On Mon, Jul 21, 2014 at 11:09:04PM -0500, Richard R. Pope wrote:
> Joe,
>      My resettimer function does this:
>          resettimer()
>          {
>              TMR1H =3D 0x0B;
>              TMR1L =3D 0xDC;
>          }
>      Doesn't that set the timer registers back to 0 and as soon as I=20
> turn the timer back on it will count to the proper value for 100mS?

No, that sets the timer to 0xbdc, or 3036 decimal.  THR1H and TMR1L
together make up a 16-bit counter, H means high byte, L means low
byte.

When the timer is allowed to run, it will count up and overflow to zero,
and when it reaches zero an interrupt will trigger and your ISR will
be started.

This technique is called preloading the timer, as opposed to free
running the timer.

The number of counts will be 0x10000 minus 0xbdc, which is 62500.

How this relates to instruction cycles will depend on a prescaler
configuration.

(By the way, if you read the timer in your ISR you will find it
advanced from zero by as many instruction cycles as were required to
get to that point in the ISR.)

--=20
James Cameron
http://quozl.linux.org.au/
--=20
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