The object of our exercise was to remove the RTC chip and crystal from the
circuit to reduce cost, unfortunately this wasn't one of the ways.

Our circuit doesn't need that close timing, as we're interested in the
passage of time units of about 30 minutes duration.



-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Isaac Marino Bavaresco
Sent: 26 August 2012 12:43
To: Microcontroller discussion list - Public.
Subject: Re: [EE] 32kHz RTC

The problem is that the PIC's WDT consumes a lot of current.
The PIC core alone in sleep consumes some 10's os nA. More than a
low-power RTC though.

To achieve really low power with PICs, it must be put to sleep with all
its peripherals off, except for the interrupt pin and interrupt-on-change.
The WDT, BOD, Comparator, etc. consume orders of magnitude more current
than the core itself.

Another problem with the RTC is that it is really imprecise. Any serious
timekeeping must not rely on it.

One solution would be using an RTC with a programmed alarm to wake up
the PIC at programmed intervals. The PIC then re-enables the necessary
peripherals, does its job, program a new alarm date in the RTC, turns
off all its peripheral and sleeps again.


Best regards,

Isaac



Em 25/8/2012 18:51, Jim Franklin escreveu:
> We run our RTC chip at 32KHz with a 0.1F supercap and that runs for about
3
> days (all we need in terms of offline time).
>
> Our idea was to have a sleeping PIC, and the WDT wakes it up every few
> minutes to keep the time ticks going - we need relative time as opposed t=
o
> actual time.=20
>
> Unfortunately in our application, we cannot use batteries so I have
> experimented with keeping a PIC s/w RTC running on a supercap, but the
best
> I could manage with 1F was about 12Hours - even in sleep the PIC takes a
> hearty current (17uA). For reference the same circuit on a battery would
> last about 468 years :)
>
>
>
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
Of
> KPL
> Sent: 24 August 2012 20:59
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] 32kHz RTC
>
> Thanks, that's what we finally came to also:)
>
>> The reason is current consumption. The lithium batteries store around
>> 240mAh and the RTC must run months to years without replacing batteries.
>>
>>
>> Isaac
>
>
>

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