11 the presentation that arpad forgot michael mirmak intel corp. september 30, 2008
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11
The Presentation That Arpad Forgot
Michael MirmakIntel Corp.
September 30, 2008
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22
Proposal for New Keywordsto Improve Buffer Impedance
Modeling
Michael MirmakIntel Corp.
September 30, 2008
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33 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
A Simple Proposal• Add two “traditional” IBIS keywords to IBIS 5.1
– [C_comp Series R]– [C_comp Series C]
• Examples
[C_comp Series R]Branch 1| units below are volts, ohms|Voltage typ min max0.0 20 NA NA0.75 20 NA NA1.5 800 NA NA [C_comp Series C]Branch 1| units below are volts, farads|Voltage typ min max0.0 4p NA NA0.75 2.33p NA NA1.5 1.63p NA NA
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44 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Concept
• This approach captures both buffer Z(f) and Z(V) variations, improving time- and frequency-domain modeling.
• It supports the “Rdie” DDR approach currently documented by JEDEC.• It avoids direct frequency-based tables, difficult extraction methods or extensive model
pre-processing. • Supporting these in today's tools should not represent any more of a burden than classic
C_comp does.• The tables here cover an I-V-like voltage range, but need not match I-V ranges for any
particular table. In many cases a pair of voltage rows for a particular [... R] and [... C] with different voltages but identical R and C values per corner would be perfectly adequate.
CComp Branch 1 Branch 2 Branch 3
…
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55 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Keyword Syntax Rules• [C_comp Series R] and [C_comp Series C]
– are optional – are hierarchically located within [Model]– may appear multiple times within any [Model]– contain only one subparameter, "Branch"– contain four columns of data after the required subparameter– columns are voltage, and typ/min/max ohms (for [... Series R])– columns are voltage, and typ/min/max farads (for [... Series C])– do not affect, override or interfere with existing C_comp values– if one is present, the other is required for any given Branch value– voltage corresponds to pad vs. pulldown/ground clamp rail (really, the bias voltage
for AC sweeps)– voltage sweep should correspond to pulldown range (-Vcc to 2*Vcc)– must not contain negative values (voltage, capacitance or resistance)
• Branch subparameter
– is required for any usage of [C_comp Series R] and/or [C_comp Series C]– contains an integer argument, positive and non-zero– no two [... Series C] tables may have the same Branch value in the same [Model]– no two [... Series R] tables may have the same Branch value in the same [Model]– If multiple tables are present, one must use the value 1 and others must use
sequential increasing Branch values
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66 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Issues• These keywords are generally subject to the same usage rules and
restrictions as C_comp (e.g., not in series devices or [Driver Schedule] scheduled models).
• No special treatment or procedure is defined here for differential buffers. A differential RC could be defined in a separate keyword structure, along with a differential C_comp.
• These keywords are not expected to be used with multi-lingual (IBIS
4.1/4.2) buffers. • IBIS AMI should be unaffected, though the use of heavily voltage-
dependent RC circuits may violate the LTI assumption in some cases. • Additional keywords or subparameters may be required should
separate RC circuits connected to the pullup rail, power clamp rail, etc. be needed.
• Impact to and usage in ECL designs is unknown.
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77 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
From April & October 2004IBIS Summits…
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88 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
How is C_comp data collected?• Common method for single-ended buffer C_comp
1. Use Vsource with known edge rate, dV/dt2. Measure the input current3. Calculate capacitance (may have to take an average)
Driver
I
Vsource
dV/dt
dt
dVCI *
)(dtdVI
C
What about for the differential case?How about pre-emphasis (wired-or structures)?
