Creating Broadband Analog Models for SerDes Applications

Transcription

1 Creating Broadband Analog Models for SerDes Applications Adge Hawes, IBM Doug White, Cisco Walter Katz, SiSoft Todd Westerhoff, SiSoft DesignCon 2009 IBIS Summit Santa Clara, CA February 5, 2009 Signal Integrity Software, Inc.

2 Agenda IBIS-AMI Models Analog Output Characteristics SerDes Reference Platforms SerDes Driver Modeling Correlation Existing Proposals The Direct Path Summary 2

3 IBIS-AMI Models An IBIS-AMI model has two parts: Analog Model Models unequalized analog device behavior Traditional IBIS table-driven models supplied as text (.IBS) files Used to characterize analog network and derive impulse response Analog model points to additional algorithmic model Algorithmic Model Models equalization and clock recovery behavior Supplied as executable code Models can operate at two different levels: INIT: impulse response processing GETWAVE: time-domain waveform processing 3

4 Analog Output Characteristics Our Target Behavior F A Low Pass Filter! Hz Hz 13 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands 13 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others may be claimed as the property of others ~ pf pf V Michael Mirmak IBIS-ATM Work Archive 1-Oct-2008 ~ pf pf Frequency-dependence of buffer behavior has been well documented Arpad Muranyi, 2003 Michael Mirmak 2004, 2008 Both transmission and reflection behaviors are frequency-dependent Existing IBIS black-box model doesn t represent broadband behavior 4

5 SerDes Analog Model Requirements Accurately model impedance and capacitance (transmission and reflection) characteristics over a wide frequency range Leverage existing IBIS format Leverage existing vendor data and processes Easy to understand and use 5

6 SerDes Reference Platforms Traditionally, IBIS models have been compared to SPICE simulations and physical measurement SerDes models are usually compared to internal vendor tools, which are correlated to measurement 6

7 Modeling SerDes Drivers Buffer model S Transmission Reflection P N Since we re comparing to SerDes vendor tools, we should understand how they model analog circuit behavior One method is to represent the driver as an ideal source in series with S-parameter data This scheme is simple to implement and models transmission / reflection characteristics across a wide frequency range 7

8 How Can We Do This in IBIS? + - [???] P An ideal source could be approximated with existing IBIS constructs S Better method is to just use an ideal source + - Analog Model N Algorithmic models handle the equalization behavior How do we include the S-parameter block? In AMI Out Algorithmic Model 8

9 Current IBIS Proposals Concept C Comp Comp Branch 1 Branch 2 Branch 3 This This approach approach captures captures both both buffer buffer Z(f) Z(f) and and Z(V) Z(V) variations, variations, improving improving timetimeand and frequency-domain frequency-domain modeling. modeling. It It supports supports the the Rdie Rdie DDR DDR approach approach currently currently documented documented by by JEDEC. JEDEC. It It avoids avoids direct direct frequency-based frequency-based tables, tables, difficult difficult extraction extraction methods methods or or extensive extensive model model pre-processing. pre-processing. Supporting Supporting these these in in today's today's tools tools should should not not represent represent any any more more of of a a burden burden than than classic classic C_comp C_comp does. does. The The tables tables here here cover cover an an I-V-like I-V-like voltage voltage range, range, but but need need not not match match I-V I-V ranges ranges for for any any particular particular table. table. In In many many cases cases a a pair pair of of voltage voltage rows rows for for a a particular particular [... [... R] R] and and [... [... C] C] with with different different voltages voltages but but identical identical R and and C values values per per corner corner would would be be perfectly perfectly adequate. adequate. Michael Mirmak has proposed an N-stage RC ladder network Could frequency-dependent behavior be adequately represented in this manner? 4 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands 4 may Copyright be claimed (C) 2008 as the Intel property Corporation. of othersall Rights Reserved.*Other names and brands may be claimed as the property of others Michael Mirmak IBIS-ATM Work Archive 1-Oct-2008 Final Stage Subcircuit Walter Katz IBIS-ATM Work Archive Multiple Presentations P N Walter Katz s IBIS Interconnect SPICE proposal could be used to encapsulate S-parameter data 9

10 The Direct Path + - DH PH H Point directly to a TOUCHSTONE file from within the buffer or component model TOUCHSTONE is already an EIA standard + - DL S PL L The S-Parameter element would be inserted between an ideal source and the pad nodes TSTONEFILE max * drv_bc.s4p DH PH DL PL TSTONEFILE typ * drv_tc.s4p DH PH DL PL TSTONEFILE min * drv_wc.s4p DH PH DL PL * Corners need further discussion Touchstone is a registered trademark of Agilent Technologies, Inc. and is used with permission. 10

11 Correlation to Reference Platforms How well would the S-parameter scheme work? Commercial tools are already using it Results for one tool were reported at the DAC 2008 Summit IBIS-AMI Model Results Correlation to IBM HSSCDR 11

12 Summary Analog buffers have frequency-dependent transmission/reflection behaviors that need to be properly represented for SerDes analysis An ideal source / S-parameter combination models these behaviors well Results correlate to established vendor tools Existing IBIS efforts can be adapted to meet this requirement, or we can point to S-parameter data directly Main issue is expediency: need is NOW Direct method already in production use 12