I can't help you with a backup circuit for 1380 but
you might want to look at the ds1302 which is
basically the same as the ds1307/ht1380 but also has
trickle charging support.  literally, all you do is
add the cap.  I'm using it with a .33F supercap and it
gets something like 600 hrs of backup.

And regarding my earlier question on the stability of
this chip in a solderless breadboard.  I took a little
more care in wiring of the circuit (mostly grounding
any unused rails) and now have a very stable circuit. 
With the current crystal and setup, I'm currently off
from the atomic clock (nist boulder) by about 3
seconds a day - 35 PPM.  A bit off from the claimed 20
PPM of the crystal but at least its consistant.  I
think the extra capacitance of the solderless BB is
partly to blame for the disparity.

Phil

Phil
--- Mauricio Jancic <jancic@ciudad.com.ar> wrote:
> Hi,
> 	I have a question for anyone out there. I'm going
> to use the HT1380
> as an RTC datasheet available at
> http://www.holtek.com.tw/pdf/consumer/1380_1.pdf
> 
> 	I was originally using the DS1307, but this one is
> cheaper. Now,
> instead of using a battery for backup I'll use a
> 0.1F x 5V supercap.
> 
> 	The circuit (the PIC connected to the RTC) is
> working at 5V, the RTC
> has an voltage range of 2~5V. 
> 
> 	How should I connect properly the SUPERCAP to the
> system in order to
> maintain the time in the RTC??
> 
> Best regards,
> 
> Mauricio
> 
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