What Makes a Better I/O Driver Model? Vadim Heyfitch Cadence Design Systems, Inc. (978)

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? IBIS Pull-Down I-V Pull-Up I-V PWR_Clamp I-V GND_Clamp I-V Pkg C_comp V-T

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 Like reading ASCII files?

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

Look and Feel (Physical View) Error(s ) Warnings(s )

Look and Feel (Object View)

Navigating Errors and Warning Click on warning message

Viewing Curves

Viewing Curves (contd)

What should we do to insure quality of the next IBIS model we use in simulation? Check original data: 1.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 2.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 Vcc2Vcc-Vcc I Vcc2Vcc-Vcc I V table = Vcc – V raw V raw V table PU PC

Qualitatively Correct I-V Picture Vcc2Vcc-Vcc I V PU PD PWR GND

Error: Clamp Diode Current – not subtracted Whats wrong with this picture?

Clamp Diode Current – properly subtracted

PVT variation Vcc2Vcc-Vcc I V Slow Typical Fast

All I-V info at once - textbook example

Suspiciously Looking Model + Vcc

V-T: dynamic behavior [Ramp] | variable typ min max dV/dt_r 1.518V/ ps 1.260V/ ps 1.731V/ ps dV/dt_f 1.759V/ ps 1.579V/ ps 1.928V/ ps R_load = Required: 80% 20 %

V-T DC levels must correlate to I-V curves Optional :

Driving in to Configurable Load

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.

Sources of useful info Papers to read: –Assessing and Improving the Quality of IBIS models, SiSoft, Inc. –IBIS Behavioral Models, Microns Technical Note TN –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, –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, –The Development of Analog SPICE Behavioral Model Based on IBIS Model, Y.Wang, H.N.Tan, 1999.

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