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Valued Contributor III
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Cyclone III LS Vccbat high current draw

Hi all, 

 

I developed board using EP3CLS70U484C8 with battery on it as supply for stored key. Battery is small about 85mAh and is connected to VCCBAT thru DS1314 chip. 

Everything works just fine except one little thing. 

The current draw from battery. 

I expected current measured in nano amps. Not more than 100 - 200nA,but when I turn off main supply and everything goes off, current draw from battery is rising from nA to approx 20 - 30uA. 

This number is far from expected value. I already checked it is going to altera chip. 

I also took some x-ray photos to see shorcuts and others issues down there but soldering looks good and was done in proffessional lab. 

 

This is half of issue. 

The second half of issue is that stored key dissapears after few seconds from power off. 

When I turn on power supply again it is already gone. I can load it again and then chip works fine again. 

 

Is it another great anty tamper feature ? 

Is it chip damadge ? 

Is it mistake in design ? 

What can create such behaviour teoretically ? 

What can erase stored decryption key besides VCCBAT < MIN condition ? 

 

I am out of ideas. 

 

Best regards 

 

Adam
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