Controlling RGB LED display with Raspberry Pi GPIO

A library to control commonly available 32x32 or 16x32 RGB LED panels with the Raspberry Pi. Can support PWM up to 11Bit per channel, providing true 24bpp color with CIE1931 profile.

Supports 3 chains with many 32x32-panels each. On a Raspberry Pi 2, you can easily chain 12 panels in that chain (so 36 panels total), but you can stretch that to up to 96-ish panels (32 chain length) and still reach around 100Hz refresh rate with full 24Bit color (theoretical - never tested this; there might likely be timing problems with the panels that will creep up then). With fewer colors you can control even more, faster.

The LED-matrix library is (c) Henner Zeller [email protected] with GNU General Public License Version 2.0 http://www.gnu.org/licenses/gpl-2.0.txt

The demo-main.cc example code using this library is released to the public domain.

NOTICE: Wiring change on 2015-07-19

The wiring to connect the RPi and the Hub75 changed on 2015-07-19 to provide improved output quality. If you have an existing wiring from an earlier version of this library, provide

DEFINE+=-DRGB_CLASSIC_PINOUT make

to the compilation to make the old wiring work. Better yet, consider changing the wiring as it provides a much more stable image. See table below for wiring.

Overview

The 32x32 or 16x32 RGB LED matrix panels can be scored at Sparkfun, AdaFruit or eBay. If you are in China, I'd try to get them directly from some manufacturer, Taobao or Alibaba.

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.

Check out the minimal-example.cc to get started using this library.

All Raspberry Pi versions supported

This supports the old Raspberry Pi's Version 1 with 26 pin header and also the newer B+ models as well as the Raspberry Pi 2 with 40 pins. The 26 pin models can drive one chain of RGB panels, the 40 pin models up to three chains in parallel (each chain 12 or more panels long).

The Raspberry Pi 2 is faster than older models and sometimes the cabeling can't keep up with the speed; check out this troubleshooting section what to do.

It is recommended to install an image with a realtime kernel (for instance this one) to minimize a loaded system having an influence on the image quality.

Types of Displays

There are various types of displays that come all with the same Hub75 connector. They vary in the way the multiplexing is happening or sometimes they are

Type	Scan Multiplexing	Program Option	Remark
32x32	1:16	-r 32
32x64	1:16	-r 32 -c 2	internally two chained 32x32
16x32	1:8	-r 16
?	1:4	-r 8	(not tested myself)

These can be chained by connecting the output of one panel to the input of the next panel. You can chain quite a few together.

Connection

You need a separate power supply for the panel. There is a connector for that separate from the logic connector, typically a big one in the center of the board. The board requires 5V (double check the polarity: what is printed on the board is correct - I once got boards with supplied cables that had red (suggesting +) and black (suggesting GND) reversed!). This power supply is used to light the LEDs; plan for ~3.5 Ampere per 32x32 panel.

The connector on the RGB panels is called a Hub75 interface. Each panel typically has two ports, one is the input and the other is the output to chain additional panels. Usually an arrow shows which of the connectors is the input.

Here you see a Hub75 connector to be seen at the bottom of the RGB panel board including the arrow indicating the input direction:

Other boards are very similar, but instead of zero-indexed color bits R0, G0, B0, R1, G1, B1, they start the index with one and name these R1, G1, B1, R2, G2, B2; the functionality is identical.

Throughout this document, we will use the one-index base, so we will call these signals R1, G1, B1, R2, G2, B2 below.

The strobe signals is sometimes also called latch or lat. We'll call it strobe here.

If you plug an IDC-cable into your RGB panel to the input connector, this is how the signal positions are on the other end of the cable (imagine the LED panels somewhere outside the picture on the left); note the notch on the right side of the connector:

The RPi only has 3.3V logic output level, but many displays operated at 5V interprets these logic levels fine, just make sure to run a short cable to the board (if you see problems, see troubleshouting paragraph). If you do run into glitches or erratic pixels, consider some line-buffering, e.g. using the active adapter PCB. Since we only need output pins on the RPi, we don't need to worry about level conversion back.

