long countdown timer, how to save power?

[ardhuru]

Easiest thing in my mind is to get an 8pin RTC, DS1602 type and use that. Then you can put the PIC to sleep and have it wake itself up every roughly XX number of seconds using the WDT timer and the WDT prescaler.

I think this is absolutely the easiest, perhaps the most accurate way of doing it too, especially since you need the LCD to light up only on manual intervention. I have not used the DS1602, but I have used the Philips PCF 8563, and the current draw is almost immeasurable.

Regards,

Anand

[Jerson]

I do not agree with using an RTC unless your project dictates it. The option of using the WDT(watchdog timer) to wake you from sleep, keep time and sleep again is the best. I use it in my battery powered security system. Use the Prescalar and WDT to decide at what interval you want to wake.

Of course, you will need to rewrite your code if you have written the timing loops using pause.

[Acetronics2]

Hi,

Find here an asm example that counts down ( 180 days ! ) ... !

sooo simple !

Alain

[Kamikaze47]

I do not agree with using an RTC unless your project dictates it. The option of using the WDT(watchdog timer) to wake you from sleep, keep time and sleep again is the best. I use it in my battery powered security system. Use the Prescalar and WDT to decide at what interval you want to wake.

Of course, you will need to rewrite your code if you have written the timing loops using pause.

At the moment my code has an interrupt every 1 second using a timer and keeps track that way. I assume that the timer does not continue while in sleep mode, so when you say "wake you from sleep, keep time and sleep again", how does it keep time? Just by adding a constant to a timer every time it wakes up? How accurate would you think that would be over a longer period of time?

I'm hoping to get an accuracy of +/- 1 min in 30 days.

[Darrel Taylor]

The Timer1 oscillator will continue running while the chip is in sleep mode.
With a 32768hz crystal, you'll get an interrupt every 2 seconds. (More accurate than the main crystal)

When the PIC wakes up (at the normal OSC speed), you simply update the time then go back to sleep. Batterys will last months.

You don't actually need to use interrupts. If the GIE bit is clear, it will just resume where it left off, so it never leaves the main loop.

DT

[Kamikaze47]

The Timer1 oscillator will continue running while the chip is in sleep mode.
With a 32768hz crystal, you'll get an interrupt every 2 seconds. (More accurate than the main crystal)

When the PIC wakes up (at the normal OSC speed), you simply update the time then go back to sleep. Batterys will last months.

You don't actually need to use interrupts. If the GIE bit is clear, it will just resume where it left off, so it never leaves the main loop.

DT

How would you keep track of the time without using interrupts?

This is the way I was thinking:

Timer1 with 32.768kHz OSC who's interrupt routine increments the time variables.

main:
[ code to check time variables, update LCD, etc ]
NAP
goto main

And have the WDT off so that it only wakes up when the Timer 1 interrupt triggers.

Does that sounds like it should work?

[skimask]

Does that sounds like it should work?

Yep, that's one good reason why Timer1 works the way it does.
There's a good explanation on DS39605F-page 107 of the datasheet along with some assembly code on how it all works.

EDIT: adding a thought... I don't know what would take more power to run and/or be more accurate (a trade off here)...
A self powered RTC with it's own internal clock that you read once in a great while with a PIC that's sleeping most of the time......OR.....
A PIC that's woken up X times per second (or less) to update a number internally and put back to sleep...

[Kamikaze47]

Any idea what Darrel was talking about when he was saying it could be done without using an interrupt?