Controlling RGB LED display with Raspberry Pi GPIO

A library to control commonly available 32x32 or 16x32 RGB LED panels with the Raspberry Pi.

The LED-matrix library is (c) Henner Zeller h.zeller@acm.org with GNU General Public License Version 2.0 http://www.gnu.org/licenses/gpl-2.0.txt

The example code is released in the public domain.

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.

The RGBMatrix class provided in include/led-matrix.h does what is needed to control these. You can use this as a library in your own projects or just use the demo binary provided here which provides some useful examples.

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, found on the circuit board:

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..11
     -l            : Don't do luminance correction (CIE1931)
     -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

There are also two examples minimal-example.cc and text-example.cc that show use of the API. The text example allows for some interactive output of text (using a bitmap-font found in the fonts/ directory), but it could also be used to connct via a pipe, say you have an program or shell-script that wants to output something; let's display the time in blue:

 (while :; do date +%T ; sleep 0.2 ; done) | sudo ./text-example -f fonts/8x13B.bdf -y8 -c2 -C0,0,255

You could connect this via a pipe to any process that just outputs new information on standard-output every now and then. The screen is filled with text until it overflows which then clears it. Or sending an empty line explicitly clears the screen (if you want to display an empty line, just send a space).

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).

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:

In action:

Using the API

While there is a demo program, the matrix code can be used independently as a library. The includes are in include/, the library to link is built in lib/. So if you are proficient in C++, then use it in your code.

Due to the wonders of github, it is pretty easy to be up-to-date. I suggest to add this code as a sub-module in your git repository. That way you can use that particular version and easily update it if there are changes:

 git submodule add https://github.com/hzeller/rpi-rgb-led-matrix.git matrix

(Read more about how to use submodules in git)

This will check out the repository in a subdirectory matrix/. The library to build would be in directory matrix/lib, so let's hook that into your toplevel Makefile. I suggest to set up some variables like this:

 RGB_INCDIR=matrix/include
 RGB_LIBDIR=matrix/lib
 RGB_LIBRARY_NAME=rgbmatrix
 RGB_LIBRARY=$(RGB_LIBDIR)/lib$(RGB_LIBRARY_NAME).a
 LDFLAGS+=-L$(RGB_LIBDIR) -l$(RGB_LIBRARY_NAME) -lrt -lm -lpthread

Also, you want to add a target to build the libary in your sub-module

 # (FYI: Make sure, there is a TAB-character in front of the $(MAKE))
 $(RGB_LIBRARY):
	 $(MAKE) -C $(RGB_LIBDIR)

Now, your final binary needs to depend on your objects and also the $(RGB_LIBRARY)

 my-binary : $(OBJECTS) $(RGB_LIBRARY)
     $(CXX) $(CXXFLAGS) $(OBJECTS) -o $@ $(LDFLAGS)

As an example, see the PixelPusher implementation which is using this library in a git sub-module.

Note, all the types provided are in the rgb_matrix namespace. That way, they won't clash with other types you might use in your code; in particular pretty common names such as GPIO or Canvas might run into clashing trouble.

Anyway, for convenience you just might add using-declarations in your code:

 // Types exported by the RGB-Matrix library.
 using rgb_matrix::Canvas;
 using rgb_matrix::GPIO;
 using rgb_matrix::RGBMatrix;
 using rgb_matrix::ThreadedCanvasManipulator;

Or, if you are lazy, just import the whole namespace:

 using namespace rgb_matrix;

Read the minimal-example.cc to get started, then have a look into demo-main.cc.

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. A low ESR capacitor close to the input is good in these cases.

On some displays, the quality of the output quickly gets erratic when voltage drops below 4.5V. Some even need a little bit higher voltage around 5.5V to work reliably.

When you connect these boards to a power source, the following are good guidelines:

• 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).

• If you still see noise, increase the voltage sligthly to 5.5V.

Technical details

The matrix modules available on the market 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; in that case, it is 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 by constantly clocking in pixels.

Limitations

There seems to be a limit in how fast the GPIO pins can be controlled. We get about 10Mhz clock speed out of GPIO clocking. Do do things correctly, we would have to take the time it takes to clock a row in as essentially the lowest PWM time (~3.4µs). However, we just ignore this 'black' time, and switch the row on once we have it. With that, we do PWM with the lowest time period of about 200ns, which brings us display update rates of ~140Hz at a PWM of 11 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 (CONFIG_PREEMPT_RT), let me know how that 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.)