Timer interrupt set at 1ms?

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Hi

I would like to setup a timer that will interrupt every 1ms.
I am using a 4MHz crystal.

If I do:
setup_counters(RTCC_INTERNAL,RTCC_DIV_4);

this will interrupt every 976us.

Can someone help me with this please?

Also while in the interrupt function- all that time does not contribute to the timer counter (i.e. the time is lost)?

Thank you
a.

[Ttelmah]

You cannot generate a 1mSec timer with the RTCC, and a 4MHz crystal. The best solution, is to use a timer that supports adjustable 'endpoints', normally Timer2. Alternatives are, to set the timer 'forward' a known amount on each interrupt. This has been discussed recently in the thread, 'PIC26F84A Timer_0 interrupt running slowly'. The other route, is to chose a crystal, that gives the required division. For instance, the 4.096MHz crystal is a standard part, and the 4MHz PICs,are actually specified to run to this (I asked Microchip, a long time ago). This then gives a nice 1mSec interrupt.
No, time in the interrupt is not 'lost' to the timer, however the count _will_ become in error, if you try to 'set it' to a fixed value inside the interrupt, since the counter will have already advanced, and possibly by a variable amount.

Best Wishes

[arrow]

Hi Ttelmah

Thank you for your suggestions. Before I get the 4.096 Xstal, can you please tell me about Timer2, or direct me to where I can find how to use it to set interupt durations?

Regards
a.

[Ttelmah]

Depends whether your chip has it!. You don't mention what the chip is. Data sheet...
MicroChip, have an application note about the various timers.
The 'point' about timer2, is that it has a hardware circuit to 'reset' it at a specified count. It can also interrupt on every 'n' counts (where 'n' is a 4 bit counter). So if your chip has it:

setup_timer2(T2_DIV_BY_4,249,10);

Will for a 4MHz clock, divide this by 16 (/4, then /4 in the prescaler), count from 0...249, and interrupt every tenth time through the count, giving an interrupt every 10mSec. Rather nice for timing. :-)
Be careful about the counter value. The 'wizard', gets this wrong, as does the help file. the chip counts from zero _to_ the value specified, and resets on the _next_ cycle. Hence for 250 counts, you need 249, yet the wizard insists on using 250. The MicroChip notes have this right As you'd - hopefully - expect!).

Best Wishes

[arrow]

Hi Ttelmah

Thank you very much- that worked 100%!

Regards
a.

[Ttelmah]

Thanks for 'coming back'. :-)

Best Wishes

[brian]

for a 4 MHZ clock

to get 1ms

setup_timer_2 (T2_DIV_BY_4, 25-1, 10);