An Introduction to IBIS Models and Signal Integrity Simulation

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An Introduction to IBIS Models and Signal Integrity Simulation. Shahana Tanveer Northrop Grumman April 13, 2004. What is SI?. GND. 1.7V. Common Signal Integrity Problems. Common Signal Integrity (SI) problems include: Undershoot/overshoot Ringing Crosstalk

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### An Introduction toIBIS ModelsandSignal Integrity Simulation

Shahana Tanveer

Northrop Grumman

April 13, 2004

Common Signal Integrity Problems

Common Signal Integrity (SI) problems include:

• Undershoot/overshoot
• Ringing
• Crosstalk
• Power/ground plane noise, ground bounce

It is important to lay out the PWB to eliminate possible SI problems.

Things to consider:

• Termination Schemes
• Layer Stack up
• Trace width/spacing/length/impedance
• Bypassing
Solving SI problems - Simulation
• SI simulation is becoming more common with the availability of quality tools that fit seamlessly with the PWB layout flow.
• SI simulation becomes more important with faster operational speeds and higher density requirements.

A line is electrically long when:

Propagation delay, tp > Rise Time, tr/4,

Where tp = Length, L/ velocity, v

Assuming v = 1.4 x 108 m/s

When tr = 2.5 ns, L = 8.75 cm

But when tr = 1 ns, L = 1.05 cm

So as edge rates are getting faster and faster with

the newer technologies, it is becoming more

important to consider transmission line effects,

termination and possible signal integrity problems

in the PWB design.

Simulation Models: SPICE

SPICE….

• SPICE simulations model a circuit at transistor level, thus SPICE models contain detailed information about the circuit and process parameters which is regarded as proprietary and IC vendors are reluctant to provide..
• Not all SPICE simulators are fully compatible.
• Although SPICE simulation accuracy is typically very good, a significant limitation with this type of modeling is simulation speed.
• SPICE has various simulator options that control accuracy, convergence and the algorithm type, and any options that are not consistent might give rise to poor correlation in simulation results across different simulators.
• IBIS, is an alternative to SPICE simulation.
Simulation Models: IBIS

IBIS: Input/Output Buffer Information Specification from the Electronics Industry Alliance

• IBIS behavioral data is taken from actual devices.
• IBIS models tend to simulate much faster than SPICE models.
• IBIS Modeling provides a simple table-based buffer model for semiconductor devices.
• IBIS models can be used to characterize I/V output curves, rising/falling transition waveforms, and package parasitic information of the device.
• IBIS models are intended to provide nonproprietary information about I/O buffers and are more easily available from different IC vendors
• Non-convergence is eliminated in IBIS simulation.
• Virtually all EDA vendors presently support IBIS models, and ease of use of these IBIS simulators is generally very good.
• IBIS models for most devices are freely available over the Internet making it easy to simulate several different manufacturers’ devices on the same board.
Elements of an IBIS Model

Element 1: Pull-down

• Describes the I/V characteristics during pull-down.
• Data for minimum and maximum current for given voltages.
• Data is taken for -Vcc to 2Vcc as that allows a behavioral model for signal reflections caused by improper termination and overshoot and undershoot situations when the protection diodes are forward biased.

Element 2: Pull-up

• Describes the pull-up state of the buffer when the output drives high.
• Data is entered using the formula Vtable =

Vcc – Voutput

• The minimum and maximum values are determined by the minimum and maximum operating temperatures, supply voltages and process variations.
• Combining the highest current values with the fastest ramp time and minimum package characteristics, a fast model can be derived. A slow model can be derived by combining the lowest current with the slowest ramp time and maximum package characteristics.

Element [1]

Element [2]

Elements of an IBIS Model

Element 3: GND and Power Clamps

• Describes the ground and power clamp diodes.
• The GND clamp curve is derived from the ground relative data gathered while the buffer is in the high-impedance state and illustrates the region where the ground clamp diode is active. The range is from -Vcc to Vcc.
• The power clamp curve is derived from the Vcc relative data gathered while the buffer is in a high impedance state and shows the region where the power clamp diode is active. This measurement ranges from Vcc to 2Vcc.

