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

[Reza Naima]

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. 

http://www.reza.net/misc/caps.jpg

Thanks!
Reza



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
> 64)
>
>  
>
> > I was also wondering, should the AREF and AVCC be attached to
> > something? 
> >    
>
> 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!
>
> Erik
>