Multiplexing 4-digit 7-segment display is flickering

Question

I connected a 4-digit 7-segment display to the GPIO-port. It uses 8 ports for the LEDs and 4 ports for multiplexing via PNP-transistors. It is running so far with a loop written in C program:

while(1) {
  for(j = 0; j < 4; j++) { // Loop through 4 digits
    clearSegments();
    writeSegments(j+1, values[j]);
    delayMicroseconds(500); // approximately 500 Hz total refresh frequency
  }
}

In general this works fine but maybe once or two times a second there is some irregular flickering. There is no XSession running and only MPD plays music. I run the program with a very high priority:

nice -n -19 /home/pi/mpd/C/main &

What else can I do?

I already did some research and found out the following options:

• Create own kernel module which scheduling tasks (refer to tldp.org).
• Enable high resolution timers by patching the kernel (refer to raspberrypi.stackexchange.com).

Both ways seem to be very complicated for just using such a simple display.

What do you suggest?

Answer 1

To avoid flickering of the segments, you really do need hard real time control of the multiplexing. Even small variations in the timing will be quite noticeable.

Using the timer on the CPU should work quite well, but as you observed it seems a lot of hassle for one simple thing. Perhaps it's worth doing if it's not just a one-off thing you are making.

Using a smart display, or making one by adding a small micro is probably the easier route. I'd probably opt for the micro since it gives the most flexibility.

Answer 2

If you avoid multiplexing there will be no flickering, but it will cost you higher current usage and extra hardware. Just search for disp7s2 in http://www.e-lab.de/AVRco/DOC_en/DocuStdDriver.pdf document. It will show you an example schematics for non-multiplexed 7-segment display driver using 74HC595 shift register and 8 resistors for each digit. The advantage of this mode is ultra low radiation and therefore no EMR problems.