DS1307 malfunction with external battery

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When external battery is connected on PIN3 of DS1307 micro controller reads all 
0xff data for all time and date.
When battery is removed it shows proper values without reseting contents.

Original issue reported on code.google.com by contact2...@gmail.com on 26 Apr 2011 at 7:43

• Merged into: #1

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Original comment by contact2...@gmail.com on 26 Apr 2011 at 7:45

• Changed state: Started

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On debugginf I found this is not a new issue but it is effect of Issue 1. 
Below text is copied from DS1307 datasheet

"When VCC  falls below 1.25 x VBAT  the device terminates an access in progress 
and resets the device address counter.  Inputs to the device will not be 
recognized at this time to prevent erroneous data from being written to the 
device from an out of
tolerance system.  When VCC  falls below VBAT  the device switches into a 
low-current battery backup mode.  Upon power-up, the device switches from 
battery to VCC when VCC  is greater than VBAT   + 0.2V and recognizes inputs 
when VCC is greater than 1.25 x VBAT."

Because of Issue1 VCC is around 3.8V and Vbat is 3.3 volt i.e. VCC<1.25 x VCC 
and hence DS1307 goes into battery backup mode in which read and write 
operation on device is prevented. As a result when microcontroller reads time 
and other values it gets 0xff. Fixing Issue 1 will automatically resolve this 
issue. Marking it as duplicate.

Original comment by contact2...@gmail.com on 27 Apr 2011 at 12:31

• Changed state: Duplicate