Controlling RGB LED display with Raspberry Pi GPIO

This is mostly experimental code.

Code is (c) Henner Zeller [email protected], and I grant you the permission to do whatever you want with it :)

Overview

The 32x32 RGB LED matrix panels can be scored at AdaFruit or eBay. If you are in China, I'd try to get them directly from some manufacturer or Taobao. They all seem to have the same standard interface, essentially controlling two banks of 16 rows (0..15 and 16..31). There are always two rows (n and n+16), that are controlled in parallel (These displays are also available in 32x16 - they just have one bank).

The data for each row needs to be clocked in serially using one bit for red, green and blue for both rows that are controlled in parallel (= 6 bits), then a positive clock edge to shift them in - 32 pixels for one row are clocked in like this (or more: you can chain these displays). With 'strobe', the data is transferred to the output buffers for the row. There are four bits that select the current row(-pair) to be displayed. Also, there is an 'output enable' which switches if LEDs are on at all.

Since LEDs can only be on or off, we have to do our own PWM. The RGBMatrix class in led-matrix.h does that.

Connection

The RPi has 3.3V logic output level, but a display operated at 5V digests these logic levels just fine (also, the display will work well with 4V; watch out, they easily can sink 2 Amps if all LEDs are on). Since we only need output pins, we don't need to worry about level conversion back.

We need 13 IO pins. It doesn't really matter to which GPIO pins these are connected (but the code assumes right now that the row address are adjacent bits) - if you use a different layout, change in the IoBits union in led-matrix.h if necessary (This was done with a Model B, older versions have some different IOs on the header; check http://elinux.org/RPi_Low-level_peripherals )

LED-Panel to GPIO with this code:

• GND (Ground, '-') to ground of your Raspberry Pi
• R1 (Red 1st bank) : GPIO 17
• G1 (Green 1st bank) : GPIO 18
• B1 (Blue 1st bank) : GPIO 22
• R2 (Red 2nd bank) : GPIO 23
• G2 (Green 2nd bank) : GPIO 24
• B2 (Blue 2nd bank) : GPIO 25
• A, B, C, D (Row address) : GPIO 7, 8, 9, 10
• OE- (neg. Output enable) : GPIO 2
• CLK (Serial clock) : GPIO 3
• STR (Strobe row data) : GPIO 4

This is the typical pinout on these LED panels

Running

The main.cc has some testing demos.

 $ make
 $ ./led-matrix
 usage: ./led-matrix <options> -D <demo-nr> [optional parameter]
 Options:
         -D <demo-nr>  : Always needs to be set
         -d            : run as daemon. Use this when starting in
                         /etc/init.d, but also when running without
                         terminal.
         -t <seconds>  : Run for these number of seconds, then exit
                (if neither -d nor -t are supplied, waits for <RETURN>)
 Demos, choosen with -D
         0  - some rotating square
         1  - forward scrolling an image
         2  - backward scrolling an image
         3  - test image: a square
         4  - Pulsing color
 Example:
         ./led-matrix -d -t 10 -D 1 runtext.ppm
 Scrolls the runtext for 10 seconds

To run the actual demos, you need to run this as root so that the GPIO pins can be accessed.

The most interesting one is probably the demo '1' which requires a ppm (type raw) with a height of 32 pixel - it is infinitely scrolled over the screen; for convenience, there is a little runtext.ppm example included:

 $ sudo ./led-matrix -D 1 runtext.ppm

Here is a video of how it looks: http://www.youtube.com/watch?v=OJvEWyvO4ro

Limitations

There seems to be a limit in how fast the GPIO pins can be controlled. Right now, I only get about 10Mhz clock speed which ultimately limits the smallest time constant for the PWM. Thus, only 7 bit PWM looks ok with not too much flicker.

The output should be luminance ('gamma') corrected, but isn't currently in the code (this can be simply done in SetPixel(), but ideally, we have more PWM output resolution, such as 10 bits).

Right now, I tested this with the default Linux distribution ("wheezy"). Because this does not have any realtime patches, the PWM can look a bit uneven under load. If you test this with realtime extensions, let me know how it works.

One experiment was to use the DMA controller of the RPi to circumvent the realtime problems. However, it turns out that the DMA controller slower writing data to the GPIO pins than doing it directly. So even if offloading this task to the DMA controller would improve the realtime-ness, it is too slow for any meaningful disply.