Simple PIC based timer needed, any advice appreciated.

[Mr Chris]

Im new to this forum so hello everyone! Sorry if this is posted in the wrong place.

I need a simple timer that will pulse for 2 seconds once in every (approx) 24 hours. Physical size must be small as it is to be fitted into existing equipment. Hence why I thought of a PIC solution. 5 volts available and it would need to trigger a miniture relay. Permanently connected to power so would need to reset and then wait a further 24 hours and send a pin high for 2 seconds again.

I know its a simple request but Ive had no luck in finding anything on the web, so if anyone could point me in the right direction that would be very much appreciated!

Chris

[ScaleRobotics]

You could use instant interrupts elapsed timer example. Then look for the flag daychange. Check out this example for 24 hour timer: http://darreltaylor.com/DT_INTS-18/elapsed.html Keep an eye on DaysChange, and you should be able to make it happen.

[Mike, K8LH]

Hi Chris,

Every 24 hours starting when? Starting with power-up?

How precise for the 24-hour interval? Is ±10 secs / day from internal oscillator ok? Or, do you need crystal precision?

Regards, Mike

[Mr Chris]

Thanks for replies.
I need this to force a hard reset on some telematics equipment. The devices do have built-in timers that can be used for a variety of events, including a system reset. However in practice they fail after a couple of months. This is why we need to have a completely independent timer that we can rely on to do its job. Like I say it would be permanently connected to 5 volts, so would need to start timing on power-up. Also after its 2 second 'on' period, restart again. The 24 hours isnt critical, just an ideal figure, once a day. If this fluctuated and became 22 hours or 26 hours, or a mixture it wouldnt ever be a problem. Neither is the 'on' time 2-5 seconds is fine.
Thanks again.
Chris

[mackrackit]

Quick and dirty

    CNT_1 VAR BYTE
    CNT VAR WORD
    HIGH PORTC.4
    PAUSE 2000
    low PORTC.4
    PAUSE 2000
START:
    FOR CNT_1 = 0 TO 24
    FOR CNT = 0 TO 600
    HIGH PORTC.4
    PAUSE 6000
    NEXT CNT
    NEXT CNT_1
    LOW PORTC.4 
    PAUSE 5000
    GOTO START

[amgen]

the devices with the 8 mhz int osc are about 1%, (12f283 for example) pretty good timing.
i would blink an led at 1 sec and use the 1 second as a time base. the led just shows that your ckt is doing something.
there are 86400 seconds/day but a var word is only 16 bit - max 65535 count so use 2 second count inc up to 43200.

start:
while count < 43200
high led
pause 1000
low led
pause 1000
count=count+1
end while
outputRSTpin=1
pause 2000
outputRSTpin=0
count=0
goto start

don f

[Mike, K8LH]

Chris,

Would you like to try it on a 55 cent 6-pin 10F200? I just simulated a 63 word program with "cycle accurate" 23:59:58 "off" time and 00:00:02 "on" time. Accuracy will be as good as you can get with the internal oscillator.

Regards, Mike

[Mr Chris]

Mike, that looks interesting. I would like to try that. Using this type of chip how will it perform months down the line. eg in 7 months will I still be getting a 2 seconds 'on' once in 24 hours? What I suppose Im trying to ask is, will it gradually lose time or make time or fluctuate +/- so keepinmg an average of 24 hours?

thanks again

[Mike, K8LH]

Hi Chris,

The example I wrote will maintain a 24 hour cycle as long as it's powered up but the internal oscillator only has a 1% tolerance at room temperature so it will drift. Over a period of several months you might be an hour off from where you started but you should still be getting an interval very close to 24 hours. If that's a problem then you probably should use an 8-pin PIC (12F629, 12F617, 12F635, 12F675, 12F683) and a crystal which should get you accuracy to within a minute or so per year.

Cheerful regards, Mike

[mackrackit]

Mike,
Do you plan on posting the code so Chris can use it or just continue talking about it?

[Dagwood]

Hi All,Very new to Pic Programming and very new to your forum. I found this thread searching Google for ideas on progrmming a PIC on a 24hour cycle. Ok the problem, I have a JDM programmer and when I try to write to a 10F200 PIC all I get is, [IMG]file:///C:/DOCUME~1/BENSHE~1/LOCALS~1/Temp/msoclip1/01/clip_image001.png[/IMG]I have tried running it from the ZIF sockets and the off board connectors????Am stuck now??? Any ideasCheersDagwood

[Demon]

Moved from Schematics.

Robert