SPI Protocol, Explained

SPI stands for Serial Peripheral Interface, a way for electronic devices like microcontrollers to communicate with sensors or displays. One device (the master) sends a piece of data, the other device (the slave) sends a piece back, and they keep going until the message is complete. SPI uses four wires: MOSI, MISO, SCK, and CS.

• MOSI (Master Out Slave In): the wire the master uses to send information to the slave.
• MISO (Master In Slave Out): the wire the slave uses to send information back to the master.
• SCK (Serial Clock): the wire the master uses to send clock signals that synchronize the transfer.
• CS (Chip Select): the wire the master uses to select which slave it wants to talk to.

For communication, the master sends a series of bits over MOSI while sending clock pulses over SCK. The slave responds over MISO. Each transfer is a “transaction,” and multiple transactions can occur in a row as long as the master keeps CS low to keep the slave selected. Once done, the master raises CS to deselect that slave and can move to another.

Applications

• Memory devices: EEPROMs, Flash, and SRAM for reading and writing data.
• Sensors: accelerometers and temperature sensors.
• Displays: OLEDs and LEDs.
• Communication modules: Wi-Fi and Bluetooth modules.
• Motor control: communicating with motor drivers.

How to use it in Arduino

Arduino has built-in hardware support for SPI. On most boards the SPI pins are labelled MOSI, MISO, SCK, and SS (sometimes CS). SPI.begin() initializes the interface; digitalWrite(SS, LOW) selects the slave; SPI.transfer() sends and receives data; digitalWrite(SS, HIGH) deselects the slave.

Advantages of SPI

• Faster than asynchronous serial and I2C.
• No start and stop bits.
• No slave addressing mechanism like I2C.
• Full duplex: separate MISO and MOSI lines, so data can be sent and received at the same time.
• Not limited to 8-bit data.
• The slave hardware can be a simple shift register.
• Supports multiple slave devices.

Disadvantages of SPI

• Requires more wires than UART and I2C (which only need two lines).
• No acknowledgement to confirm data was received.
• No error-detection protocol.
• The master must control all communication (slaves cannot talk directly to each other).
• Usually needs a separate CS line per peripheral, which is awkward with many slaves.
• Supports only a single master.
• Suitable only for short distances.

Example Arduino code

#include <SPI.h>

int csPin = __;  // CS pin of the SPI device

void setup() {
  SPI.begin();                  // Initialize SPI interface
  pinMode(csPin, OUTPUT);       // Set CS pin as output
  digitalWrite(csPin, HIGH);    // Deselect the device
  Serial.begin(9600);
}

void loop() {
  digitalWrite(csPin, LOW);             // Select the device
  SPI.transfer(0x55);                   // Send data to the device
  int response = SPI.transfer(0x00);    // Receive data from the device
  digitalWrite(csPin, HIGH);            // Deselect the device

  Serial.print("Response: ");
  Serial.println(response);
  delay(1000);
}

Here the Arduino sends a byte (0x55) to an SPI device, reads a byte back, and prints it every second. You may need to adjust the SPI mode and clock speed for your specific device.

Originally published on sslabs.in.