For a single chain of LED-panels, we need 13 IO lines, which fit all in the header of the old Raspberry Pis. Newer Raspberry Pis with 40 pins have more GPIO lines which allows us to connect three parallel chains of RGB panels.

For reference, this is how the numbering on the Raspberry Pi looks like:

This is the same representation used in the table below, which helps for visual inspection.

Wiring diagram

For each of the up to three chains, you have to connect GND, strobe, clock, OE-, A, B, C, D to all of these (the D line is needed for 32x32 displays; 32x16 displays don't need it); you find the positions below (there are more GND pins on the Raspberry Pi, but they are left out for simplicity).

Then for each panel, there is a set of (R1, G1, B1, R2, G2, B2) that you have to connect to the corresponding pins that are marked [1], [2] and [3] for chain 1, 2, and 3 below. If you only connect one panel or have one chain, connect it to [1] (:smile:); if you use parallel chains, add the other [2] and [3].

To make things quicker to navigate visually, each chain is marked with a separate icon:

[1]=:smile:, [2]=:boom: and [3]=:droplet: ; signals that go to all chains have all icons.

Connection	Pin	Pin	Connection

                         -    |   1 |   2 | -
         :droplet: **[3] G1** |   3 |   4 | -
         :droplet: **[3] B1** |   5 |   6 | **GND** :smile::boom::droplet:

😄💥💧 strobe | 7 | 8 | [3] R1 💧 - | 9 | 10 | - 😄💥💧 clock | 11 | 12 | OE- 😄💥💧 😄 [1] G1 | 13 | 14 | - :smile::boom::droplet: A | 15 | 16 | B 😄💥💧 - | 17 | 18 | C 😄💥💧 😄 [1] B2 | 19 | 20 | - :smile: [1] G2 | 21 | 22 | D 😄💥💧 (for 32 row matrix, 1:16) 😄 [1] R1 | 23 | 24 | [1] R2 😄 - | 25 | 26 | [1] B1 😄 - | 27 | 28 | - 💥 [2] G1 | 29 | 30 | - :boom: [2] B1 | 31 | 32 | [2] R1 💥 💥 [2] G2 | 33 | 34 | - :boom: [2] R2 | 35 | 36 | [3] G2 💧 💧[3] R2 | 37 | 38 | [2] B2 💥 - | 39 | 40 | [3] B2 💧

In the adapter/ directory, there are some boards that make the wiring task simpler.

Running

The demo-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.

$ make
$ ./led-matrix
Expected required option -D <demo>
usage: ./led-matrix <options> -D <demo-nr> [optional parameter]
Options:
        -r <rows>     : Panel rows. '16' for 16x32 (1:8 multiplexing),
                        '32' for 32x32 (1:16), '8' for 1:4 multiplexing; Default: 32
        -P <parallel> : For Plus-models or RPi2: parallel chains. 1..3. Default: 1
        -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>)
        -b <brightnes>: Sets brightness percent. Default: 100.
        -R <rotation> : Sets the rotation of matrix. Allowed: 0, 90, 180, 270. Default: 0.
Demos, choosen with -D
        0  - some rotating square
        1  - forward scrolling an image (-m <scroll-ms>)
        2  - backward scrolling an image (-m <scroll-ms>)
        3  - test image: a square
        4  - Pulsing color
        5  - Grayscale Block
        6  - Abelian sandpile model (-m <time-step-ms>)
        7  - Conway's game of life (-m <time-step-ms>)
        8  - Langton's ant (-m <time-step-ms>)
        9  - Volume bars (-m <time-step-ms>)
        10 - Evolution of color (-m <time-step-ms>)
        11 - Brightness pulse generator
Example:
        ./led-matrix -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). Even though it is just an example, it can be useful in its own right. For instance, you can connect to its input with a pipe and simply feed text from a shell-script or other program 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).

