SPI: home page

[last updated: 2025-05-12]
SPI software
SPI breadboard trial
SPI vs. i2c
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• Description:
  • SPI (Serial Peripheral Interface) is a synchronous serial data protocol used by microcontrollers for communicating with each other or with one or more peripheral devices.
  • Maximum range is generally considered to be about 10', though lower clock speeds give more range, and bus driver circuitry can extend it dramatically, reportedly up to 100m or more.
  • There is one master/controller and may be multiple slaves/peripheral devices.
    If there are multiple slaves, they are differentiated and managed by each having its own Chip Select line.
    That's the theory, at least, though I have not yet figured how to implement it in practice...
  • A given communication (data transfer) is always initiated by the master/controller.
  • Using the Arduino SPI.h library, a communication from the master to the slave is done by executing:
    SPI.transfer([byte of data to be sent]);

    Each such transfer consists of a single byte of data, though many SPI.transfer(...) 's can be done in sequence under program control for transferring bigger chunks of data.

  • A single byte will be returned from slave to master with each byte that gets sent from master to slave.
    If you wish to use the byte of data that is returned from slave to master, do this:
    byte dataRcvd = SPI.transfer([byte of data to be sent]);

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• Hardware:
  • 3 lines common to all peripherals:
    MOSI - Master Out, Slave In (COPI - Controller Out, Peripheral In)
    MISO - Master In, Slave Out (CIPO - Controller In, Peripheral Out)
    SCK - Serial Clock
  • and 1 line for selecting which peripheral device you're communicating with:
    SS - Slave Select (or CS - chip select) - low to select, high to ignore
  That is, if you have more than one SPI device on the same MCU, then connect the MOSI lines together, same for MISO and SCK,
  then specify a separate SS line for each device.
  • MOSI/COPI line of master MCU connects to MOSI/COPI of slave
    MISO/CIPO of master connects to MISO/CIPO of slave
    SCK of master connects to SCK of slave
  • A dedicated output from the master connects to the SS of the slave. It will be controlled by the master software.
• Arduino models often have (different) hard-wired (ie. not changeable) pin assignments for SPI signal lines.

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