TMR1 100Hz Exactly Interrupt Routine

[manumenzella]

Hi.
Im working on a datalogger project that needs to take a reading every exactly 1 second. I want to make TMR1 interrupt every 10 ms, so that every 100 TMR1 interrupts, I get exaclty one second. This is the routine I got to write. I have not tested yet in a PIC, because I run out of them after burning to of them because of wrong polarity . LOL. By the way, I am using 16F877A. This is the code.

/////////////////////////////////////////////////

INT:
@ BCF T1CON,0 'STOP TIMER1
@ MOVLW 0xC3 'LOAD WITH 65527 BECAUSE
@ MOVWF TMR1H '50000 (20MHz / 4 / 100) - 2 CYCLES
'TO ENTER INT
@ MOVLW 0x48 '- 6 CYCLES TO LOAD THE REGISTERS, AND
@ MOVLW TMR1L 'TURN IT ON AGAIN
@ BSF T1CON,0 'TURN TIMER1 ON AGAIN

X = X + 1 'INCREMENT REG
PIR1.0 = 0 'CLEAR INT FLAG
CONTEXT RESTORE 'RESTORE REGs AND RETFIE

/////////////////////////////////////////////////

Can you please tell me if its OK??? Thanks for your help, and sorry for my english. Manuel

[manumenzella]

Hey its me again. I have just realized that in the number to load to TMR regs, i have to take into account the cycles needed to save all the regs. But anyway, I just want to know what you guys think about the general of the routine. Would it work??? Would it be accurate??? Thanks again.
Manuel

[paul borgmeier]

Why 10 mS, 100 Counts? You are thinking like a person and not a computer. Computers like powers of 2s not 10s.

Why not use a 4.194304MHz Xtal (DigiKey has them in stock for $0.70 ea), let Timer 1 run free, set the prescaler to 8, and count TMR1 overflows. 2 Overflows = 1.000 sec. This gives you lots of time to do other stuff in between interrupts.
(math = 4/4.194304*65536 * 8 * 2 = 1000000 uS = 1 S)

Oh yeah, you also do not have to deal with accounting for the timing for saving registers etc because the timer is free running.

[manumenzella]

Hi Paul
Thanks for your help!!!!!!!!!
Youre right, im quite new to programming and still not used to power of 2!!
Anyway, which crystal do you recommend to use to run close to 20MHz???
Thanks a million
Manuel

[Acetronics2]

100 Hz ...

might have very precise 100 Hz timebase from mains ... just a trafo and two rectifiers !!!

just an idea ...

Alain

[Bruce]

Have a look here http://www.pbpgroup.com

Darrel has some excellent PBP examples.