One of the Raspberry Pi’s many features is that it’s easy to connect to displays: it has both a modern HDMI connector and a phono socket dispensing old-school composite video. However, there are times when one doesn’t want a large monitor around but would rather have a few seven-segment displays instead.

Whilst one could build that from scratch, one can also buy small modules with eight seven-segment displays, eight red-green LEDs and eight push buttons for about $7 (as of August 2012). I bought mine from dealextreme1 but there might be other sources.

The boards are basically just the LEDs and switches, plus a TM1638 driver chip. The chip sits on a two-wire serial bus which makes it fairly easy to connect the boards to a computer/microcontroller of your choice. Of course, one needs a little bit of software, so I wrote some.

Arduino woz ’ere.

People have already worked out how to do all this on the Arduino:

On the Raspberry Pi

Hardware issues

The only important difference between the Arduino and the Raspberry Pi in this case is that the Arduino’s a 5V beast but the Pi prefers 3.3V. Happily though the dealextreme board appears to cope perfectly well with the lower supply voltage.


My code isn’t really a port of Ricardo’s Arduino library: I wanted a different API. However, I did copy his nice 7-segment font, and his code was very helpful when it came to understanding the data-sheet.


Before you start, you’ll need Mike McCauley’s nice bcm2835 library.6

You can then grab my TM1638 library from github7 in a couple of ways.

If you’ve got autotools then you can just clone the repository:

$ git clone
$ cd pi-tm1638
$ autoreconf -vfi						
$ ./configure						
$ make							
$ sudo make install					

You might find this easier and faster though:

$ wget
$ tar xzvf pi-tm1638-1.0.tar.gz
$ cd pi-tm1638-1.0
$ ./configure
$ make
$ sudo make install

Generic AVR support

In a pleasingly symmetric way, Filipe Moraes has ported this back to generic AVR chips, and added a scroll feature. You can read about it in Brazilian Portuguese on his blog,8 or grab the code from


If you’ve got doxygen installed the compilation process leaves HTML files in doc/html. Otherwise feel free to browse them on github.10


Three example programs are included with the software, and you’ll find them all in the examples directory:

N.B. All three examples hard code the pin numbers into the executable. So to run the examples you’ll need to make the following connections:

Raspberry PiTM1638 Board
3.3VP1-01VccPin 1
GPIO 17P1-11DIOPin 4
GPIO 21P1-13CLKPin 3
GPIO 22P1-15STB1Pin 5

Raspberry Pi Revision 2

Dominik Eschenmoser pointed out that if you’re using revision 2 of the Pi hardware, you have to chance the clock pin from GPIO21 to GPIO27.


Once you’ve sorted out the hardware, doing simple things with the hardware is easy.

Let’s look at one of the example programs, which turns your Raspberry Pi into a digital clock. Stripping out the comments and error checks, here’s the code:

#include <stdio.h>					
#include <stdlib.h>					
#include <stdbool.h>					
#include <time.h>					
#include <bcm2835.h>					
#include <tm1638.h>					
int main(int argc, char *argv[])			
  tm1638_p t = tm1638_alloc(17, 21, 22);
      time_t now    = time(NULL);			
      struct tm *tm = localtime(&now);			
      char text[10];					
      snprintf(text, 9, "%02d %02d %02d", 		
                  tm->tm_hour, tm->tm_min, tm->tm_sec);	
      tm1638_set_7seg_text(t, text, 0x00);		
  return 0;						

If you’ve installed the tm1638 library, and saved the code above as clk.c, to compile and run it:

$ gcc -std=c99 clk.c -o clk -lbcm2835 -ltm1638
$ sudo ./clk

You need to run the program as root (which is what the sudo does above), so that the code can talk to the GPIO hardware.

Having done that little lot, you should get something like this: an automatic wireless clock: