William "Chops" Westfield wrote:
> I want to calculate:
>
>    (ticks * <microsecondsPerTick>) / 1000
>
> to return the number of milliseconds elapsed based on a count
> maintained by a timer ISR.  microsecondsPerTick is a nice constant
> number like 1024 or 2048, so the multiplication becomes a shift, and
> factoring out twos I get:
>
>    (ticks * (microsecondsPerTick/8)) / 125
>
> "ticks" is at least 40 bits, however, and the multiplication MUST NOT
> OVERFLOW (been there, trying to fix that!), so I'm looking at at at
> least a 48bit intermediate value.

Do you have access to the ISR? I would be inclined to do the following,
rather than (or in addition to) simply counting the raw ticks:

    /* microseconds can be a 16-bit integer */
    microseconds += microsecondsPerTick;
    while (microseconds >= 1000) {
        microseconds -= 1000;
        ++milliseconds;
    }

-- Dave Tweed
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