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Welcome to Spiceguy.net.
Today is: Monday May 19, 2025
 IBIS Modeling
1. Introduction
IBIS stands for Input/Output Buffer Information
Specification
Purpose of an IBIS model: A way to present the
electrical characteristics of an input, output, or I/O buffer behaviorally,
i.e., without revealing the underlying transistor or process information.
The Input/Output Buffer Information Specification (IBIS) was developed by the
IBIS Open Forum, which is affiliated with the Electronic Industries Alliance (EIA).
The first spec version 1.0 came out in April 1993, and the current version is
5.0 from August 2008. IBIS specifies a standard form to present
information in ASCII format, using special files. This information
describes the behavior of various I/O buffers that send electrical signals
outside the silicon chip, or receive such signals.
This type of information includes:
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Output I-V curves for output buffers, in LOW and
HIGH states.
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Input I-V curves of the power clamp or ESD diodes.
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Rise and fall time of the buffer when switching (dV/dt_r
and dV/dt_f).
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Values for input or output die capacitance "c_comp".
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V(t) curves, describing the exact form of
transitions from LOW to HIGH states, and from HIGH to LOW states, for a
specified load.
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Electrical parameters of the packages.
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The characteristics of the package (the values of
the lead inductance, resistance, and capacitance)
2. My IBIS Flow
using HSPICE and S2IBIS3
My IBIS Flow document (not shown) consists of the
following sections describing how to generate an IBIS model. The
Validation step is a SPICE simulation of the actual circuit together with the
IBIS model which demonstrates the integrity of the IBIS model as shown in the
last simulation plot below.
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Introduction and
Pre-modeling information
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Procedure to Generate IBIS file Manually
using HSPICE Simulations
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Specific Instructions for Manual SPICE Simulation to derive IBIS parameters
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IBIS model Validation
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Procedure to Generate IBIS file Using "S2IBIS3"
Java Script, developed at North Carolina State University
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Syntax Check of your IBIS model
3. Sample
Simulations from IBIS Flow
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HSTL 800MHz I/O Simulation Waveform with Output Fixture (Fast /
Typical / Slow)
This simulation is for a
HSTL 800MHz I/O Buffer's Rising and Falling Waveform used for the IBIS
model. The simulation includes the IBIS spec test fixture R/L/C
components at the measurement point. Curves are shown for
Fast/Typical/Slow conditions for before and after the IBIS test fixture.
(HSTL
800MHz Waveform Simulation for IBIS Model: Click to
Enlarge.) (IBIS Test Fixture: Click to Enlarge.)
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PCI-X
100MHz I/O Simulation Waveform with Output Fixture (Fast /
Typical / Slow)
This simulation is for a
PCI-X I/O Buffer's
Rising and Falling Waveform used for the IBIS model. The simulation
includes the IBIS spec Test Fixture R/L/C components.
(PCI-X Waveform Simulation for IBIS Model: Click Image to Enlarge)
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PCI-X **VALIDATION** Simulation Waveform with Output
Test Fixture (Typical / Slow /
Fast)
This is the IBIS Validation simulation is for a
PCI-X I/O Buffer's
Rising and Falling Waveforms. The previous simulation is rerun with
the IBIS model instantiated. The IBIS signal is shown in
cyan, and
the actual circuit is shown in red. For this setup we see excellent
correlation with only 250pS difference in one transition (Falling edge).
The IBIS model matches the logic voltage levels, as well as the slew rate of the actual
circuit.
(PCI-X Validation of
IBIS Model vs. Actual Circuit: Click Image to Enlarge)
4. Excerpts from
Custom HSPICE Top-Level IBIS Simulations
These are portions HSPICE top level source code used to
simulate various IBIS scenarios and generate data for the IBIS model.
A) HSPICE top level deck to measure PCI-X clamp currents
for IBIS model
(Click Image to Enlarge)
B) HSPICE top level deck to measure C_comp
for IBIS model using AC Analysis
(click image to enlarge)
C) HSPICE top level deck to
measure IBIS Rising and Falling Waveforms
(click image to enlarge)
D) HSPICE top level deck
to run IBIS Validation
This shows how to simulate the IBIS model
together with the actual netlist for model validation.
(click image to enlarge)
E)
Additional SPICE Code for IBIS Spec Test Fixture used for Rising/Falling
Waveforms
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IBIS
RLC test fixture
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IBIS T-Line test fixture (optional)
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