Micros and the check for less than 255

Projects Programming
1

Looking at the micros() core code I see a check for the timer value to be less than 255. What is the purpose of that check. I found the bug that it was trying to fix but still don't understand it. I was hoping to write similar code for timer1 for a project I'm working on and wondering if I need to include something like this to handle combining the timer value and overflow count together.

Source code in question: ArduinoCore-avr/wiring.c at 24e6edd475c287cdafee0a4db2eb98927ce3cf58 · arduino/ArduinoCore-avr · GitHub

2

It's hard to comment on, unless you identify it.

Have you seen this?
https://www.gammon.com.au/timers

The core code was not written to learn from. It's better to build from the ground up, using tutorials and always the processor data sheet in hand.

3

Take a look at the code and think about what might go wrong if t==255, on an 8 bit timer, and the check were not done.

	t = TCNT0;
	if ((TIFR & _BV(TOV0)) && (t < 255))
		m++;

If that is not clear, then the tutorial linked in the post above will certainly help.

4

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