[SI-LIST] Re: Driver Impdance from IBIS
- From: "Jiang, George Q" <george.q.jiang@xxxxxxxxxx>
- To: "Nijagunamurthy, Hithesh (GE Intelligent Platforms)" <hithesh@xxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 14 Dec 2010 13:07:06 +0000
MentorGraphics has a tech note MG23434 regarding how HyperLynx calculates the
drive impedance. Hope it helps.
George Jiang
Delphi Electronics & Safety
The following is what is in MG 23434
___________________________________________________
The Terminator wizard is calculating the impedance of the driver by checking
the delta V / delta I in 1/10 volt increments along both the pull-up and
pull-down VI tables. It keeps the lowest impedance it sees for each table and
then averages the two together to come up with the driver impedance.
If you look at the pull-up and pull down table data graphically in the IBIS
editor, you will see that both the pull -up and pull-down tables are quite
non-linear, so there is a wide range of impedances present along each line.
The pull-up table shows impedances ranging from about 32 ohms to an average of
88 ohms (when you draw straight line from 0 volts to 3.3 volts).
The pull down table shows impedances ranging from about 28 ohms to an average
of 96 ohms (when you draw a straight line from 0 volts to 3.3 volts
The help text in the IBIS editor defines the output impedance the following way:
*****************************************************************
Driver output impedances are not always specified in the IC datasheet. Lacking
a manufacturer's specification for impedance, there are several ways to obtain
it, described in the following sections.
Measure from Published V-I Curves
Many manufacturers are now publishing (or have readily available) V-I curves
for their output-buffer stages (high and low). If you can obtain such curves,
output impedance is easy to calculate.
To measure impedance from a V-I curve:
1. Draw a straight line along the linear portion of the V-I curve,
before the current "flattens out" or saturates.
2. Take two points on the line. Measure delta(v) and delta(i) between
the points.
3. Then calculate the output impedance as Zout = delta(v) / delta(i)
*************************************************************************
When this method is used and a line is drawn along the most linear portion of
the graphs from 0.0 volts to 1.1 volts, the impedances come out to be 40.2 ohms
for the pull-down and 44.8 ohms for the pull-up.
Xilinx must be calculating the average driver impedance differently then
Terminator Wizard. If you draw a line from 0 volts to 1.6 volts on each graph,
then you get about 50 ohms. This must be close to what Xilinx is doing.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Nijagunamurthy, Hithesh (GE Intelligent Platforms)
Sent: Tuesday, December 14, 2010 7:40 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Driver Impdance from IBIS
Hi All,
How do you find the driver impedance information from the IBIS file.
In the V/I table, is the listed voltage just V or Vcc- V
The way I find impedance is
For pull up -
(Vcc-V in table)/I
For pull down -
(V in table)/(I in table)
What voltage do you consider? 1.00 or 1.5?
Some IBIS files mention if V in table is Vcc-V others don't.
For PCI devices like power pc, Intel 21555 bridge, the impedance values
I get is around 11 to 18 ohms.
Thanks,
-Hithesh
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