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99 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
IBIS for SerDes Pre-Emphasis
A B C D
TX+
TX-
Pre-EmphasisA = P C = PB = P D = P
De-EmphasisA = P C = PB = P D = P
Main Boost
Pullup – Main (+ & -) Pullup – Boost (+ & -)
Ground ClampsMain + & -
1) Extract C_compfor entire buffer
2) Split C_compacross Main, Boost
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1010 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Results of Crude C_comp AccountingMain+Boost1 V-t Curve
-1.000E-01
0.000E+00
1.000E-01
2.000E-01
3.000E-01
4.000E-01
5.000E-01
6.000E-01
5.400E-08 5.500E-08 5.600E-08 5.700E-08 5.800E-08 5.900E-08 6.000E-08
Time (s)
Vo
ltag
e (
V) /o
/obIBIS PIBIS N
10-90% Edge Rates
Transistor 7.46E-10IBIS 9.44E-10Difference 1.98E-10Percentage 26.54%
Transistor vs. Combination IBIS into Rload
C_comp = 8pF
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1111 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Proposed Fix• Method 2: Adjust all V-t curves for the total C_comp
value before using the models for simulation– Each V-t curve reflects total C_comp load– Adjust each curve (Main, Boost) for total C_comp– Set IBIS C_comp for Main, Boost to 0 pF– Add external cap equal to original total C_comp
images and equations from A. Muranyi
Vwfm_pu (V-t)
Vfx_pufixture
Current through fixtureis function of current through pullup AND
through die cap
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1212 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Another New Approach• First C_comp adjustment attempt made two assumptions
– Differential buffers can be “split” in two– Each can be modeled adequately with a single C_comp
• Additional data calls at least one assumption into question– Single C_comp does not permit frequency dependence– A differential component appears at some frequencies
• Proposal: Stay in AC Domain for C_comp Measurement– Attempt to match AC behavior of simulation model– Once model correlates in frequency of interest, add data into
IBIS model– This may stretch IBIS 3.2/4.0 beyond available keywords
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1313 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Our Target Behavior
VHz
F
~ 14-15 pF
~ 5-7 pF
A LowPass
Filter!
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1414 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Recall L. Giacotto’s Slides…• 2002: Luca Giacotto presented twice on buffer impedance
starting with observations by A. Muranyi
Two caps,two resistors
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1515 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Original Giacotto-Muranyi C_comp Model
Pad
CComp = 7pF
C1 = 8.3pF
45 Ω R1
LinearizedI-V Curve
One side of our differentialbuffer (built-in pulldown,
current source is AC high-Z)
Near DC, only 45 Ω;At low AC, caps look more
like ~ 14 pF
Value of R1 depends onfrequency of drop in
capacitance to ~ 7 pF
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1616 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Model vs. AC Analysis of Buffer Capacitance Matching Chart
Vbias=0
0.00E+00
2.00E-12
4.00E-12
6.00E-12
8.00E-12
1.00E-11
1.20E-11
1.40E-11
1.60E-11
1.80E-11
1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09 1.00E+10
Frequency (Hz)
Cap
acit
ance
(F
)
USB transistor model
R1 = 45
R1 = 1k
R1 = 5k
R1 = 10k
transistor model
Ignoring voltage…
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1717 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Modified G-M ModelPad
CComp = 3p
C2 = 8.3p
45 Ω R1 R2
C3 = 4p
LinearizedI-V Curves
Near DC, only 45 Ω;At low AC, caps still add
to ~ 14 pF
Adding new elementsto cause new “break”
in frequency
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1818 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Modified Model AC Analysis
Capacitance Matching Chart
Vbias=0
0.00E+00
2.00E-12
4.00E-12
6.00E-12
8.00E-12
1.00E-11
1.20E-11
1.40E-11
1.60E-11
1.80E-11
1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09 1.00E+10
Frequency (Hz)
Ca
pa
cit
an
ce
(F
)
USB, typical, PsideR1 = 45, R2 = 0R1 = 5k, R2 = 0R1 = 1k, R2 = 0R1 = 10k, R2 = 0R1 = 5k, R2 = 20
transistor model
Ignoring voltage…
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1919 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
• Model is a fair match up to ~ 2 GHz• Problems arise
– A single C_comp value is no longer adequate– No IBIS keywords support this structure– Still need to “back out” these elements from V-t curves
Model Assessment
Pad
CComp = 3p
C2 = 8.3p
45 Ω R1= 5 kΩ
R2 = 20 Ω
C3 = 4pLinearizedI-V Curves
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2020 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Original C_comp Adjustment
Pad
CCompRfixture
Diff.Driver
Idriver
Icap Ifixture
• Idriver(t) = Icap(t) + Ifixture(t)
• Icap = C_comp * dV/dt where dV/dt is instantaneous V-t slope
• Ifixture = V-t/Rfixture, taken at every time point
• “Cap-less” V-t curve = Idriver(t) * Rfixture
– Driver is pullup curve set with internal pulldowns
– Vfixture = 0, since there is no active, driving pulldown
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2121 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Modified Approach
Pad (V-t curves)
CComp
Rfixture
Diff.Driver
Idriver
Icap
Ifixture
•Very tricky differential equation…•Easier solution – use EDA tool to generate data!
