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 
 


-----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

-- 
w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com

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