[SI-LIST] Re: quenching ugly spikes?

My guess is that you are having problems because of heavy ringing and =
possibly crosstalk (especially on those data lines +2 -> -1V -> +5V?? =
That's a lot of swinging). If you interface's timing is flexible (can =
you work with 10-15ns delays?)I would put some large series resistor =
(70-100ohms) at the drivers so that the voltage swings going down the =
interconnects are as small as possible reducing the amplitude of both =
reflections and crosstalk noise.

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On Behalf Of Henrik Gild
Sent: Thursday, October 27, 2005 2:14 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] quenching ugly spikes?

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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