Do you think you'll be able to catch that blink? The LED is on for only a few clock cycles. At 16MHz, that's pretty short.
You're right gonna change that.
dig4 has some random value. So, it gets incremented. OK, I guess.
The rest of the manipulation of dig1, dig2, dig3, and dig4 (none of which have explicitly been set) occurs only if dig4 equals RANGE.
They usually get initialized automatically with the value 0 afaik, so it works fine, but yes, it is a good idea to initialize it .
Nothing? How do you know that?
because the array "digit" gets initialized this way:
uint8_t digit[4]= { ZERO,ZERO,ZERO,ZERO};
So even if the main code fails, it should display four zeros.
EDIT: It seem s that the the different values (see loop) are being set , but not muxed by the interrupt.