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What Makes a Better I/O Driver Model?. Vadim Heyfitch Cadence Design Systems, Inc. (978) 446-6455 [email protected] Content: What are we going to discuss?. Personal Introduction What is the intended audience? Who should care? What is an IBIS Driver model? Why verify?

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what makes a better i o driver model

What Makes a Better I/O Driver Model?

Vadim Heyfitch

Cadence Design Systems, Inc.

(978) 446-6455

[email protected]

content what are we going to discuss
Content: What are we going to discuss?
  • Personal Introduction
  • What is the intended audience? Who should care?
  • What is an IBIS Driver model?
  • Why verify?
  • Model Integrity (MI) - Cadence Tool for verification, editing
    • What can it do?
  • What to watch out for: typical problems?
    • Qualitative criteria of IBIS model “goodness”
what is an ibis driver model
What is an IBIS Driver model?

GND_Clamp

I-V

PWR_Clamp

I-V

V-T

Pull-Up

I-V

IBIS

Pkg

Pull-Down

I-V

C_comp

why verify
Why verify?
  • IBIS speed ~>25x of SPICE lends it for solution space exploration (aka pre-layout analysis)
  • Time spent upfront pays off
  • Lots of bad IBIS models out there
    • Only 42% passed IBISchk3 without errors or warning
model integrity mi module a graphical look at ibis models
Model Integrity (MI) Module - a graphical look at IBIS models

‘Like reading ASCII files?

model integrity mi module what can it do
Model Integrity (MI) Module – What Can It Do?
  • Ibischk3 Golden Parser: GUI to navigate errors and warnings.
  • Visual representation and navigation of (multiple) .ibs file structure
  • Editing, Sorting, …
  • Translation to:
    • IBIS to DML
    • QUAD to DML
    • Espice to Spice
  • Viewing curves
    • V-I for Pullup and Pulldown, GND_Clamp, PWR_Clamp
    • V-T
navigating errors and warning
Navigating Errors and Warning

Click on warning message

what should we do to insure quality of the next ibis model we use in simulation
What should we do to insure quality of the next IBIS model we use in simulation?
  • Check original data:
    • all I-V curves (i.e., static behavior):
      • V-Range of I-V data: - Vcc <> + 2Vcc
      • Pulldown I-V WITHOUT GND_clamp
      • Pullup I-V WITHOUT PWR_clamp
      • GND_clamp
      • PWR_clamp
    • V-T (i.e., dynamic behavior):
      • How many V-T curves? None, 2, 4?
      • V-T does not contradict I-V ?
      • dV/dT_r/f (even if V-T available)
pullup and pwr clamp translating the raw data
Pullup and PWR_clamp – translating the raw data

I

I

-Vcc

-Vcc

Vcc

Vcc

2Vcc

2Vcc

PC

PU

V raw

V table = Vcc – V raw

V table

qualitatively correct i v picture
Qualitatively Correct I-V Picture

I

-Vcc

Vcc

2Vcc

PWR

PU

V

PD

GND

error clamp diode current not subtracted
Error: Clamp Diode Current – not subtracted

What’s wrong with this picture?

pvt variation
PVT variation

I

-Vcc

Vcc

2Vcc

Slow

Typical

Fast

V

v t dynamic behavior
V-T: dynamic behavior

Required:

[Ramp]

| variable typ min max

dV/dt_r 1.518V/360.974ps 1.260V/534.096ps 1.731V/327.019ps

dV/dt_f 1.759V/167.287ps 1.579V/534.096ps 1.928V/155.338ps

R_load = 50.0000

80%

20%

slide21
V-T

Optional:

DC levels must correlate to I-V curves

what do simulators do with an ibis model
What do simulators do with an IBIS model?
  • Simulator specific
  • If 4 V-T available (Rising and Falling w/f into pull-up and pull-down R_fixture) – it’s the best! Can extrapolate between I-V curves.
  • Read the last two papers listed in references.
sources of useful info
Sources of useful info
  • Papers to read:
    • “Assessing and Improving the Quality of IBIS models”, SiSoft, Inc.
    • “IBIS Behavioral Models”, Micron’s Technical Note TN-00-07.
    • “Correlating IBIS and Hspice Buffer Models”, Todd Westerhoff, CISCO Systems.
    • “A Critique of IBIS Models Available For Download on the Web: Part 1”, SiQual Inc., Jan. 22, 2002.
    • “Introduction to IBIS Modeling of Fiber Optic Transceivers”, Mark Chang, Agilent Technologies.
    • “Extraction of Transient Behavioral Model of Digital I/O Buffers from IBIS”, P.F.Tehrani, Y.Chen, J.Fang, 1996.
    • “The Development of Analog SPICE Behavioral Model Based on IBIS Model”, Y.Wang, H.N.Tan, 1999.
what have we talked about today
What have we talked about today?
  • What is an IBIS Driver model?
  • Why verify?
  • Model Integrity (MI) - Cadence Tool for verification, editing
    • What can it do?
  • What to watch out for: typical problems?
    • Qualitative criteria of IBIS model “goodness”
  • Pointed to sources for further reading
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