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micros() problems #1

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larsimmisch opened this issue Dec 29, 2011 · 0 comments

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larsimmisch commented Dec 29, 2011

I'm beginning to suspect that the Arduino::micros function (which is pretty much the original) isn't behaving vey well.

Simplified:

    static uint32_t micros()
        {
        uint32_t m;
        uint16_t t;
        uint8_t oldSREG = SREG;

        cli();
        t = TCNT0;

        if ((TIFR & _BV(TOV0)) && (t == 0))
            t = 256;

        m = timer0_overflow_count;
        SREG = oldSREG;

        return ((m << 8) + t) * (64 / (F_CPU / 1000000L));
        }

Once m reaches 2**24, the value wraps around to t * 4 (assuming a 16MHz clock).

And timer0_overflow_count itself wraps around at 2^32, so the function resets twice over its value range.

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