MAX7219 Driver (Python 3)
Original project (Python 2): https://github.com/rm-hull/max7219
Interfacing LED matrix displays with the MAX7219 driver in Python using hardware SPI on the Raspberry Pi.
Suport:
- multiple cascaded devices
- LED matrix and seven-segement variants
Python Usage
For the matrix device, initialize the matrix class:
import max7219.led as led
mx = led.matrix()
mx.show_message("Hello world!")For the 7-segment devce, initialize the sevensegment class:
import max7219.led as led
sevenseg = led.sevensegment()
sevenseg.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 cascaded=...
parameter, and generally methods which target specific devices will expect a
deviceId=... parameter, counting from zero.
For more information, see https://max7219.readthedocs.io/ (original version doc)
Pre-requisites
By default, the SPI kernel driver is NOT enabled on the Raspberry Pi Raspian image.
To enable follow this steps:
1. Run sudo raspi-config
2. Use the down arrow to select 9 Advanced Options
3. Arrow down to A6 SPI.
4. Select yes when it asks you to enable SPI,
5. Also select yes when it asks about automatically loading the kernel module.
6. Use the right arrow to select the <Finish> button.
7. Select yes when it asks to reboot.
GPIO pin-outs
The breakout board has two headers to allow daisy-chaining:
| Board Pin | Name | Remarks | RPi Pin | RPi Function |
|---|---|---|---|---|
| 1 | VCC | +5V Power | 2 | 5V0 |
| 2 | GND | Ground | 6 | GND |
| 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 Python library
Use pip to install
For Raspian:
$ sudo apt-get install python3-dev python3-pip
$ sudo pip3 install spidev RPimax7219
For Arch Linux:
# cd RPimax7219
# pacman -Sy base-devel python
# pip install spidev RPimax7219
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.
Examples
Run the example code as follows:
$ sudo python3 examples/matrix_test.py
or
$ sudo python3 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: http://quick2wire.com/non-root-access-to-spi-on-the-pi
References
- http://hackaday.com/2013/01/06/hardware-spi-with-python-on-a-raspberry-pi/
- http://gammon.com.au/forum/?id=11516
- http://louisthiery.com/spi-python-hardware-spi-for-raspi/
- http://www.brianhensley.net/2012/07/getting-spi-working-on-raspberry-pi.html
- http://raspi.tv/2013/8-x-8-led-array-driven-by-max7219-on-the-raspberry-pi-via-python
- http://quick2wire.com/non-root-access-to-spi-on-the-pi
License
The MIT License (MIT)
Copyright (c) 2016 Richard Hull (c) 2016 Fernando Manfredi
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.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
