Avago Display LedDisplay Library Bug


I’ve been trying to contact with Tom Igoe since he was the developer of this library but I couldn’t find some email to write him. Therefore I think the apropiate place to post the bug is here.

I found an old avago 4 character display from a nortel modular router board so I decided to play a bit with it and arduino and it gave me a lot of fun.

What I found was that the library was originally writen for a 8 character display and (maybe) never tested against a 4 character display, until I tried it out to find it didnt work. Always showed blank display, and running some tests and debugs I found that the display worked if, instead using myDisplay.home(); used myDisplay.setCursor(4);.

I worked a couple of days with that little trick, until an idea crossed my mind like a bolt: the display was a shift register and the library allways sent a 40 lenght array without checking the display length; it was too much for a 20 needed display, so the display showed positions 4 to 7 instead 0 to 3 (20 to 39 instead 0 to 19).

Anyway, here is the patch that fixes this little issue:

--- LedDisplay_orig/LedDisplay.cpp      2009-04-17 22:12:49.000000000 +0200
+++ LedDisplay/LedDisplay.cpp      2009-10-06 18:05:14.000000000 +0200
@@ -250,12 +250,14 @@
 // this method sends 320 bits to the dot register:
 void LedDisplay::loadDotRegister() {
+  // define max data to send, patch for 4 length displays by KaR]V[aN
+  int maxData = displayLength * 5;
   // select the dot register:
   digitalWrite(registerSelect, LOW);
   // enable writing to the display:
   digitalWrite(chipEnable, LOW);
   // shift the data out:
-  for (int i = 0; i < 40; i++) {
+  for (int i = 0; i < maxData; i++) {
     shiftOut(dataPin, clockPin, MSBFIRST, dotRegister[i]);

Hope to see it fixed soon.

Thanks for the effort, Arduino Community and keep up the good job!

PS: As the main arduino.cc page says about this library:

loadDotRegister() - sends 320 bits to the dot register

320 bits is for 8 character display, 4 character display must send 160 bits.


after reading the forum a little bit more i have to say this topic is in the worng forum and should be under softwaree instead of hardware so please admin, move it to the right forum.

My apoligies.


One question is it possible to display special charaters like ã or ç

if yes who i'm waiting for my display to arrive and i need this ones


I extended the code a bit to support two 2 or 4 of the 8 character displays. Four displays end to end give a 32 character display.
Let me know if you want the modifications.

HCMS-2976 has BLUE leds!
see http://www.avagotech.com/pages/en/led_displays/smart_alphanumeric_displays/serial_interface/hcms-2976/