– Create netlist as shown– Drive PWL source with original V-t data set(s), at pad– CCCS into load = Rfixture provides “adjusted” V-t curves
C1 C2
R1R2
I1I2
V1 V2
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2222 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Assessment• We can now adjust V-t curves for AC C_comp
– Enabling each portion of differential buffer…– Take a separate V-t curve (Main Only, Boost only)– Take AC domain frequency response for entire buffer
– Single-ended response only analyzed so far
– Match AC domain with model & extract adjusted V-t curve
• Can we repeat the process for differential behavior?
VHz
F
A HighPass
Filter!
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2323 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
What About Differential C_comp?
Pad
CCompP = 3p
C2P = 8.3p
45 Ω R1P
= 5kR2P
= 20
C3P = 4p
CCompN = 3p45 Ω R1N
= 5kR2N
= 20
C2N = 8.3p
Pad
C3N = 4p
To match ourtarget…
RD ~ 2 Ω
At high Hz,C2P andC2N addup, acting
differentially
Bias = 0V
We need Rdiff, not Cdiff!
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2424 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Differential C_comp (Bias = 0 V)
Bias = 0 V
0.00E+00
2.00E-12
4.00E-12
6.00E-12
8.00E-12
1.00E-11
1.20E-11
1.40E-11
1.60E-11
1.80E-11
1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09 1.00E+10
Frequency
Cap
acit
an
ce
(F
)
Cp - Transistor Model
Cdiff - Transisor Model
Cp - GMM Model
Cdiff - GMM Model
Diff. C_comp resembles a pass-band
filter (BW ~ 1/Rd)
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2525 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
What About Differential C_comp?
Pad
CCompP = 3p
C2P = 8.3 p
45 Ω R1P
= 5kR2P
= 20C3P
= 2.33 p
CCompN = 3p45 Ω R1N
= 5kR2N
= 20
C2N = 8.3 p
Pad
C3N
= 2.33 p
To match ourtarget…
RD ~ 2 Ω
At high Hz,C2P andC2N addup, acting
differentially
Bias = 0.75 V
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2626 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Diff. C_comp (Bias = 0.75 V)
Bias = 0.75 V
0.00E+00
2.00E-12
4.00E-12
6.00E-12
8.00E-12
1.00E-11
1.20E-11
1.40E-11
1.60E-11
1.80E-11
1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09 1.00E+10
Frequency (Hz)
Cap
acit
ance
(F
)
Cp - Transistor Model
Cdiff - Transistor Model
Cp - GMM Model
Cdiff - GMM Model
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2727 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
What About Differential C_comp?
Pad
CCompP = 2.6 p
C2P = 4.86 p
45 Ω R1P
= 5kR2P
= 800C3P
= 1.63 p
CCompN = 2.6 p45 Ω R1N
= 5kR2N
= 800
C2N = 4.86 p
Pad
C3N
= 1.63 p
To match ourtarget…
RD ~ 2 Ω
At high Hz,C2P andC2N addup, acting
differentially
Bias = 1.5 V
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2828 Copyright (C) 2008 Intel Corporation. All Rights Reserved.*Other names and brands may be claimed as the property of others
Diff. C_comp (Bias = 1.5 V)
Bias = 1.5
0.00E+00
2.00E-12
4.00E-12
6.00E-12
8.00E-12
1.00E-11
1.20E-11
1.40E-11
1.60E-11
1.80E-11
1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09 1.00E+10
Frequency (Hz)
Ca
pac
itan
ce (
F)
Cp - Transistor Model
Cdiff - Transistor Model
Cp - GMM Model
Cdiff - GMM Model