SMI LEDs

This project allows you to drive 8 or 16 WS2812 LED strips with one single Raspberry Pi!

The basis for this project is Jeremy P Bentham's rpi_smi_leds project that uses the RPi's SMI memory interface to accurately drive 8 or 16 LED strips -- flicker free and without a lot of CPU overhead -- simply amazing. For all the gory details on how this works, read Jeremy's excellent blog post about his code:

https://iosoft.blog/category/neopixel/

However, Jeremy's code was just that -- some code, but with zero documentaion on how to build it (several fixes were needed) or how to install it on a RPi. This project addresses those shortcomings and packages that code into a deployable project.

You can choose to link to a static C library build with CMake and include it in your C/C++ project, or you can choose the python module, which can be installed from pip.

Design philosophy

This module is for advanced users of LED strips and thus it may be missing some of the features you'd expect in other modules (e.g. setting indiviual LED values). For the moment, this module assumes that you are an advanced user of LEDs and thus likely already have some tools around creating LED sequences in your toolkit.

The only way to update the leds is via the led_set() function which takes a buffer of bytes exactly NUM_STRIPS * NUM_LEDS * 3 in size with LED data and sends it to the driver. The led_send() function actually shifts the LEDs out to the LED strips.

There is some code in the driver for setting indiviual pixels, but it wasn't working, so I've not exposed that function. But if people want that, someone should be able to make it work.

RGB data structure

For this setup, it is assumed that you have 8 or 16 equally sized LED strips.

Each time you call led_set, MUST send the same number of bytes:

• 8 * num_leds * num_strips * 3 bytes for an 8 strip setup
• 16 * num_leds * num_strips * 3 bytes for a 16 strip setup

If you have LED strips with differing lengths, set num_leds to the length of your longest LED strip. Then, for all strips that are shorter than the maximum length pad the RGB with zeros, so that you're always sending the same number of bytes for each of your strips. The extra zeros will be ignored by the shorter strips.

If you have fewer than 8 or 16 LED strips: Pad the RGB data sent to the strips out to 8 (or 16) strips. If you have 4 strips, send 4 strips of data and then 4 strips of zeros for a 8 strip setup.

Installation

Raspberry Pi Setup

In order for the smi_leds to work, the i2s system that is used to provide audio on the RPi must be disabled. The easiest way to do this is to edit /boot/firmware/config.txt (previously used to be /boot/config.txt) and to change the line:

#dtparam=i2s=on

to

dtparam=i2s=off

Don't forget to uncomment the line. Once you've made this change, reboot your RPi.

Prerequisites

To install this library, you'll need to have the following packages installed:

• python-dev-is-python3: Python dev headers, formerly python3-dev
• cmake

On a recent RPi install, this line should work:

sudo apt install -y build-essential python-dev-is-python3 cmake

Install the C/C++ static library

Prerequisites

To install this library, you'll need to have the following packages installed:

• cmake
• build-essential

apt install -y build-essential cmake

Installation

To install smi_leds for 8 LED strands:

mkdir build
cd build
cmake ..
make
sudo make install

To install for 16 channels, pass an option to cmake:

mkdir build
cd build
cmake .. -DENABLE_16_CHANNELS:BOOL=ON
make
sudo make install

Building your own code

To build your own code, run:

gcc -I /usr/local/include -L /usr/local/lib my_test.c -lsmi_leds

Install Python 3 module

Prerequisites

To install this library, you'll need to have the following packages installed:

• python-dev-is-python3: Python dev headers, formerly python3-dev
• cmake
• build-essential

sudo apt install -y build-essential python-dev-is-python3 cmake

Installation

To install via pip for 8 LED strands:

pip install smileds

To install via pip for 16 LED strands:

export LED_NCHANS=16
pip install smileds

Hardware Setup

The image below shows the GPIO pins that will drive the LED strips:

Remember that each one of these outputs will likely need to be level-shifted to 5V (unless your LED strips are 3.3V tolerant) and have a 300 Ohm resisor in-line if you'd like to follow best practices.

Here is the same information in a more convenient table:

SD0 through SD17 are the GPIOs that will drive your LED strips.

Software Setup

C

The full example in examples/example.c. This fragment sets all the LEDs to white at 25% brightness:

    // Allocate a buffer, set it to all 0xFF (white)
    uint8_t buffer[NUM_LEDS * NUM_STRIPS * 3];
    memset(buffer, 0xFF, NUM_LEDS * NUM_STRIPS * 3);

    // initialize the smi_leds module, starting with a 25% brightness
    leds_init(NUM_LEDS, 25);
    
    // Transfer our local buffer to the SMI buffer
    leds_set(buffer);

    // Shift the SMI buffer out to the LEDs
    leds_send();

Python 3

The python version is quite similar:

    # initialize the smi_leds module, starting with a 25% brightness
    smileds.leds_init(num_leds, 25)

    # Build the LED data array
    leds = bytearray()
    for strip in range(num_strips):
        for led in range(num_leds):
            leds += bytearray((255, 255, 255))

    # Set the buffer into to the smileds module
    smileds.leds_set(leds)

    # Shift them out to the strips
    smileds.leds_send()

Demo video

Here is a super short demonstration video for this module in action:

Future work

This module could use various improvements over time if people are interesting in developing it further:

• Test more PRi models (RPi 5, in particular)
• Consider adding gamma correction
• Debug the set_pixel() function to allow changing single pixels

Also, if someone else would like to take over the development of this module, I would be happy to pass it on.