Pic MCU - Calculating Timer0 Overflow Rate

[kolbep]

Good Day.
I know there is a really simple answer to this one. But the flu has hit me to an extent where my brain does not want to do much.
What I am trying to do is work out how many times in 1 second Timer0 will generate an overflow interrupt, if the Osc is running at 16Mhz.
My intent is to make a phase angle dimmer, and I would like to calculate the time to fire the triac using  timer interrupts. But I am stumped.
What is the correct formula using:
Osc Frequency, Prescaler, Counter Bits (8/16), to work out the overlflows per second.

Here is how far I got, until I got stumped :

Thanks.
P

[jucole]

I think with most pics you need to divide the crystal clock by 4 to get the internal clock;  so - 16Mhz gives you an internal 4 Mhz "fosc/4" clock;  also I think the counter overflow occurs on the next clock cycle so in an 8 bit counter example it would appear to delay for 256 cycles, and then jump to your ISR.

[jc101]

Pop over to MikroE and download their free tool...

http://www.mikroe.com/timer-calculator/

...very handy.

[kolbep]

thnks
the calculator is exactly what i needed

[miguelvp]

I know you have the calculator now, but for overflow you should use 256 instead of 255 and 65536 instead of 65535, since those are the number of values your counter will have until the next iteration.

[dannyf]

What I am trying to do is work out how many times in 1 second Timer0 will generate an overflow interrupt

Another way to think of it is to calculate the intervals between interrupts. 16Mhz crystal means 4Mhz cpu frequency on the pic. each tick is 0.25us. At 1:2 prescaler and 8-bit counter (assuming no reloading), that's 0.25us * 2 * 256 = 128us. or 1000000us / 128us ~= 8000 times per second.

I don't know what your application is trying to do but as you can see, sometimes it yields fractional counts -> an impossibility. And over time, such fractional counts can lead to timing inaccuracy.

There are ways around that, if long-term timing accuracy is an issue for your application.