As you maybe already know, the max7219 and max7221 chips are serially interfaced (SPI) 8-bit LED drivers. These chips work perfect on 7 segment displays if they have common cathode pins, because they are designed for those display. If you have common cathode 7 segment display you get in touch with the benefit of these led drivers, because they handle the multiplexing in an easy way. They have built-in bcd decoder for easily displaying information and if you don’t want to use the decoder you can byte-wise set every digit of your display. For example, if you want to display the number „0“ without bcd decoding, you create the bit mask as follows:
The bit mask for number „0“ is: 0x0111111
To display this number you have to address (e.g.) digit1 and send your „number byte“ afterwards. That works perfectly.