What Makes a Better I/O Driver Model?

1
What Makes a Better I/O Driver Model?

• Vadim Heyfitch
• Cadence Design Systems, Inc.
• [phone]
• vadimh_at_cadence.com

2
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

3
What is an IBIS Driver model?
V-T
Pull-Up I-V
IBIS
Pkg
Pull-Down I-V
C_comp
4
Why verify?

• IBIS speed gt25x 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

5
Model Integrity (MI) Module - a graphical look
at IBIS models
Like reading ASCII files?
6
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

7
Look and Feel (Physical View)
Warnings(s)
8
Look and Feel (Object View)
9
Navigating Errors and Warning
Click on warning message
10
Viewing Curves
11
Viewing Curves (contd)
12
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 ltgt 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)

13
Pullup and PWR_clamp translating the raw data
PC
PU
V raw
V table Vcc V raw
V table
14
Qualitatively Correct I-V Picture
15
Error Clamp Diode Current not subtracted
Whats wrong with this picture?
16
Clamp Diode Current properly subtracted
17
PVT variation
Slow Typical Fast
V
18
All I-V info at once- textbook example
19
Suspiciously Looking Model
20
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
21
V-T
Optional
DC levels must correlate to I-V curves
22
Driving in to Configurable Load
23
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) its the best!
  Can extrapolate between I-V curves.
• Read the last two papers listed in references.

24
Sources of useful info

• Papers to read
• Assessing and Improving the Quality of IBIS
  models, SiSoft, Inc.
• IBIS Behavioral Models, Microns 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.

25
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