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Author Topic: bizarre problem with atmega8 + ds1307  (Read 1656 times)
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deveshsamaiya
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« on: August 21, 2012, 07:51:59 19:51 »

Friends, i am facing this bizarre problem with my circuit. I have made a simple circuit connections to interface DS1307 time keeper chip with  Mega8 its working fine when i am providing power from USB, but it is not giving expected results when i power it up using a smps wall socket power adapter. Any one having any idea what could be the reason?
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pickit2
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« Reply #1 on: August 21, 2012, 09:15:38 21:15 »

wall wart have a usb plug?
ic in debug mode?
home built pcb, if so show how power in, connects to ic.
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Codeman
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« Reply #2 on: August 22, 2012, 01:56:26 01:56 »

check the ripple of the wall adapter with a scope. Are you using 100nf decoupling caps?
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medik
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« Reply #3 on: August 22, 2012, 05:02:09 05:02 »

the problem is from the smps power supply. you need to use  capacitors say 16v 100uf and 0.1uf at the MCU VDD and VSS pins. you may put 25v 1000uf at the voltage regulator/smps end. I had such encounter sometime ago and the capacitors worked.
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sakbal
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« Reply #4 on: August 25, 2012, 10:23:14 22:23 »

Did you decouple the 5V supply to the DS1307 with a 100nF cap. (As close as possible to the DS1307)?
SMPS wall power adapters, especially the cheap ones, are renown for their high ripple noise.
 
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solutions
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« Reply #5 on: August 26, 2012, 01:08:21 01:08 »

Without a schematic, and wall wart specs, all of the above posts are rules of thumb, but may be useless.

You could, for instance, have a ground loop if the adapter is not isolated. It could also be pulling too much current from the adapter while the USB is a lower impedance source. It could also...well, you get my (and Pickit2's) point by now.

Keeping a circuit secret that doesn't work is pointless. You can black out your other stuff or just use the Windows7 Snipping Tool to grab the relevant stuff to post here as a pic.
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