Image Viewer

One of the possibly useful demo applications is an image viewer that reads all kinds of image formats, including animated gifs. It is not compiled by default, as you need to install the GraphicsMagick dependencies first:

 sudo aptitude install libgraphicsmagick++1-dev
 make led-image-viewer

Then, you can run it with any common image format, including animated gifs:

sudo ./led-image-viewer myimage.gif

It also supports the standard options to specify the connected displays (-r, -c, -P).

Chaining, parallel chains and coordinate system

Displays panels have an input connector, but also have an output port, that you can connect to the next display in a daisy-chain manner. There is the flag -c in the demo program to give number of displays that are chained. You end up with a very wide display (chain * 32 pixels). Longer chains affect the refresh rate negatively, so if you want to stay above 100Hz with full color, don't chain more than 12 panels. If you use a PWM depth of 1 bit (-p), the chain can be much longer.

The original Raspberry Pis with 26 GPIO pins just had enough connector pins to drive one chain of LED panels. Newer Raspberry Pis have 40 GPIO pins that allows to add two additional chains of panels in parallel - the nice thing is, that this doesn't require more CPU and allows you to keep your refresh-rate high, because you can shorten your chains.

So with that, we have a couple of parameters to keep track of. The rows are the number of LED rows on a particular module; typically these are 16 for a 16x32 display or 32 for 32x32 displays.

Then there is the chain length, which is the number of panels that are daisy chained together.

Finally, there is a parameter how many parallel chains we have connected to the Pi -- limited to 1 on old Raspberry Pis, up to three on newer Raspberry Pis.

For a single Panel, the chain and parallel parameters are both just one: a single chain (with no else in parallel) with a chain length of 1.

The RGBMatrix class constructor has parameters for number of rows, chain-length and number of parallel. For the demo programs and the image view, there are command line options for that: -r gives rows, -c the chain-length and -P the number of parallel chains.

The coordinate system starts at (0,0) at the top of the first parallel chain, furthest away from the Pi. The following picture gives an overview of various parameters and the coordinate system.

Remapping coordinates

You might choose a different physical layout than the wiring provides.

Say you have 4 displays with 32x32 and only a single output like with a Raspberry Pi 1 or the Adafruit HAT -- 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:

In action:

How can we make this 'folded' 128x32 screen behave like a 64x64 screen ?

In the API, there is an interface to implement, a CanvasTransformer that allows to program re-arrangements of pixels in any way. You can plug such a CanvasTransformer into the RGBMatrix to use the new layout (void RGBMatrix::SetTransformer(CanvasTransformer *transformer)).

Sometimes you even need this for the panel itself: In newer panels (often with 1:4 multiplexing) the pixels are often not mapped in a straight-forward way, but in a snake arrangement for instance. The CanvasTransformer allows you to work around that (sorry, I have not seen these panels myself so that I couldn't test that; but if you come accross one, you might want to send a pull-request with a new CanvasTransformer).

Back to the 64x64 arrangement:

There is a sample implementation class LargeSquare64x64Transformer that maps the 128x32 pixel logical arrangement into the 64x64 arrangement doing the coordinate mapping. In the demo program and the led-image-viewer, you can activate this with the -L option.

Using the API

While there is the 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.

If you are writing your own Makefile, make sure to pass the -O3 option to the compiler to make sure to generate fast code.

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.

Troubleshooting

Some panels don't handle the 3.3V logic level well, or the RPi output drivers have trouble driving longer cables, in particular with faster Raspberry Pis Version 2. This results in artifacts like randomly showing up pixels, color fringes, or parts of the panel showing 'static'.

If you encounter this, try these things

• Make sure to have as short as possible flat-cables connecting your Raspberry Pi with the LED panel.

• In particular if the chips close to the input of the LED panel read 74HC245 instead of 74HCT245 or 74AHCT245, then this board will not work properly with 3.3V inputs coming from the Pi. Use an adapter board with a bus-driver that acts as level shifter between 3.3V and 5V. (In any case, it is always a good idea to use the level shifters).

