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

From: Mattias Svensson
Subject: Re: [avr-gcc-list] supply pins on atmega32
Date: Wed, 15 Sep 2004 14:27:37 +0200
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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.

Indeed they are common, but each pin on the outside is likely connected to a different pad on the chip inside. Not connecting some could lead to different VCC levels on the chip. To get the best result all VCC pins should be connected.

For testing it could work though... not sure of the result.
Let us know how it goes!

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

Yes, and IMHO ground bounce is not depending on the actual clock speed the chip is running on, but the load that is switched. The clock speed only controls how often the bounces will occur.

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.

If you are using the ADC that uses AVCC, it should be connected through a RC filter to VCC. See the data sheet for "noise cancelling techniques". If you are not using the ADC, but only the digital port you can connect AVCC directly to VCC.

/Mattias S

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