Using TMR0 to keep time

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- 23rd November 2011, 03:46
BraveZero

Using TMR0 to keep time

I'm trying to use TMR0 for one second periods, but can't seem to get it. I'm using a PIC16F887 with a 4MHz clock. From what I understand you take (((4MHz / 4) / 256) / prescaler) which will give you the total overflows per second. I know that it won't be an exact second every time it advances, but it should correct itself over time. I'm using a prescaler of two and this is what I have so far:

(1MHz / 256) / 2 = 1953.125
1953.125 * 8 = 15625

Code:

While (INTCON.2 == 0) : WEND INTCON.2 = 0 Time = Time + 8 if (Time > 15625) Then Time = Time - 15625 gosub Second endif

Is there something wrong with what I did? I can't seem to get the clock to be very accurate.
- 23rd November 2011, 04:12
prstein

Re: Using TMR0 to keep time

Download and use Mister_E's PicMultiCalc from http://www.picbasic.co.uk/forum/cont....-PICMultiCalc.

It totally rocks and will calculate the settings you want.

Best Regards,
Paul
- 23rd November 2011, 05:25
BraveZero

Re: Using TMR0 to keep time

Quote:

Originally Posted by prstein

Download and use Mister_E's PicMultiCalc from http://www.picbasic.co.uk/forum/cont....-PICMultiCalc.

It totally rocks and will calculate the settings you want.

Best Regards,
Paul

Alright, so the number I got for each interrupt was 512 uS. With the program I got 513-527 uS. I realized I didn't include the instruction cycle in the equation which adds on a uS per cycle, any idea on how I could figure out the instruction cycle number?