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.