Raspberry PI MAX7219 driver¶
Interfacing LED matrix displays with the MAX7219 driver [PDF datasheet] in Python (both 2.7 and 3.x are supported) using hardware SPI on the Raspberry Pi. The particular LED matrix I bought can be acquired for a few pounds from Banggood. Likewise 7-segment displays are available from Ali-Express. There are many other outlets selling both types of devices on Ebay and other such places.
This library supports:
- multiple cascaded devices
- LED matrix and seven-segement variants
For the matrix device, initialize the
import max7219.led as led device = led.matrix() device.show_message("Hello world!")
For the 7-segment devce, initialize the
import max7219.led as led device = led.sevensegment() device.write_number(deviceId=0, value=3.14159)
The MAX7219 chipset supports a serial 16-bit register/data buffer which is clocked in on pin DIN every time the clock edge falls, and clocked out on DOUT 16.5 clock cycles later. This allows multiple devices to be chained together.
When initializing cascaded devices, it is necessary to specify a
parameter, and generally methods which target specific devices will expect a
deviceId=... parameter, counting from zero.
For more information, see https://max7219.readthedocs.io/
By default, the SPI kernel driver is NOT enabled on the Raspberry Pi Raspian image. You can confirm whether it is enabled using the shell commands below:
$ lsmod | grep -i spi spi_bcm2835 7424 0
_[Depending on the kernel version, this may report spi_bcm2807 rather than spi_bcm2835 - either should be adequate]_
And that the devices are successfully installed in
$ ls -l /dev/spi* crw------- 1 root root 153, 0 Jan 1 1970 /dev/spidev0.0 crw------- 1 root root 153, 1 Jan 1 1970 /dev/spidev0.1
If you have no
/dev/spi files and nothing is showing using
lsmod then this
implies the kernel SPI driver is not loaded. Enable the SPI as follows (steps
taken from https://learn.sparkfun.com/tutorials/raspberry-pi-spi-and-i2c-tutorial#spi-on-pi):
- Use the down arrow to select
9 Advanced Options
- Arrow down to
- Select yes when it asks you to enable SPI,
- Also select yes when it asks about automatically loading the kernel module.
- Use the right arrow to select the <Finish> button.
- Select yes when it asks to reboot.
After rebooting re-check that the
lsmod | grep -i spi command shows whether
SPI driver is loaded before proceeding. If you are stil experiencing problems, refer to the official
Raspberry Pi SPI troubleshooting guide for further details, or ask a new question - but please remember to add as much detail as possible.
The breakout board has two headers to allow daisy-chaining:
|Board Pin||Name||Remarks||RPi Pin||RPi Function|
|3||DIN||Data In||19||GPIO 10 (MOSI)|
|4||CS||Chip Select||24||GPIO 8 (SPI CE0)|
|5||CLK||Clock||23||GPIO 11 (SPI CLK)|
Note: See below for cascading/daisy-chaining, power supply and level-shifting.
Installing the library¶
Note: The library has been tested against Python 2.7 and 3.4. For Python3 installation, substitute
python3-pip in the instructions below.
Install the latest version of the library directly from PyPI:
$ sudo apt-get install python-dev python-pip $ sudo pip install max7219
Alternatively, clone the code from github:
$ git clone https://github.com/rm-hull/max7219.git $ cd max7219 $ sudo pip install -e .
Next, follow the specific steps below for your OS.
$ cd max7219 $ sudo apt-get install python-dev python-pip $ sudo pip install spidev $ sudo python setup.py install
# cd max7219 # pacman -Sy base-devel python2 # pip install spidev # python2 setup.py install
Cascading, power supply & level shifting¶
The MAX7219 chip supports cascading devices by connecting the DIN of one chip to the DOUT of another chip. For a long time I was puzzled as to why this didnt seem to work properly for me, despite spending a lot of time investigating and always assuming it was a bug in code.
- Because the Raspberry PI can only supply a limited amount of power from the 5V rail, it is recommended that any LED matrices are powered separately by a 5V supply, and grounded with the Raspberry PI. It is possible to power one or two LED matrices directly from a Raspberry PI, but any more is likely to cause intermittent faults & crashes.
- Also because the GPIO ports used for SPI are 3.3V, a simple level shifter (as per the diagram below) should be employed on the DIN, CS and CLK inputs to boost the levels to 5V. Again it is possible to drive them directly by the 3.3V GPIO pins, it is just outside tolerance, and will result in intermittent issues.
Despite the above two points, I still had no success getting cascaded matrices to work properly. Revisiting the wiring, I had connected the devices in serial connecting the out pins of one device to the in pins of another. This just produced garbled images.
Connecting the CLK lines on the input side all together worked first time. I can only assume that there is some noise on the clock line, or a dry solder joint somewhere.
Run the example code as follows:
$ sudo python examples/matrix_test.py
$ sudo python examples/sevensegment_test.py
Note: By default, SPI is only accessible by root (hence using
sudo above). Follow these instructions to create an
spi group, and adding your user to that group, so you don’t have to run as root.
Pull requests (code changes / documentation / typos / feature requests / setup) are gladly accepted. If you are intending some large-scale changes, please get in touch first to make sure we’re on the same page: try and include a docstring for any new methods, and try and keep method bodies small, readable and PEP8-compliant.
- Thijs Triemstra (@thijstriemstra)
- Jon Carlos (@webmonger)
- Unattributed (@wkapga)
- Taras (@tarasius)
- Brice Parent (@agripo)
The MIT License (MIT)
Copyright (c) 2016 Richard Hull
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
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