i don't know the ht1380, but have done the same thing with a pcf8573
from philips, using a 6600uf cap. 
this chip can get vcc over a diode, and disables it's i2c interface on a
power fail (don't remember how this is detected).
therefore, it can be connected to the i2c as usual.
tino

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>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>On Behalf Of Mauricio Jancic
>Sent: Thursday, May 12, 2005 4:17 PM
>To: 'Microcontroller discussion list - Public.'
>Subject: [EE] RTCs and SUPERCAPs
>
>
>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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