max 7219 not giving serial out data

I’ve been following this thread and looking at the example code and checking the MAX7219 datasheet. The code made sense and looked like it should work and I was ready to conclude some sort of hardware / timing problem between displays 2 and 3 when suddenly it clicked.

With four displays linked together data is effectively being clocked into a 64 bit shift register.

First 16 bits of DATA are sent then LOAD is strobed so the data is loaded into display #1.

Next 32 bits are sent (16=DATA, 16=NOP) and LOAD is strobed. The data is loaded into display #2 and display #1 gets nop.

However the original 16 bits for display #1 get shifted down the chain into display #3 so display #3 gets loaded with the display #1 data. It will briefly show a copy of display #1 until the actual display #3 data is sent which is why it flickers.

Data clocked into the shift register is not cleared after a LOAD pulse so 64 bits must always be sent to ensure all displays get loaded with NOP except for the selected display.

Something like:

Send16:
    for x=4 to 1
        if x == displayNumber
             SendDATA
        else 
             SendNOP