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 to
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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KPL
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