RE: [SI-LIST] : AGP impedance compensated drivers?

Volk, Andrew M (andrew.m.volk@intel.com)
Wed, 10 Jun 1998 08:49:00 -0700

Andrew -

The timing margins are very tight on AGP-4X and the trace length
requirements are not shrinking over AGP-2X. To maintain the same
trace lengths with tighter timing skew, it was necessary to go to
tighter layout requirements, tighter board impedance control and
tighter driver characteristics. It is felt that the specified range
of driver strengths that delivers the allowable flight time skews on
the interconnect is smaller than the range a typical process produces.
Therefore, as a part of enabling vendors to build AGP-4X systems, a
list of references to papers that described impedance compensated
buffers was provided as a design aid.

The amount of compensation that is required will vary according to
process and design technique. Note also, that there is a new
"curvature" spec for AGP-4X buffers. Both timing skew and ringback
can be reduced by making the buffer more linear. Adding a series
resistor in the buffer output will linearize the buffer (reduce
curvature). If this resistor is sufficiently stable and precise, it
can also significantly reduce the impedance variation, reducing or
removing the required compensation.

Andrew Volk

-
**********************************************************************
Andrew M. Volk E-mail: Andrew.M.Volk@intel.com
Principal Engineer
Graphics Components Division MS: FM5-64
Intel Corporation
1900 Prairie City Road Phone: (916) 356-5102
Folsom, CA 95630 FAX: (916) 356-3051
**********************************************************************

______________________________ Reply Separator
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Subject: [SI-LIST] : AGP impedance compensated drivers?
Author: "Andrew Ingraham" [SMTP:Andrew.Ingraham@digital.com] at MSXGATE
Date: 6/9/98 6:52 AM

There is a document on the AGP web site (agpforum.org) that says AGP-4X will

require special "impedance compensated" drivers.

What is this all about? I don't recall seeing anything in the AGP spec
implying
a need for impedance compensated drivers. What sort of impedance
compensation
is Intel talking about? Why is it needed?

http://www.agpforum.org/downloads/ctl_impd.pdf

Thanks in advance,
Andy Ingraham

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