS).

Hook up the corresponding pins on the Master and Slave devices.

One Master device can connect to multiple Slave devices, as long as it has enough chip select pins. The Master controls which Slave device receives the data by setting the corresponding chip select pin to LOW or HIGH.

The Raspberry Pi 4 has 7 SPI busses . If you want to know how to access the SPI busses 1 to 6 then leave a comment below.

Let’s have a look at the SPI bus 0.

As you can see in the pinout graphic above pin 23 is the SCLK pin on the Raspberry Pi. MISO is pin 21 and MOSI is pin 19. The bus 0 has two CS pins which are pins 24 and 26.

If you want to use the bus 0, make sure to enable it first.

Execute the command

sudo raspi-config

Then select 3 Interface Options and click I4 SPI. Finally press Yes and Ok to enable SPI.

SPI Modes

There are four different data transmission modes that you should be aware of. They differ in clock polarity and clock phase.

Both can be either 0 or 1. Regarding clock polarity, a 0 means a LOW idle state and a 1 means a HIGH idle state. A 0 clock phase means that data is sent on the falling edge, while a 1 means that the data is sent on the rising edge. Before they can send data, the Master and Slave have to agree to one of the four modes.

Advantages

The biggest advantages are the high speed, easy hardware and software set up and low power requirement.

Disadvantages

The serial peripheral interface is not really extensible in range and number of devices since the Master neads a chip select pin for each device. Also there is no hardware Slave acknowledgement, so the Master does not know if the data is recieved by anyone.

Use cases

Most SPI devices are embedded devices or LCD-Displays. In general, it is used in areas where few devices and fast data transmission are required.

It allows the Raspberry Pi to interface all kinds of sensors and devices. Nearly all Raspberry Pi compatible devices use the SPI or the I2C interface.

SD cards use the SPI interface as well so even the SD card reader on the Raspberry Pi has some internal MISO and MOSI pins.