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Topic: Big capacitor near microcontroller - Circuit stops working (Read 3389 times) previous topic - next topic

brenin

Hello guys,

I have a circuit and when I put a big capacitor (330uF) near the atmega, the atmega gets stuck after running for a while. When I discharge the capacitor the circuit works again for a while and then stops again. On oscylloscope, i saw that the circuit stops working after the capacitor charges.

If I take that capacitor out, the circuit works perfectly.

I have an atmega168 running at 8mhz, with external crystal, and in the board there are also shift-registers and uln2803 drivers to control a LED matrix.

Does anyone knows what's happening? The circuit is working, as I took the capacitor out, but I'd like to know what happened so i don't make the same mistake in future projects.

Thanks for any help...

CrossRoads

What is the big cap connected to? From your parts listed, I can't see any reason for it to be across anything but power & ground.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

brenin

Exactly, it`s connected between power and ground... I forgot to mention, the capacitor is eletrolytic

CrossRoads

Electrolytic, so  it's polarized? Not installed backwards?
Got a schematic we can look at?
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

brenin

Already checked that... it's on the right direction...

CrossRoads

So, back to reply #3: Got a schematic we can look at?
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

brenin

Here's the schematic...

DATA_ROW, DATA_COL, etc, etc, goes to the shift registers... The board supports a rf, but it's not installed, so it's not the problem...

Actually, I don't even know why I put that capacitor there, but now i'd like to know what's happening

and hey... thanks for the reply... I wish we can solve that

westfw

Quote
On oscylloscope, i saw that the circuit stops working after the capacitor charges.

a 300uF cap directly across the power rails should charge pretty much instantly, right?

It almost sounds like you have the cap in between the AVR and one of the power rails.  Except I calculate the time would still be very short...

brenin

#8
Mar 18, 2011, 06:44 am Last Edit: Mar 18, 2011, 06:46 am by brenin Reason: 1
At AREF, it's just decoupling capacitor as in the datasheet... Anyway, I am not using the AD...

Maybe it's not the charging that makes it stop... I thought that was the reason, cause when I saw it discharged, the circuit worked again... it doesn't work too long by the way

Coding Badly

Quote
At AREF...

Sorry I didn't get my post removed in time.  Please pretend you never saw it.   :smiley-red:

CrossRoads

Surface mount part eh?

Are you sure the cap is connected to where you think it is?  What does the cap charge up to when it stops working?
Once VCC is over ~3.9 volts, the processor should just run, and if it continues to  5V it should not make a difference.

If the power supply has enough current, the cap should charge up almost immediately if it is actually connected to VCC.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

brenin

It charges pretty fast...

the atmega, the 100n caps and the shift registers are smd... the capacitor itself is pth

I'm starting to think that the behavior after I discharge the capacitor is just a coincidence, and it's charging is not actually the reason of the problem...

BenF

There appears to be a number of issues with your schematic/wiring. For starters I notice the following:


- AREF should be pin 21
- Avcc should be pin 20
- Gnd should be 8 and 22
....

You better check them all and match the corrected schematic to your wiring.

CrossRoads

The pinouts are correct for a 32-pin quad flat pack part.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

brenin

CrossRoads is right... it smd part, so the pins are a bit different from the regular dip part...

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