This approach is indeed what I use for very long run-time requirements on
battery operated platforms.  I run the RTC unregulated off the battery and
shut everything off via a MOSFET, when the RTC alarms it brings the rest
of the board up via the MOSFET.

On Aug 26, 2012, at 7:43 AM, Isaac Marino Bavaresco wrote:

> 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.
>=20
> 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.
>=20
> Another problem with the RTC is that it is really imprecise. Any serious
> timekeeping must not rely on it.
>=20
> 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.
>=20
>=20
> Best regards,
>=20
> Isaac
>=20
>=20
>=20
> 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 abou=
t 3
>> days (all we need in terms of offline time).
>>=20
>> 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 =
to
>> actual time.=20
>>=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 b=
est
>> 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 :)
>>=20
>>=20
>>=20
>>=20
>> -----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
>>=20
>> Thanks, that's what we finally came to also:)
>>=20
>>> The reason is current consumption. The lithium batteries store around
>>> 240mAh and the RTC must run months to years without replacing batteries=
..
>>>=20
>>>=20
>>> Isaac
>>=20
>>=20
>>=20
>=20
> --=20
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