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Re: [avr-gcc-list] supply pins on atmega32

From: Reza Naima
Subject: Re: [avr-gcc-list] supply pins on atmega32
Date: Wed, 15 Sep 2004 03:38:24 -0700
User-agent: Mozilla Thunderbird 0.7 (Windows/20040616)

Thanks for all the prompt answers. I was able to cut one of the traces I had that was bridging a VCC and GND pin -- which leaves only one VCC pin, the AVCC pin, and one GND pin connected -- and it seems to work. Well, I'm able to program it, and have it spit stuff out via the UART. I'll fix it up in the "final" version, however.

One other question regarding bypass caps -- So, as there are several VCC pins, should there be multiple bypass caps so that they are all close to the relative pins - or would it be sufficient to put them all next one one another, and just run traces between all the VCC pins? The TQFP IC is only 11mm square, so it seems like a bit of an overkill to put the bypass caps all around (if distance is the important factor).

I'm also expecting some lower frequency EMF noise -- like around 1-10khz, so I'm using a .1 and 1 uF caps. I feel like this is a bit of "black magic" as I've not found any references which correlates the capacitance of a bypass cap to the frequency range in which it is effective. I'm including a link to an image of the PCB w/ the bypass caps and xtal placement. I would love some criticism as how to improve the layout w.r.t. the caps. C1 is 1uF and C11 is 0.1uF. Ignore C5.


Erik Christiansen wrote:

On Tue, Sep 14, 2004 at 07:07:13PM -0700, Reza Naima wrote:
First off, the atmega32 has 3 VCC pins, and 4 GND pins. Do they all have to be supplied for the circuit to work, or can I get by with just one?

The data sheet refers to just one VCC in the pin description, so I'd bet
they're common. A continuity check, with no other pins connected, and
using less than 600 mV, should resolve any doubts, though. I'd still
connect and bypass each, especially if you're running at high speed.

Quadrupling ground impedance, by connecting only one ground pin is
probably asking for trouble. At 16 MHz, the flanks of output signals are
likely to generate internal ground bounce, especially on capactive loads.

At least the 32 doesn't seem to have the external bus. I've seen VCC
spiking to +7V, when driving a few inches of bus at 16 MHz, without
adequate bypassing (on all VCC pins) or series bus termination. (On the

I was also wondering, should the AREF and AVCC be attached to

My recollection is that AVCC needs to be within 300 mV of VCC, so I'd
play safe. The data sheet insists that AREF not be connected when the
internal reference is used (IIRC), so there's no worry there.

Good luck!


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