CCP1 rollover

[dan king]

I'm using CCP1 tied to an IR RX/TX pair to measure the speed of an object passing by (18F2553). I've never used the CCP module before but I do have working code modified from the code library that works but my issue is, how do I handle the pulse count delta when an object passes by that is slower than the 16 bit timer can store? I have TIMER1 set to the slowest count rate (div_8). Since the delta is generated by the difference of the start and stop edges, I can't even monitor if TIMER1 rolled over since the start edge may have been near 65535 at the starting (falling in my case) edge.

Any insight is appreciated.

Thanks,

Dan

[Ttelmah]

The CCP doesn't roll over. The timer does.
When the timer rolls over, it can generate an interrupt.
Increment a counter in the interrupt for the timer.
Then when the CCP interrupts, you can use a 32 bit value from the CCP (low 16bits) combined with the counter from the timer interrupt (make32, or combine the values using a union).
Just make sure the timer interrupt has higer priority than the CCP one. The CCP 'holds' the logged time, so can be read at any point until the next edge.

[dan king]

Would that require that the initial TIMER1 value be 0 at the instant the starting CCP interrupt occurred? The code sample I'm using allows TIMER1 to free run and the ISR captures the start and stop values from TIMER1 so the first TIMER1 interrupt may not indicate an overflow for the first time through. Can I set_TIMER1(0) at the start of a capture in the CCP ISR?


I will generate a small test program to show my code shortly.

I understand the CCP doesn't roll over, poor word choice on my part.

Thanks for the fast response.

[dan king]

Excellent, that works. Thanks for the guidance.

Dan

[Ttelmah]

Good.

No, you don't want to set the timer to zero in the first ISR (actually this is OK for slow signals, but otherwise can cause problems, since it takes time to get into the ISR). All of these approaches are best done by working holding the 'last' and 'current' values, and then calculating using the 'delta' between these.

Though you have got it working, it is worth looking at an approach like:

[dan king]

I just used the principle of your example extending the timing to 32 bits and the results are excellent. That makes much better sense in that I don't need to worry about the state of the timer1 overflow and just let everything run. Thanks again for the help.

Dan