James,

This is a real common thing to try - there should be lots of good examples
on PIClist.com.

90% of my projects run with this architecture:

I will usually have a timer interrupt that goes off some fraction of a
second, and count them.  Say 0.1 seconds.
Every 10 interrupts I will set a flag, or else increment a variable called
"secondscounter".  The timer interrupt does little else but keep track of
time.  I usually code in C, which has a bit of overhead at each interrupt,
so I will often hand-calibrate a timer interrupt in a simulator or in a
real PIC to make sure the timer interrupt is accurate enough.

The rest of the application simply runs in a superloop.  If a subroutine
needs to know what time it is, secondscounter is always available.  Say it
is a timer for a baking cycle.  The user presses a button to start a
baking cycle, secondscounter is set to zero.  Every time through the
superloop, the bake subroutine looks at the time and decides if it is time
to shut off yet.

I never need to know timing closer than one second.  This architecture
would be inaccurate for sub-second timing and useless for millisecond
timing.  Probably also inaccurate for timing longer than 24 hours.  But it
is great for keeping track of a few minutes with one second accuracy.

BTW, what kind of projects are you designing?  From your email I assume it
is with a toy manufacturer?


-- Lawrence Lile





"M. Adam Davis" <adampic@UBASICS.COM>
Sent by: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
05/20/2004 03:18 PM
Please respond to pic microcontroller discussion list


        To:     PICLIST@MITVMA.MIT.EDU
        cc:
        Subject:        Re: [PIC:]  Timing intervals (in the range of minutes)


It depends on how accurate the intervals need to be.  Generally, though,
I'd use one of the 16bit timers with prescaler and a slow 32kHz
crystal.  It needs to have an interrupt which will wake the processor
from sleep.

-Adam

James Tu wrote:

> What is the best way to use the PIC to time intervals.  I basically
> want to
> create a state machine and want it to be partially driven by time and
> partially driven by user input.
>
>
> James Tu
> james@2-bit-toys.com
>
> --
> http://www.piclist.com hint: To leave the PICList
> mailto:piclist-unsubscribe-request@mitvma.mit.edu
>
>
>

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu



--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads