time trigger

qaxobi27b · Joined: 30 Apr 2011 Posts: 2

I would like to make a time delayed trigger..

something like a delay but the problem with the delay is that it stops all the program for the specified amount of time, i just want a to count during the program is running and after like 3 seconds i do that function. is there a way to do this?

note: I tried the get_timer() command but it counts up to 64K and resets more than once in a second.

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

This is usually done with timer interrupts. 3 seconds is a long time, so you will have to set a timer to interrupt say every 30ms and count 100 interrupts.
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qaxobi27b · Joined: 30 Apr 2011 Posts: 2

temtronic · Posted: Sat Apr 30, 2011 5:30 am

In the Code Library there's a Subject '...software based Real Time Clock..'(sorry , haven't figured how to paste the link...)

Works like a charm,currently I have it setup to trigger the Dallas DS18b20P temp sensors which need 750ms to 'do their thing' in the 'background'.
Having the ISR do the 'dirty work' allows 'Main' to do a LOT more than just sit in a 3/4 second delay loop,twiddling bits !

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Here's an example of how to make a timer interrupt every 10 ms.

pic_micro · Joined: 07 Feb 2011 Posts: 26

Can you explain how the 256-195 relate to 10ms?
Can you show the formulas

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The Timer is clocked at the instruction cycle rate, which is 1/4 of the PIC
oscillator frequency. So with a 20 MHz oscillator, the timer is clocked at 5 MHz.

We're using a pre-scaler of 256. So the instruction clock is divided by
256 before it gets to the Timer. The actual Timer clock is 5 MHz / 256 =
19531 Hz.

If we want the Timer to run at approximately 100 Hz, then we can see by
inspection that we need to divide 19531 by 195, which gives approximately
100 Hz.

5440 · Joined: 08 Feb 2009 Posts: 11

I recently made a similar system where I used a 1ms interupt, where it just increments a 16 bit counter. I wrote my own ISR to make it tight, took some time but got it done. I had an issue with the fast stack that threw me for a loop as I am using priority ints and had the retfie 1 and retfie 0 in wrong spots.

I then made two functions, one that is called when a new time value is required or must be called again say in blinking a LED. It looks at the current counter value (buffered) then adds the appropriate time while checking for counter overflow. The other function is called every scan which checks all the timers (configured as a timer array of type struct). I'm using about 10 of these timers for any time up to about 65 sec on an PIC18F @ 20MHZ. I use them to blink LEDs, debounce switches etc. The timer struct has control bits configured much like in an AB PLC 5 that are checked elsewhere to see if timer is done, still timing etc.

Working fine so far, just have to watch using it for very short time values less around the 1-5ms ranges as my current scan time is in the .8ms range.