TMR1 100Hz Exactly Interrupt Routine

[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

[manumenzella]

Sorry in my last post, it should have been:
The variable should have been loaded with 65.000!! NOT 60.000
And when we add a thousand, do we get just 0x0000 or 465 (65000 + 1000 - 65535)??? Thanks
Manuel

[paul borgmeier]

1st question divide by 0x010000
2nd question = 464

Do you really need 1 second intervals and how exact do you need to be?

[manumenzella]

Thanks paul. I am planning to build a temperature datalogger, that will collect data every two minutes for a month or two, so I need to get it as accurate as possible. Also, in my country, Argentina, there are not RTC availables....

Now following Ronsimpson way, (the second one, with the 20.000 MHz), the variable to which we add 859 on every interrupt, I should never clear it, right??? Or should I clear it every time the variable overflows???

Sorry for my english, and thanks again.
Manuel

[paul borgmeier]

>>collect data every two minutes for a month
Check out
http://www.picbasic.co.uk/forum/showthread.php?t=2129
It would be easy to add in the data logger every two minutes.

With Ron's approach, do not stop the timer, just add fast after the interrupt.

Why 20MHz for a datalogger?

[manumenzella]

Hey
Thanks so much for the links. There great because the dont have interrepts and they are not too complicated!!!
Yes youre right 20MHz is WAY too much for this project. I could use only 32KHz and bring down the current draw.
I am going to use an LM34 or LM35 to measure temperature, and I was thinking of shutting it down while im waiting to save power. How long before making the actual measurement do you think I should turn it on so that the sensor has stabilized???
Thanks
Manuel