Element [3]

Elements of an IBIS Model

Element 4: Ramp

• Describes the ramp time for the pull-up and pull-down devices. Ensures proper AC operation of the model.
• The min and max columns represent the minimum and maximum slew rates for the buffers.
• The values represent the intrinsic values of the transistors with all package parasitics and external loads removed.

Element 5: Package

• Adds the component and package parasitics.
• C_comp is the capacitance of the die itself, excluding the package capacitance.
• Package characteristic resistance, inductance and capacitance are added by R_pkg, L_pkg, and C_pkg, respectively.

Element [4]

Element [5]

Putting it all together – the IBIS File

A standard IBIS model file consists of three sections:

• Header Info–this section contains basic information about the IBIS file and what data it provides.
• Component, Package, and Pin Info –this section contains all information regarding the targeted device package, pin lists, pin operating conditions, and pin-to-buffer mapping.
• V-I Behavioral Model–this section contains all data to recreate I-V curves as well as V-t transition waveforms, which describe the switching properties of the particular buffer.
Putting it all together

Component

Model I-V Data

Caution!

Not all models can be trusted out of the box!

|***********************************************************************

| RTSX-S IBIS Model (RT54SX32S and RT54SX72S)

|***********************************************************************

[IBIS ver] 3.2

[File name] rtsxs.ibs

[File Rev] 1.1

[Date] March 1, 2004

[Disclaimer] All V/I data was verified for accuracy against bench measurements. The measurements were done on typical production parts. 3.3V PCI model has not been verified against silicon measurements. Please check Actel IBIS page for updates at http://www.actel.com/

|******************************************************************************

| IBIS file 6325q83f.ibs created by Jason Lew

|******************************************************************************

[IBIS ver] 2.1

[File name] 6325q83f.ibs

[File Rev] 2.x

[Date] April 9, 2003

[Source] From Lab mesurement at Quicklogic.

[Disclaimer] This information is for modeling purposes only, and is not guaranteed.

Be aware of what you are using!

• Make certain that models come from trusted sources and are verified
Setting up the Simulation
• Import CAD data into Simulation tool (translation is seamless if using the simulator tool supported Layout tool). This incorporates trace routing, via, power plane information into simulation.
• Set up simulation environment with different parameters (as shown below).

to devices

Generate Stimulus

Define Board Construction

Specify Noise Rules

Viewing Results

Various Result options:

Simulation Waveforms

Violation Reports

Simulation Examples

Example 1: Actel AX driving Xilinx input

• 1V overshoot and undershoot
• Simulation helped choose termination value, topology
• Use simulation to test different IO buffers for optimum buffer selection that meets both timing and SI requirements.

AX High Slew Driver

Violates Device Abs Max

AX Slow Slew Driver

Cannot meet device timing

AX Fast Slew Driver with 45 Ohm Termination: Cleans Signal

Simulation Examples

Question:How would RTSX-S behave in this environment?

• Use RTSX-S Buffer model with same routing
• Simulate both High Slew and Slow Slew RTSX-S

SX-S High or Slow slew: Same

rising edge, slower only on falling edge

SX-S with 45 Ohm Termination:

Cleans Signal

Simulation Examples

Example 2: RTSX-S on multi driver signal (other end Xilinx Virtex)

• Overshoot and undershoot violates Abmax
• Use Simulation to try different Termination schemes:

- Series termination at RTSX-S

- RC termination at Xilinx

Unterminated RTSX-S driver

Simulation Examples

With 45 Ohm at RTSX-S – overshoot and undershoots removed

45 Ohm at RTSX-S, RTSX-S driving

45 Ohm at RTSX-S, Xilinx driving (12S driver)

Simulation Examples

RC termination at Xilinx:

• Reduces overshoot and undershoot
• But reveals problems with the Xilinx driver

12S driver

RC termination, RTSX driving

RC termination, Xilinx driving

16F Driver