Just the tutorial called the Datasheet.
See Table 2 for the register addresses. Write to the bottom 5 in setup(), then leave them alone except for maybe intensity if your sketch is changing brightness of the attached LEDs.
Setup SPI with the default settings.
#include <SPI.h> goes at the top of your sketch
byte csPin = 10;
pinMode (csPin, OUTPUT);
then load the 5 registers per the code example I gave above.
In loop, write to registers 1 to 8 with the data you want displayed. If using 0,1,2,3,4,5,6,7,8,9, H,E,L,P, -, blank, you can use Decode Mode and just the data per Table 5.
No library needed, just a little reading of the datasheet.
Register, 8 bits that data is written into. Just like any other register.
I don’t use any library for MAX7219 because the ones I have seen do not use the SPI port of the 328P chip for fast transfers out to the chip, and instead bit-bang the control lines out on other pins.
MAX7219-MAX7221.pdf (451 KB)