Timer Questions

[rwskinner]

Wow, Maybe the question was phrased incorrectly.

I now have an interrupt being fired by timer1 at approx. 1 second. I had to keep guessing at the value I loaded into the timer until I got close to what I needed.

I would rather understand the formula for determining how to get the proper value to load into the timer. Can anyone shed some light on this?

How to take the clock speed, determine the prescaler value, and the counts that need to be preloaded in order for the timer to overflow and cause an interrupt.

[BigWumpus]

As an example:

Timer1 (16 Bit) is used and feed with:

I use an 4MHz resonator.
f_OSC/4 (this is the "normal" countrate für a PIC!)
Presacler: 1/8 (Set in T1CON or so)

Then I get 4.000.000 / 4 = 1.000.000
/ 8 = 125.000 counts per second

So, i can handle up to 0,5 seconds (62500 counts) in this 16-bit-timer.

I load the Timer1 with -62500 (=0BDCh), clear TMR1IF and start TIMER1.

When it reaches 0000, the TMR1IF is set, I will stop TIMER1, count down my WORD-variable in order to build the watchdog-timer, clear the TMR1IF and start TIMER1 again.

If you are using higher speeds, then the numbers must be changed.

It is quick-n-dirty !

[Bruce]

With an 8MHz osc, Timer1 will increment by 1 every 500nS with a prescaler
of 1:1.

It's a 16-bit timer so it can count from 0 to $FFFF or 0 to 65,535. Once it
reaches 65,535 and rolls-over back to 0, it will generate the interrupt if the
interrupt is enabled.

That means you have 65,536 Timer1 ticks before roll-over. 65,536 * 500nS
= 32.768mS before each Timer1 interrupt.

1 second/32.768mS = 30.517. Once Timer1 has over-flowed 30 times, you
know a 30 * 32.768mS (0.98304 seconds) period has expired.

If 0.98304 seconds is close enough to your "approximate" period of 1 second,
then you could do something simple like this;

Setup Timer1 with a 1:1 prescaler, clear Timer1, start the timer. Then count
each over-flow by monitoring the Timer1 interrupt flag bit. PIR1.0.

Just monitor PIR1.0. Once this flag bit is set, clear it, then increment your
variable;

IF PIR1.0 THEN
    PIR1.0 = 0
    MyVar = MyVar+1
ENDIF

If MyVar = 30 & no serial data has been received from your external device,
then reset the external device.

If the external device responds before MyVar = 30, then reset MyVar to 0 to
restart your countdown.

You can do this without interrupts by just watching the Timer1 interrupt flag
bit. This flag bit is automatically set each time Timer1 rolls-over, whether you
have the Timer1 interrupt enabled or not.

If you need more time, then just use the prescaler.

Example: Setup Timer1 prescaler to 1:8. This gives you 8 * 65,536 * 500nS
(8 * 32.768mS) for 0.262144 seconds before the flag bit is set. You would
then only need to increment your variable to 4 (4 * 0.262144 = 1.048576
seconds) for your approximate 1 second period.

You only need a 1 byte variable for the over-flow counter, your program
never gets interrupted while it's doing other things, and you'll be darn close
to your 1 second period before resetting the external device.

If you need better precision, you can let the timer free run and roll-over x
number of times until the roll-over count is just short of your required time
period.

After x number of roll-overs, stop the timer, and load it with a value that will
cause the last roll-over at the remaining time required for your 1 second
period.