I use the watchdog.  Set it for max delay (around 2.3 seconds nominal) and
go to SLEEP.  The watchdog will wake you up.  In mine I do two consecutive
SLEEPs, so I get a new reading a little more than once every 5 seconds.

Of course this won't do you a lot of good if you need to log anything
resembling an accurate timestamp, but I have gotten several months out of
three AA cells this way.  Mine was a 16F84 and DS1620, by the way - I
think a 1620 would be fine with a slow clock, and it's only two more pins
than an 18S20 and simpler software.

Dale
--
"Curiosity is the very basis of education and if you tell me that
curiosity killed the cat, I say only the cat died nobly."
          - Arnold Edinborough


On Thu, 7 Feb 2002, marco genovesi wrote:

> Hi,
> my 16F84 -based datalogger (16F84 running at 4MHz with a DS1820) have a
> current
> consumption of about 0.7 ma  at 4.5V, 3AA cells (when sensor is off).
> I'm not sure, but It is at least 1 month of continuous use without excessive
> discharge of the cells.
> This is enough for my most uses but not for all...
> For long-time measuremets I would put the logger in "sleep": at this time I
> have two (or more?) ways:
>
> - Use a 32 Khz crystal clock.
> - Use an external circuit ( a 4060 CMOS  with 32 Khz crystal ) that send an
> interrupt to wake-up the PIC.
>
> The first solution isn't valid for DS1820 (timings, i think) but in this
> event I can change with a DS1721.
> I must also modify all the code delays, but this is obvious, but the board
> is more compact.
> The second is more complex to assemble but is still cheap and the pic's
> clock remains 4Mhz.
>
> My questions:
>
> 1) Is 32 Khz solution too limited if used for other types of measure?
> ( for example, i want use other sensors with frequency output )
> 2) Are there other CHEAP solutions for low current consumption?

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