yet another PIC timer question

Hello, I discovered these boards earlier this week and found it very useful, so thanks to everyone already!

I am having a hard time understanding timer1 for a PIC18F4520. I am running an 8MHz oscillator and I want to have it interrupt every 10 ms.

My understanding of this is that the timer would increment the counter (with a 1:8 prescaler) every 4uS:

Period = (1/(8MHz/4) x 8) =4uS

Using it in 16 Bit mode, the counter will count is 65,536 times before it reaches overflow, so I would be getting an interrupt every 262.144mS.

To get a 10 mS interrupt I only want the counter to count 10mS/4uS = 2500 times.

Do I just subtract 2500 from 65536 to get 63306 and place it in the TIMER1H and TIMER1L registers in order to get my required delay?

Thank you in advance.

Hi and Welcome,

Yes thats basically what you do, though have not checked your maths.

In your Main code you preload TMR1H, and L if you are particular, then start TMR1 and the Interrupts.

In the ISR, after testing if TMR1IF is now set , you preload TMR1H / L again then Clear TMR1 IF.

Dont forget that on entering the ISR, global interrupts are automatically turned off and turn on again with the Retfie, so don't spend too much time in your ISR or you might miss another TMR1 IF

Basically yes, thats what you do. The interrupt is generated as the timer counts from 65535 to 65536 (which is effectively zero because you run out of bits to count with). So in order to create a delay you use the number of counts it takes to reach that point You might have to take into account the time to execute code to initially load the counter and to respond to the interrupt to get exact timing.

By my calculation, excluding extra execution times:
with prescaler 1:8 use 63036 (not 63306 as you stated)
with prescaler 1:4 use 60536
with prescaler 1:2 use 55536
with no prescaler (1:1) use 45536

If your delay is repeating, it's a good idea to reload TMR1H first so there is less chance of another interrupt occuring while you are loading TMR1L although disabling the timer while you load it is safest.

Brian.

great thank you!

I am unable to test the code right now, which is why I was asking. The timing doesn't need to be exact since this is just for toggling the clock for a stepper motor.

And thanks for catching my typo, luckily it didn't make it's way into my conversion to hex.

This is my resulting code so it seems all will be well.

Looks good to me.

Brian.