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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 vadimh@cadence.com. 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?

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  1. What Makes a Better I/O Driver Model? Vadim Heyfitch Cadence Design Systems, Inc. (978) 446-6455 vadimh@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? GND_Clamp I-V PWR_Clamp I-V V-T Pull-Up I-V IBIS Pkg Pull-Down I-V C_comp

  4. 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

  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) Error(s) Warnings(s)

  8. Look and Feel (Object View)

  9. Navigating Errors and Warning Click on warning message

  10. Viewing Curves

  11. Viewing Curves (cont’d)

  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 <> + 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 I I -Vcc -Vcc Vcc Vcc 2Vcc 2Vcc PC PU V raw V table = Vcc – V raw V table

  14. Qualitatively Correct I-V Picture I -Vcc Vcc 2Vcc PWR PU V PD GND

  15. Error: Clamp Diode Current – not subtracted What’s wrong with this picture?

  16. Clamp Diode Current – properly subtracted

  17. PVT variation I -Vcc Vcc 2Vcc Slow Typical Fast V

  18. All I-V info at once- textbook example

  19. Suspiciously Looking Model + Vcc

  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) – it’s 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”, 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.

  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

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