Re: [SI-LIST] : [SI-LIST]:Drivers to minimize reflections

Scott McMorrow (scott@vasthorizons.com)
Wed, 08 Sep 1999 12:21:13 -0700

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Bobby,

Definitely use as slow an edge rate device as you possibly
can and then place an RC or just C on the output to roll off the edge
rate to 5 to 10 times the delay to the device farthest away.
Then transmission line effects will not be an issue. Somewhere
on the order of a 10ns rise time will do the job quite well for your
board. Distribute the loads across the routed net and you will
be able to route the pretty much any way that you'd like.

Since the rise time is fairly slow, make sure that these lines are
well seperated from other lines that might cause crosstalk.
I'd suggest a 3x track width spacing.

regards,

scott

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

Bobby Hubertus wrote:

> Dear SI gurus, > I am designing a board (approx. 6"x8.5") that has a number of buffered > signals. The critical signals have a relatively high fanout (up to about > 20) that go all over the board. My goal is to eliminate > reflections/glitches in these signals which could cause double-clocking. > Is my best approach to look for a slow edge rate device (not incredibly > fast signals here, i.e. 2MHz clock lines) to allow me to treat the loads as > a lumped-load which doesn't require terminations? Or am I better off > splitting the line and terminating each signal? If I split it enough, I > can do point-to-point with series terminations on each line, but this will > use more board space. Or I can do it point-to-multipoint, grouping the > loads that are physically near each other and adding an RC termination at > the end of each group. > > BTW, signal levels are 3.3V. Any particular logic family ideally suited > for this job? > > Thanks in advance, > Bobby Hubertus > Design Engineer > BBS Telecom > bobbyh@bbstelecom.com > > **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

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