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

From: Andy Warner
Subject: Re: [avr-gcc-list] supply pins on atmega32
Date: Wed, 15 Sep 2004 08:34:18 -0500
User-agent: Mutt/1.2.5i

Reza Naima wrote:
> [...]
> 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).

You should decouple each pair of power/ground pins as close
to the package as possible. You should also make sure the power
and ground PCB traces are as direct as possible, and perhaps
even wider than default signal nets. The reasoning behind
this is that chips like the AVR really only internally draw
current when they are switching something between states,
when they need that current, they need it fast, and they
need a lot of it (compared to when nothing is switching.)
To make sure that the voltages inside the chip don't sag when
this current is suddenly required, you need to provide a
low-impedance supply. This means no long, thin power traces
and distributed decoupling. Even though capacitors in parallel
add up, one large capacitor is not the same as several smaller
ones distributed where they are most needed. If you're
re-spinning the board anyway, pay close attention to the
power nets if you're letting the autorouter do the work.

If you don't do this, the board may work fine in most
situations, maybe even all situations. However the board
is likely to start misbehaving in erratic and unpredicatable
ways, which *will* drive you nuts. Ask anyone who's been through

Since you've already cut traces to get your board to work, I'd
recommend jumpering the power/ground supply around the chip
with rework/wirewrap wire, to connect all the Vcc/Gnd pins.
Lift pins if you need to and solder directly to the pin.

A lot also depends on what you're driving with I/O pins, if you're
sourcing 20mA from I/O pins on the other side of the chip from
the power pin(s) you have connected, you're asking for trouble.

Andy Warner             Voice: (612) 801-8549   Fax: (208) 575-5634

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