Modern C++ API for BitWizard SPI relay card for Raspberry Pi

Introduction

On a lengthy journey to automate the central heating of my house with https://domoticz.com/, I bought by error several SPI versions of the BitWizard relay card for the Raspberry Pi instead of the GPIO version of the card... So instead of a native support of the GPIO pins of the Raspberry Pi by https://domoticz.com/ I had to find some way to deal with the SPI version...

Unfortunately I did not find a library compatible with my modern C++ palate, so here is a small modern C++ library of my own to deal with this card. It may be useful for some other people, so here it is.

Anyway, it is not complete waste of time: I will recycle and extend this SPI library in some other projects.

This has been tested with Raspbian GNU/Linux 11 (Bullseye) with G++ 10.2 on a Raspberry Pi 3.

The library

It is a pure header library to limit usage headache, using only C++14 STL and Linux API. Fortunately, not a single malloc(), free(), new, delete or pointer is used here. Pure organic vegetarian source code.

Basically, you just have to include include/bw_spi_relay.hpp to use it and the documentation is the comments from the public methods of the raspberry_pi::bit_wizard::spi_relay4.

There is a small demo in examples/main.cpp.

The controling daemon uses also some Boost libraries and {fmt} you can get by installing the libboost-all-dev and libfmt-dev package for example.

To use SPI on the Raspberry Pi, it has to be configured to enable it, for example by running raspi-config.

To compile it and run it: :

cd examples
make
./main

and it will animate the relays for a few seconds.

The controller/daemon program for the relays

In the examples, there is also a program to control the relays independently, in a persistent way.

So you can use this program in a Domoticz script for example.

To use it you have first to launch the controlling process attached to the relays with: :

./bw_spi_relay_control --daemon

Then to control a relay, use for example: :

./bw_spi_relay_control --reset
./bw_spi_relay_control --relay 3 --on
./bw_spi_relay_control --relay 1 --off
./bw_spi_relay_control --relay 1 --state 1
./bw_spi_relay_control --relay 1 --state 0

You can stop the daemon with: :

./bw_spi_relay_control --stop

To get the list of the commands: :

./bw_spi_relay_control --help

Related links

Some useful information:

• http://www.bitwizard.nl/shop/raspberry-pi?product_id=148
• http://www.bitwizard.nl/wiki/Raspberry_Relay
• https://github.com/rewolff/bw_rpi_tools/blob/master/bw_tool/bw_tool.c
• https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#serial-peripheral-interface-spi
• https://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus
• linux/Documentation/spi/spidev
• /usr/include/linux/spi/spidev.h
• in some old kernel sources there is also linux/Documentation/spi/spidev_test.c
• http://wiringpi.com/
• https://github.com/JoachimSchurig/CppGPIO
• http://www.airspayce.com/mikem/bcm2835/
• http://www.hertaville.com/interfacing-an-spi-adc-mcp3008-chip-to-the-raspberry-pi-using-c.html