[SI-LIST] Re: quenching ugly spikes?

Henrik,

You suspect the right things.

Your big problem is that you don't have much of anything for a defined=20
return path, so it is composed from your other signals.  You need a lot=20
more than two grounds at either end of a wide bus.  Why not borrow IDE133=20
cables with interleaved grounds.  Your signaling rates are similar to=20
theirs, and those connectors are plentiful and cheap.

Your signals are pretty slow, so you will only need to terminate if the=20
path is over abut 8cm.

Books that could help you immensely:

"High Speed Digital Design", Johnson and Graham
"Signal Integrity Simplified", Bogatin

Good luck.

Steve.

At 11:13 PM 10/27/2005 +0200, Henrik Gild wrote:
>Hi!
>
>As a hobby I'm designing an Ethernet/USB board for those Atari computer
>enthusiasts who are still active. My board sits on top of an accelerator
>board containing an MC68060 and a couple of CPLDs, connecting to my board
>through two 2.54mm (0.100") 2x25 pin connectors (one for data lines and one
>for address/control). My problem is that I get really ugly signals from the
>accelerator board. They are supposed to be 3.3V signals but I see spikes=
 all
>the way up to 6V on the address lines and the data lines start off with a
>small 2V top, then down to -1V and finally up to a 5.5V spike before
>settling at 3.3V after a total of 18ns. I assume that these symptoms are
>caused by two things:
>
>1) There are no series resistors on any lines on the accelerator board, and
>the data lines are as long as 22cm (about 8.5"). The rise times are
>somewhere in the 2-3ns region, so I suppose the lack of series termination
>is not a good thing. I have series resistors on my board and when my board
>drives the signals they look much nicer.
>2) There are only two ground pins at each end of both connectors, in total
>four per connector. The lack of nearby ground pins for the data and address
>lines isn't good either?
>
>What do you think I can do about this? I've tried terminating using diodes
>(though only 1N4148) but that didn't change anything. I'm thinking of using
>chokes on the lines. Could that be a good solution?
>
>Both my board and the accelerator board are four layer boards with=
 dedicated
>ground and power layers.
>
>I hope you can help me out! :)
>
>Thanks!
>/Henrik Gild=E5
>
>
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