Explanation for micros function

Question

I was trying to understand the working of micros() function internally and I had a look at the function in Arduino.h

This is the code:

unsigned long micros() {
 unsigned long m;
 uint8_t oldSREG = SREG, t;
 cli();
 m = timer0_overflow_count;
 #if defined(TCNT0)
 t = TCNT0;
 #elif defined(TCNT0L)
 t = TCNT0L;
#else
 #error TIMER 0 not defined
#endif
#ifdef TIFR0
 if ((TIFR0 & _BV(TOV0)) && (t & 255))
 m++;
#else
 if ((TIFR & _BV(TOV0)) && (t & 255))
 m++;
#endif
 SREG = oldSREG;
 return ((m << 8) + t) * (64 / clockCyclesPerMicrosecond());
}

Can someone please explain me what this line is doing :

 if ((TIFR0 & _BV(TOV0)) && (t & 255))
 m++;

Answer 1

If the timer 0 has overflowed (TIFR0 & _BV(TOV0)) and there is at least a count of 1 in the timer count register (t & 255) then increment the local copy of the overflow count.

That overflow count is used later on to add an upper 8 bits to what is basically an 8 bit counter to turn it into a 16 bit one ((m << 8) + t).

• Parthiv P: can you please explain me "timer0_overflow_count" too?
• Majenko: It's the current count of timer overflows - it's incremented in an interrupt routine elsewhere.

Answer 2

Since at the start of this function interrupts have been disabled by the cli() function it is possible that the timer TCNT0 has overflowed yet this event has not been processed and included in the value of timer0_overflow_count that has been read into m. Therefore the line that you are asking about is checking for this condition and incrementing the local copy m if necessary before using it to calculate the number of microseconds since startup.