Robin Abbott wrote:
> You can a software correction which automatically applies an offset and
> places it in EEPROM. However for production you might find it a pain
> doing this automatically over a 20 hour period. You would need an
> accurate pulse source as well - say once per hour. I can let you know
> some more ideas on this if you drop an email.
>
> Robin Abbott
> Forest Electronics - the home of the WIZ-C PIC RAD C Compiler system
> robin.abbott@fored.co.uk
> www.fored.co.uk 
>   
If you can, try to obtain a 50 or 60hz signal, rather than a crystal. 
The 50/60hz pulse is incredibly
accurate, usually within a second per day, non-accumulative.

--Bob

>  
>
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Brooke Clarke
> Sent: 17 July 2006 16:46
> To: piclist@mit.edu
> Subject: [PIC] TMR2 Not keeping Time?
>
>
> Hi:
>
> I'm using Timer 2 as an 8 ms tick interrupt clock.  The 16F88 gets it's 
> external clock from a 1.00000000000000 MHz source.  But after running 
> for 20 hours the clock is 4 to 5 seconds fast.  Any ideas why?  Timer 2 
> is setup:
>
> ;  Fin  Fclk     Pre  Cntr  Freq    Post    Int Hz    Int ms
> ;  1M   250k     4    250   250     2       125       8  
>
>     banksel T2CON
>     movlw    0x0D    ; 2 post, Cntr On, 4 Pre: 1 MHz in
>     movwf    T2CON
>     banksel PR2
>     movlw    .251    ;    count  to 250 for 10, 5, 1 MHz
>     movwf    PR2
>     banksel PORTA
>
> The mid range reference manual says that after TMR2 gets to PR2 it's 
> reset to FF and the 16F88 manual says it's reset to 0.  Ten minutes of 
> running shows that PR2 needs to be set to .251 not the 250 that seemed 
> correct.  I like the idea that the counter gets automatically reset.  In
>
> a prior clock using Timer 1 it was a hassle to load the registers and 
> keep track of the latency, but it did work.
>
> Have Fun,
>
> Brooke Clarke
>
>   

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