TMR1 100Hz Exactly Interrupt Routine

[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.

[manumenzella]

Hi. Thanks for all the help.
Is the line frequency accurate enough to make a clock??? Hope it is!!!!! Great idea anyway. Thanks.
Manuel

[ronsimpson]

DigiKey has a 19.6608mhz crystal. If I did the math right...

math = 19,660,800/4/65536 =75 cycles
Your interrupt will happen 75 times/second. In software count to 75 and then add 1 to the second number.

or

last time I used a 20,000,000 crystal
20,000,000/4/65536=76.29395hz In software dividing by 76 is not good but close.
The interrupt happens 76.29395 times/second.
I used a 16bit variable and on every interrupt added 859 to the number. The variable overflows once per second. I think the math is off by .001%

[manumenzella]

hi ronsimpson!!! thanks for your help. i have two questions:
to calculate the interrupts with TMR1, I thought you had to divide the value by 66535 (0xFFFF). But, as you say, its 66536 because 0x0000 counts, right?
And one last thing. Suppose that a WORD variable is loaded with 60,000
Now we add to that variable, say 1000. Which is going to be the next result??? Is it going to be 0000 because it overflowed, or is it going to be 4535??? (60,000 + 1,000 - 65,535)???
Thanks for your help. Really quick answers you get in this forum!!!!!!!
Manuel