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 buses and chip selects can be specified by using other GPIOs. This is also known as bit-bang SPI.
Onion makes available two sample device tree overlay package to enable software SPI. These packages set up a software-based SPI bus on two different sets of GPIOs:
- The
onion-dt-overlay-sw-spipackage uses GPIOs 14-17 - The
onion-dt-overlay-sw-spi-altpackage uses GPIOs 0-3 which are more easily accesible
You can install either package to test out a software SPI bus on your device.
Users that need to use different pins can create their own customized version based on this package.
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
The package can be installed using opkg. Ensure the Omega2 is connected to the internet and update the package lists:
opkg update
Then install the desired package:
opkg install onion-dt-overlay-sw-spi
or
opkg install onion-dt-overlay-sw-spi-alt
Only install one of these package!
How it's used
Once the onion-dt-overlay-sw-spi or onion-dt-overlay-sw-spi-alt 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 | onion-dt-overlay-sw-spi GPIO | onion-dt-overlay-sw-spi-alt GPIO |
|---|---|---|
| SCK | 14 | 3 |
| MOSI | 16 | 1 |
| MISO | 15 | 0 |
| CS | 17 | 2 |
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
- https://github.com/OnionIoT/OpenWRT-Packages/blob/openwrt-23.05/onion-dt-overlay/src/sw-spi-alt.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.