• A temporary hack to make HC245 inputs work with the 3.3V levels is to supply only like 4V to the LED panel. But the colors will be off, so not really useable as long-term solution.

• If you can't implement the above things, or still have problems, you can slow down the GPIO writing a bit. This will of course reduce the frame-rate, so it comes at a cost.

For GPIO slow-down, the following line in the lib/Makefile is interesting:

 DEFINES+=-DRGB_SLOWDOWN_GPIO=1

The default value is 1, if you still have problems, try the value 2. If you know that your display is fast enough, try to comment out that line.

Then make again.

Inverted Colors ?

There are some displays out there that use inverse logic for the colors. You notice that your image looks like a 'negative'. In that case, uncomment the folling DEFINES line in lib/Makefile by removing the # at the beginning of the line.

 #DEFINES+=-DINVERSE_RGB_DISPLAY_COLORS   # remove '#' in the beginning

Then, recompile

 make

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 old PC power supplies that often provide > 20A on the 5V rail. Also you can get dedicated 5V high current switching power supplies for these kind of applications (check eBay).

The current draw is pretty spiky. Due to the PWM of the LEDs, there are very short peaks of a couple of 100ns to about 1ms of full current draw. Often, the power cable can't support these very short spikes due to inherent inductance. This can result 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 not to loose more than 50mV from the source to the LED matrix. So that would be 50mV / 3.5A = 14 mΩ. For both supply wires, so 7mΩ each trace. A 1mm² copper cable has about 17.5mΩ/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)

• While a star configuration for the cabeling would be optimal (each panel gets an individual wire from the power supply), it is typically sufficient using aluminum mounting brackets or bars as part of your power solution. With aluminum of 1mm² specific resistivity of about 28mΩ/meter, you'd need a cross sectional area of about 4mm² per panel and meter.

  In the following example you see the structural aluminum bars in the middle (covered in colored vinyl) dualing as power bars. The 60A/5V power supply is connected to the center bolts (display uses about 42A all LEDs on):

• Often these boards come with cables that have connectors crimped on. Some cheap cables are typically too thin; you might want to clip them close to the connector solder your proper, thick cable to it.

• 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; in the picture above I am using 1x3300uF per panel and it works fine).

Now welcome your over-engineered power solution :)

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.

Then, with 'output enable', we switch the LEDs on for the amount of time for that bit plane. This is using some hardware support from the Pi to generate precise timings (but not if you use an old pinout).

Since LEDs can only be on or off, we have to do our own PWM by constantly clocking in pixels.

CPU use

These displays need to be updated constantly to show an image with PWMed LEDs. This is dependent on the length of the chain: for each chain element, about 1'000'000 write operations have to happen every second! (chain_length * 32 pixel long * 16 rows * 11 bit planes * 180 Hz refresh rate).

We can't use hardware support for writing these as DMA is too slow, thus the constant CPU use on an RPi is roughly 30-40%. Keep that in mind if you plan to run other things on this computer (This is less noticable on Raspberry Pi, Version 2 that has more cores).

Also, the output quality is suceptible to other heavy tasks running on that computer - there might be changes in the overall brigthness when this affects the referesh rate. In general, it is a good idea to use a Linux kernel with realtime extensions.

Limitations

If you are using the RGB_CLASSIC_PINOUT, then we can't make use of the PWM hardware (which only outputs to a particular pin), so you'll see random brightness glitches. I strongly suggest to change to the now default pinout.

The system needs constant CPU to update the display. Using the DMA controller was considered but after extensive experiments ( https://github.com/hzeller/rpi-gpio-dma-demo ) dropped due to its slow speed..

There is an upper limit in how fast the GPIO pins can be controlled, which limits the frame-rate. Raspberry Pi 2's are generally faster.

Fun

I am always happy to see users successfully using the software for wonderful things, like this installation by Dirk in Scharbeutz, Germany: