Controlling RGB LED display with Raspberry Pi GPIO

This is mostly experimental code.

Code is (c) Henner Zeller h.zeller@acm.org, and I grant you the permission to do whatever you want with it :)

Overview

The 32x32 or 16x32 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 16x32 - they just have one bank of 16 or two banks of 8)

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 (There is no D needed if you have a display with 16 rows with 1:8 multiplexing)
• OE- (neg. Output enable) : GPIO 2
• CLK (Serial clock) : GPIO 3
• STR (Strobe row data) : GPIO 4

Here a typical pinout on these LED panels; picture next to the connector:

Chaining

Displays also have an output port, that you can connect to the next display in a daisy-chain manner. There is a parameter in the demo program to give number of displays that are chained. You end up with a very wide display (chain * 32 pixels).

You can as well chain multiple boards together and then arrange them in a different layout. Say you have 4 displays with 32x32 -- if we chain them, we get a display 32 pixel high, (4*32)=128 pixel long. If we arrange the boards in a square, we get a logical display of 64x64 pixels.

For convenience, we should only deal with the logical coordinates of 64x64 pixels in our program: implement a Canvas interface to do the coordinate mapping. Have a look at class LargeSquare64x64Canvas for an example and see how it is delegating to the underlying RGBMatrix with changed coordinates.

Here is how the wiring would look like:

Running

The main.cc has some testing demos. Via command line flags, you can choose the display type you have (16x32 or 32x32), and how many you have chained. (Previous versions of this software required to do modifications in the source, that is now all dynamically configurable).

 $ make
 $ ./led-matrix
 usage: ./led-matrix <options> -D <demo-nr> [optional parameter]
 Options:
     -r <rows>     : Display rows. 16 for 16x32, 32 for 32x32. Default: 32
     -c <chained>  : Daisy-chained boards. Default: 1.
     -L            : 'Large' display, composed out of 4 times 32x32
     -p <pwm-bits> : Bits used for PWM. Something between 1..7
     -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 (e.g. cron)
     -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

CPU use

These displays need to be updated constantly to show an image with PWMed LEDs. For one 32x32 display, every second about 500'000 pixels have to be updated. We can't use any hardware support to do that - thus the constant CPU use on an RPi is roughly 30%. Keep that in mind if you plan to run other things on this computer.

Also, the output quality is suceptible to other heavy tasks running on that computer as the precise timing needed might be slipping. Even if the system is otherwise idle, you might see occasional brightness variations in the darker areas of your picture. Ideally, this would run on a system with hard realtime guarantees (There are Linux extensions for that, but haven't tried that yet).

A word about power

These displays suck a lot of current. At 5V, when all LEDs are on (full white), my LED panel draws about 3.4A. That means, you need a beefy power supply to drive these panels; a 2A USB charger or similar is not enough for a 32x32 panel; it might be for a 16x32.

If you connect multiple boards together, you needs a power supply that can keep up with 3.5A / panel. Good are PC power supplies that often provide > 20A on the 5V rail.

The current draw is pretty spiky. Due to the PWM of the LEDs, there are very short peaks of about 4 microseconds to about 1ms of full current draw. Often, the power cable can't support these very short spikes due to inherent inductance. This can results in 'noisy' outputs, with random pixels not behaving as they should. On top of that, the quality of the output quickly gets erratic when it drops under 4.5V (I have seen panels that only work stable at 5.5V). So having a capacitor close is good.

So when you connect these boards to a power source, keep the following in mind:

• Have fairly thick cables connecting the power to the board. Plan to not loose more than 50mV from the source to the LED matrix. So that would be 50mV / 3.5A = 14 mOhm. For both supply wires, so 7mOhm each trace. A 1mm² copper cable has about 17.5mOhm/meter, so you'd need a 2.5mm² copper cable per meter and panel. Multiply by meter and number of panels to get the needed cross-section. (For Americans: that would be ~13 gauge wire for 3 ft and one panel)

• It is good to buffer the current spikes directly at the panel. The most spikes happen while PWM-ing a single line. So let's say we want to buffer the energy to power a single line without dropping more than 50mV. We use 3.5A which is 3.5Joule/second. We do about 140Hz refresh rate and divide that in 16 lines, so we need 3.5 Joule/140/16 = ~1.6mJoule in the time period to display one line. We want to get the energy out of the voltage drop of 50mV; so with W = 1/2CU², we can calculate the capacitance needed: C = 2 * 1.6mJoule / ((5V)² - (5V - 50mV)²) = ~6400µF. So, 2 x 3300µF low-ESR capacitors in parallel directly at the board are a good choice (two, because lower parallel ESR; also fits easier under board). (In reality, we need of course less, as the highest ripple comes with 50% duty cyle thus half the current; also the input is recharching all the time. But: as engineer plan for maximum and then some).

Using the API

While there is a demo program, the matrix code can be used independently as a library. (TODO: document more).

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 display.