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SPI

Introduction​

The Omega2 has a built-in hardware SPI controller allowing it to communicate with SPI-enabled peripherals. It offers 2 Chip Selects. However, the internal flash storage uses Chip Select 0, so only Chip Select 1 is free for external devices.

Context​

The Serial Peripheral Interface (SPI) is a four-wire synchronous communication protocol, largely used to connect microprocessors or microcontrollers to sensors, memory, and other peripherals.

SPI SignalMeaning
SCKSystem Clock
MOSIMaster Out, Slave In – Data sent from the Master to the Slave
MISOMaster In, Slave Out – Data sent from the Slave to the Master
CS/SSChip Select/Slave Select

SPI is a synchronous data bus, meaning that one of the lines is a clock, used to synchronize the bits being sent on the data lines.

The protocol is based on the Master-Worker architecture.

  • The Master will generate the System Clock and the Chip Select signals.
  • Each worker requires its own Chip Select connection to the Master.

Hardware​

The Omega2 has one SPI interface available:

  • The interface supports half-duplex transmissions and can operate in host-mode only.
  • The maximum SPI clock frequency is 40 MHz.
  • The flash storage occupies SPI Chip Select 0 and cannot be used; external devices can be connected to SPI Chip Select 1.
  • External devices connected to the SPI bus can affect the boot sequence under certain conditions. (Refer to special pins - SPI pins for details.)
danger

The Omega2 does not support full-duplex SPI transmissions. This is due to a hardware bug in the underlying MT7688 SoC used in the Omega2.

The SPI and CS1 pins are highlighted on the Omega2/2S diagrams below.

omega2-pinout spi-cs1 pins

Software​

The Omega2 SPI interface is available at /dev/spidev0.1.

  • spidev0 is the Omega2's SPI bus number.
  • 1 indicates the device ID, which corresponds to the worker connected to the Omega2's CS1 pin.

Interacting with the SPI bus​

The following features are available on the SPI bus.

Python​

Onion recommends using the python3-spidev Python module for SPI communication.

The python3-spidev module provides a series of functions that implement SPI communication through the Linux device interface. It also provides an Omega2-specific xfer3 function that implements a half-duplex write-then-read SPI transmission.

For installation and usage instructions see: https://github.com/OnionIoT/python-spidev#python-spidev.