Counting Time Without Interrupts

rhaguiuda · Joined: 05 Sep 2007 Posts: 46 Location: Londrina - Brazil

How is the best way to count time, without using timers or delays?

For example, if I need a 1ms delay?

Thanks.
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Ttelmah · Guest

At the end of the day, there are really three ways of counting time:

1) Count machine cycles. This is what the CCS 'delay' uses.
2) Count with an internal hardware timer. You can either use interrupts with this, or just poll it.
3) Count externally. For this, you need hardware other than the PIC (most RTC chips support 'alarms' at programmed intervals or times).

Now, if you have actually ruled out '1' and '2', you are left with adding hardware. This can range from using (for example), the charge time on a capacitor, through using a digital counter of some sort.

There are some 'odd' solutions that might apply, but at heart they are all based on one of the above. For instance, if you program the UART, to 100bps, and then send a character, the transmitter buffer will go empty after 1mSec. However this is just a variant of '2', but using a counter that is not normally targetted at 'timing'.

Best Wishes

rhaguiuda · Joined: 05 Sep 2007 Posts: 46 Location: Londrina - Brazil

Ttelmah, thanks for repply.

I liked the second suggestion.
So I can use the timer1 counter, for example, to count the time I need. All I have to do is make sure it won't generate interrupts (by not allowing it to overflow).

Thanks again!
_________________
Give a man a fish and you'll feed him for a day. Teach a man to fish, and you'll feed him for a lifetime.

Ttelmah · Guest

You can let it overflow.

The timers when they overflow, set _interrupt flags_. An 'interrupt' (actual call to an interrupt handler), _requires_ three things to be true, before it'll occur:
1) A device interrupt flag being set.
2) It's interrupt enable being set.
3) The global interrupt enable being set.

Now, the timer hardware sets '1', but it is up to you to set '2',and '3'. Assuming you are using interrupts on other devices, '3' will be set, but provided you ensure '2' is not set, no actual interrupt will occur. Indeed you can use the interrupt flag 'bit', as if this was an extra bit of the hardware timer counter, giving you potentially 17bit of counting, (for a 16bit timer).

Also, if you want, you can use the interrupt flag, as a hardware 'marker' to say that a particular time has been reached. Soo (for instance):

John P · Joined: 17 Sep 2003 Posts: 331

Quite correct, but note that if you use an interrupt, the flag that sends you to the interrupt (tmr3IF, or whatever) gets cleared automatically, so you won't e-enter the interrupt. If you just monitor the flag in software, it's your code that has to clear the flag! Failing to do this means that the program will keep responding to the flag.