Software SPI BUS
Introduction
The SPI hardware on the Omega2 only supports 2 devices using CS0 (already used by internal flash) and CS1.
If a use case requires an additional SPI device, software-based SPI chip selects can be specified by using other GPIOs. This is also known as bit-bang SPI.
Onion makes available a sample device tree overlay package to enable software SPI. This package sets up a software-based SPI bus on GPIO’s that were chosen arbitrarily for demonstration. You can install the onion-dt-overlay-sw-spi package to test out a software SPI on your device.
Users that need to use different pins can create their own customized version based on this package.
Drawback: Lower Bus Speed.
Since this is a software-based bus, it will not be as fast as a hardware SPI bus. This is adequate for most SPI devices, but it is not recommended for data-intensive use cases like driving a display.
Installation
Ensure the Omega2 is connected to the internet, and install the package using opkg:
opkg update
opkg install onion-dt-overlay-sw-spi
How it's used
Once the onion-dt-overlay-sw-spi package is installed, there will be a software bus available at /dev/spidev1.0.
The following table describes which pins are used for which SPI signal:
| SPI Signal | GPIO |
|---|---|
| SCK | 14 |
| MOSI | 16 |
| MISO | 15 |
| CS | 17 |
For further instruction on using the SPI Bus see the SPI article.
Source Code
The DTS fragment that enables the SPI Bus functionality can be found in the OnionIoT/OpenWRT-Packages GitHub repository at: https://github.com/OnionIoT/OpenWRT-Packages/blob/openwrt-23.05/onion-dt-overlay/src/sw-spi.dts
The package definition can be found at: https://github.com/OnionIoT/OpenWRT-Packages/blob/openwrt-23.05/onion-dt-overlay/Makefile
It is part of the onion-dt-overlay package.