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External RTC or External 32 khz on OSC pins

 
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Eugeneo



Joined: 30 Aug 2005
Posts: 155
Location: Calgary, AB

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External RTC or External 32 khz on OSC pins
PostPosted: Fri Sep 29, 2006 3:04 am     Reply with quote
The 18LF2525 has a low power OSC circuit that will run up on timer1 and overflow. After looking at the datasheets, I would guess a complete power draw of about 40-60 ua (including a 15ua quansient regulator current). The system has 2 sources of power, a 20 mm watch battery and 4 AAs.
Provided the system will not power up from sleep if the AAs drop below a certain voltage, my question is, how long will the watch battery last?
Right now its configured with a DS1302 (only 500 na) with no indication of depletion on the watch battery.

I'm leaning towards thinking that this is just more trouble than it's worth after I make the conversion.

Also after looking at the code example in the datasheet, I've noticed the assemby code pre-loads the timer with a 32768 right after the interrupt. With PCH the preloaded value will be more but will it remain constant?

Thanks in advance
PCM programmer



Joined: 06 Sep 2003
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PostPosted: Fri Sep 29, 2006 12:16 pm     Reply with quote
Look at a data sheet for an AA battery. Here's one for the Energizer E91:
http://www.saatcioglu.com.tr/pdf/fotograf/piller/e91.pdf
At the top right, it says:
Quote:
Average Capacity: 2,850 mAh (to 0.8 volts)

Then look at the graph in the lower right corner, labeled:
Quote:
CONSTANT CURRENT DISCHARGE

It shows that for a 1 ma discharge rate, it could really last about 2850
hours, until it got down to 0.8v volts. I don't know what your final
cutoff voltage is. It may not be 0.8v per cell. It may be 1.2v.
But you could extrapolate your usage from that graph.

The reason I gave a link to the data sheet from that website, is because
it's about the only site that has the 1102 "H" version of the E91 data sheet.
All the battery companies have "dumbed down" their data sheets a lot,
over the last couple of years. The graphs don't go down to 1 ma
anymore. The lowest current drain they show is 10 ma. So this older
data sheet is much better.
Eugeneo



Joined: 30 Aug 2005
Posts: 155
Location: Calgary, AB

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PostPosted: Fri Sep 29, 2006 5:23 pm     Reply with quote
Thanks for the info PCM.
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