Joe,
     I just sent a reply to James with most of the code including the=20
ISR and handler. I believe that this will work but I am still just=20
confused enough that I am not sure. I turn the timer off. I disable=20
interrupts. I reset the timer values to the their original values. I=20
re-enable the interrupts and then I turn the timer back on. Is this the=20
way to do it? Yes, the timer is set up for a 1:8 prescaler. I play will=20
with the other values after I get this basic part working. I like to get=20
a basic program going and then modify only one part at a time. I find=20
that to be easier for me.
Thanks,
rich!

On 7/21/2014 11:59 PM, IVP wrote:
>>              TMR1H =3D 0x0B;
>>              TMR1L =3D 0xDC;
>>          }
>>      Doesn't that set the timer registers back to 0 and as soon as I
>> turn the timer back on it will count to the proper value for 100mS?
> The timer counts up from 0000 to FFFF and then wraps around to
> 0000. The instruction which sends it over FFFF to 0000 also sets
> the interrupt flag, TMR1IF.
>
> Only TMR2, the PWM timer, can be thought of as "counting to" a
> particular value, in that case a hardware match. Otherwise, all the
> timers count up to and through FFFF (or FF for 8-bit timers)
>
> 0x0BDC =3D 3036 decimal.
> 0x0000 is not only 0, but also 0xFFFF + 1, or 65536 decimal
>
> So, in the context of count-up-to-0000, 3036 represents 65536-
> 3036, or -62500.
>
> That is, 62,500 cycles are required to get the timer from 0x0BDC
> up to 0000, passing through FFFF
>
> At 5,000,000 Hz (20MGz crystal) instruction frequency, 100ms is
> 500,000 cycles, whch is too big to hold in the 16-bit counter of
> TMR1H : L. A count of 62,500 cycles without a pre-scaler is only
> 12.500ms
>
> There are two options -
>
> (1) Use a pre-scaler. This will allow a higher count but resolution is
> coarser. A pre-scaler of 1:8 will give you a count value of 500,000/
> 8 =3D 62,500. Which you have. Apologies if I missed you setting the
> pre-scaler in any of the code you posted.
>
> (2) Change 0x0BDC to 0x3CB0 (-50,000 decimal) and generate an
> interrupt every 10ms, without using the pre-scaler
>
> Joe

--=20
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