Native SPI DMX Plugin

This plugin samples the Serial Peripheral Interface (SPI) at 2MHz to
receive DMX data. You need to connect only the MISO pin to the receive
pin of a transceiver chip like the MAX485 or SN75176.

Repeatedly, blocks of 4096 bytes are read as a batch operation and parsed
afterwards. We must hope that a DMX packet is contained in a block as a
whole. Since often a packet will be cut off by the block end, the higher
channels are updated less frequently than the lower ones. However, you can
set a larger block size if you want a higher refresh rate for the high
channels (which results in more stuttering in all channels, as the time
between two SPI read operation increases).

A detailed description of the development process and the techniques used
can be found in Flo Edelmann's Bachelors Thesis:
http://mnm-team.org/pub/Fopras/edel17/PDF-Version/edel17.pdf

Raspberry Pi configuration

On the Raspberry Pi, you need to enable SPI and increase spidev's
read / transmit buffer size:

• Run sudo raspi-config and enable SPI.
• Add spidev.bufsiz=65536 to /etc/cmdline.txt (in the same line). This
  is the maximum value. The value specified must be at least the block
  length set below. The default spidev.bufsiz is 4096.
• Reboot.
• Check cat /sys/module/spidev/parameters/bufsiz that it now returns the
  new value.

Only SPI device /dev/spidev0.0 is available via the GPIO pins.

Config file: ola-spidmx.conf

enabled = true
Enable this plugin (DISABLED by default).

device_prefix = <string>
The prefix of files to match in /dev. Usually set to spidev. Each match
will instantiate a device. Multiple values are supported by repeating this
line.

Per Device Settings

Note: Substitute <device> with the full device name, e.g.
/dev/spidev0.0.

<device>-blocklength = 4096
How many SPI bytes (= DMX bits) should be received (optional). The default
is 4096, but 8192 is recommended. You may need to make further
configurations on the Raspberry Pi (see above).