A few weeks ago, I was scanning the Deal Extreme website, and ordered a few different LED displays, not because of any pressing need, but because I wanted to have some display options available if I needed some for a project. I was especially interested in cheap LED displays. One of the ones I ordered was this 8 digit, 7 segment display, which seemed to have a five pin interface. Without thinking about it too much, I went ahead and ordered it.

Today, they arrived. Like a lot of things that come from dealextreme, this one had been shipped somewhat haphazardly: the right angle header pins were significantly bent, but a little gentle prodding moved them back into place. I then spent a few minutes trying to decide what the module actually needed.

Unlike the modules I’ve had previously based upon the TM1637/TM1638/TM1640, these modules did not have a controller onboard that could latch all the segments and hold values. Instead, this board has 2 8 bit shift registers (namely, the 74HC595). Basically, one shift register controls which of the 8 digits are powered up (a high value in the 8 bit word indicates that the corresponding digit has a power on its cathode (which is common to all the segments of the LED) and the other 8 bit word tells it which segment is turned on (actually, it’s inverted: a 1 indicates the corresponding segment is off).

But here lies the rub: you only have 8+8 control lines, but you have to control 56 (actually 64 if you include the decimal points) segments. The way you get around this is to display a single digit at a time, but only do it for a short period of time. If you flip between digits fast enough, your persistence of vision will merge them into a consistent display.

The usual way to do this with an Arduino would be to use the shiftOut statement to clock the data out on some serial pins, and then call delay (or delayMicrosecond) to wait a bit, then shift the data out for the next digit, etc… But that makes the loop() difficult to write: you have to interleave both the display and all the other work that you have to do, with careful attention to timing.

There is a better way: interrupts. In particular, I used the TimerOne library (available on the Teensy as well as the Arduino libraries) to setup a timer interrupt. The idea is to contain a display buffer of 8 bytes, one byte per digit, which specifies which of the 8 segments is on. When the interrupt occurs, it clocks out the cathode and then the segments for the specified digit as quickly as it can, and then increments the display digit by one, wrapping around if necessary. Because this happens in an interrupt, it means that display happens without any real use intervention. When you want to change what is displayed on the display, you just change the contents of the segment buffer and then the interrupt fires next time, the appropriate digit will
be displayed.

The basic code worked the first time I typed it in (modulo plugging jumpers into the wrong pins on the Teensy, and with the digits occurring in the reverse order than I desired). A moments tweaking yielded this nice, steady display:

Eight digit, seven segment display! Working! #Arduino #teensy pic.twitter.com/EFqDQR2lTK

— Mark VandeWettering (@brainwagon) June 2, 2015

I’ve decided that rather than posting code fragments embedded here, I will instead post links to a github repository that holds the necessary code. As I extend and make improvements, you’ll be able to get updated code there.