Wednesday, March 16, 2011

Project 21 - LED Dot Matrix Display - Scrolling Message

In the continuing story of 8x8 LED displays not working for me, Project 21 is no different. I've checked, double-checked, and triple-checked my wiring and can find nothing wrong with it.

I'm using the C- display as specified, but all I can manage is to get a nice little light display on the 8x8. There is something going on... just not sure what it is.

There also was a little bit of extra code in the program that I had to remove and fix in order to even upload the program to the Arduino. I'm including a screen capture here - there's a small symbol indicating wrap-around of the code... remove it, hit the delete key to pull the little bit of wrap-around code back to the previous line... and the program at least uploads properly.

I also did a side by side comparison of the data sheet for my 8x8... compared it to the author's and it's identical (see screen capture). You can view my data sheet here.

I'm using the small 8-pin headers to raise the display off the breadboard - I've made certain that all the pins are connecting properly but given that all the LEDs light up, I don't think there's an issue with the headers. My best guess is a wiring problem that I'm just not seeing or maybe a damaged IC, although I was very careful when handling the MAX7219 chip.

I'm going to play around with the wiring a bit and take a more detailed look at the data sheet as I think that may be my issue.


  1. I think I know the reason:

    row = row << 1

    Change to:

    row = row >> 1

    Should be bitshift RIGHT not left.

    Try that, and thanks for reviewing my book.


  2. Thanks, Mike - I've moved on BUT BUT BUT - if there are any readers who attempt this project with the code correction PLEASE consider taking video and posting to YouTube - I'd love to post a follow-up here with a video showing the project working!


  4. Hello,
    I am confused, as the initial post is for project 21 but the comments it
    seems are for project 19 or 20..
    I have 19 and 20 OK, but cannot get 21..The initial post corrected the IDE error but it still
    refuses to lite the matrix..Am I correct that the authors matrix for this project should be the columns are cathodes and the rows are the anodes? 19 and 20 was C+ and project 21 is C-
    I hope that is correct..

    1. Update to my reply..I used the 100 ohm iset resistor and my display had a bright flash then went out..I suspect it sucked my 5volts from the USB to something lower..I changed it to 47k from the Max data sheet chart, and it now scrolls..However the letters are backwards and it scrolls from left to right rather than the conventional right to left..Everything else is normal though..I have checked my matrix pin for pin with that of project 19 and it matches..This project in the text calls for a column Row (+) and common cathode(-)column.. Thats what I have.
      If I look thru a mirror it shows perfect letters and scans from right to left..Weird !!

  5. This comment has been removed by the author.

  6. The schematic of the dot matrix in the book is column anode, so the row pins should connect to DIG pins (current sink), not SEG pins. .... ignore the other comment.

  7. In my case, the code seems to have some problem which I couldnt find yet. the scrolling message starts and then as soon as the B reaches the other side of the LED matrix everithing goes blank. reset button works, but the problem happens again

  8. I found this, and it works