Download I/O Buffer Modeling

I/O Buffer Modeling Class
2 lectures
Prerequisite Reading – Chapter 7
IBIS spec will be used as
reference
Additional Acknowledgement to Arpad Muranyi, Intel Corporation
1
Additional Information
 URLs
IBIS home page:
http://www.eigroup.org/ibis/ibis.htm
IBIS 3.2 spec:
http://www.vhdl.org/pub/ibis/ver3.2/
IBIS-X: http://www.eda.org/pub/ibis/futures/
 Tools
Golden Parser:
http://www.eda.org/pub/ibis/ibischk3
Visual IBIS editor, SPICE-to-IBIS tool on IBIS
web site. We will use this free tool.
http://www.mentor.com/hyperlynx/visibis.cfm
2
Key Topics









What is a model?
Importance of accurate models
Types of buffer models
IBIS and the portions of an IBIS model
How model data is generated
How to calculate VOL and VOH from a model
Package modeling in IBIS
IBIS HSPICE example
Bergeron diagrams
3
Theories, Modeling, and Reality
“I take the positivist viewpoint that a physical theory is
just a mathematical model and that it is meaningless to
ask whether it corresponds to reality. All that one can
ask is that its predictions should be in agreement with
observation. “ 1
1 Steven W. Hawking, September 30 1994, Public Lecture
on “Time and Space”
 Electrical models can be derived in two ways
From physical structures and properties
From observed behavior
 It is irrelevant whether the electrical models
correspond to physical reality.
 It only needs to predict behavior.
Hence all models are behavioral
4
What is a Model?
?
5
?
 Electrical representation of a physical device
 For example, a transmission line can be modeled as:
 A package can be modeled as a combination of transmission
lines and lumped elements.
 An input or output buffer can be modeled in various ways as
well.
Importance of Accurate Models
 T-lines, package, connectors, vias, return paths, etc.

can all be modeled to extreme detail, but if the
input (stimulus) is not accurate, it’s wasted.
Garbage in, garbage out.
 It is extremely important for engineers to

understand the origins of model data, be familiar
with modeling types and limitations, and doublecheck models, whether they create them or they
receive them from someone else!
Also, know how your tool uses model data!
6
How do we model I/O buffers?
Description
7
Intellectual
Property
Simulation
Speed
“Sweep-ability”
Very Little
Fast
Very
Little
Fast
Somewhat
Lots
Slowest
limited
RHigh
Linear
Models
RS
More detail
RLow
Behavioral
Models
Linear or non-linear
I-V and V-t data
Transistor
Circuit /
Netlist
All buffer details including
driving transistors, pre-driver
circuitry, receiver diff. amp,
etc.
Basic C-MOS Buffer Model
Output / Driver
Pull-up
Device
Pull-down
Device
8
Input / Receiver
ESD Diodes
+
Inherent Diodes in Transistors
Pad Capacitance
Review Lattice Diagram Analysis
r
r
source
V(source)
0
Time
Vlaunch
Vlaunch
9
A signal can be
V(load) determined by just
knowing Vlaunch,
rload, and rsource plus
0
delay
load
N ps
Vlaunch rload
Vlaunch(1+rload)
2N ps
Time
Vlaunch rloadrsource
Vlaunch(1+rload +rload rsource)
3N ps
Vlaunch r2loadrsource
Vlaunch(1+rload+r2loadrsource+ r2loadr2source)
4N ps
Vlaunch r2loadr2source
0
V(load)
V(source) Zo
Vs
Rs
TD = N ps
Vs
Rt
5N ps
10
Refining Buffer Assumptions
 The original assumption was that Vlaunch, rload
and rsource are constant in time and linear.
 Most buffers are not linear.
In other words, there is a current dependent
voltage that changes with the time varying
voltage.
We call these “I-V” curve elements instead of
resistors, capacitors, or inductors
Vintial
Vs 
ZL
ZL  Z0
rload
ZL  Z0
ZL  Z0
and
ZL
Zload ( V  I)
and
ZS
then
Vintial
Zload ( V  I)
Vs 
r
Zload ( V  I)  Z0 load
ZS  Z0
ZS  Z0
rsource
Zsource ( V  I)
then
Zload ( V  I)  Z0
rsource
Zload ( V  I)  Z0
Zsource ( V  I)  Z0
Zsource ( V  I)  Z0
Beginning of Behavioral Buffer Modeling
11
Consider that Vs is Vs(t) and V is V(t), so Vintial, rload, and
rsource are Vinitial(t), rload(t), and rsource(t). Also, the
propagation functions can be described in a similar manner.
Hence the voltage and current response and for all nodes
in the network can be determined by replacing the buffer
with the appropriate “I-V” impedance functions and don’t
require the actual transistor models for the buffer.
 This was the basis for
a buffer specification
that was created in
the early 90’s called
IBIS
IBIS and Other Model Types
 IBIS = I/O Buffer Information Specification
 The beginnings of IBIS occurred at Intel during

Pentium Pro days. Engineers wanted a way to give
buffer information to customers, and decided on I-V
curves. The initial IBIS spec was created shortly
thereafter. IBIS went through many iterations,
eventually adding V-t curves (rev 2.1) and other
features like staged devices (rev 3.0). The current
revision is 3.2.
Other I-V/V-t model types include:
Various simulator vendors have their own internal models.
However most will convert IBIS to their internal format.
We often use controlled switched resistors (V-t curves of
sorts) in SPICE.
 Colloquial Terminology ~ V-t = V/T = V(t);
I-V = I/V = I(V)
12
13
What is in an IBIS file?
 First IBIS is a standard for
describing the analog
behavior of the buffers of
digital devices using plain
ASCII text formatted data
IBIS files are really not
models, they just contain the
data that will be used. Casually
they may be referred to as a
models but are really
specifications.
Simulation tools interpret this
behavioral specification to
implement their own models and
algorithms
Key
areas
of
spec
Key Portions of an IBIS Model
14
ESD Diodes
+
Inherent Diodes in Transistors
Output / Driver
Vcc
Pull-up
Device
I(V)
V(t)
I(V)
I(V)
V(t)
I(V)
Pull-down
Device
Vss may
be 0V
Input / Receiver
P
a
c
k
a
g
e
P
a
c
k
a
g
e
Die Pad Capacitance
Vcc
I(V)
I(V)
Vss may
be 0V
MOS I-V Curves
15
 Impedance of a buffer is dynamic during transitions - between fully open


and fully driving (RON).
Example – let’s take a look at a high-to-low transition below.
In the next few slides we will learn how we can model this dynamic
V-I characteristic.
VGS
VCC
VOUT (t=0) = VCC
VGS (t=0) = 0
Source
VT
Gate
Drain
Drain
Gate
Vcc
+
VGS
-
ID
Source
ID
Triode
(Ohmic)
+
time
0 1 2 3 4 5
VDS =
VOUT
Saturation
t=3
t=4
-
t=5
Vss
t=2
VCC
t=0, t=1
(no current
below Vt)
Assume pulled up to Vcc at t=0
Generating pull down I-V Data
Pull-down I-V
Measurement or Simulation Setup
16
I
+I
Driving
LOW
(N-channel
curve)
Pull-up
Device
on
Pull-down
Device
off
Sweep V
–Vcc to 2Vcc
V
Output / Driver
I(V)
V(t)
I(V)
I(V)
V(t)
I(V)
Current is
positive above
Vss per
definition if I
flows
17
Generating Ground Clamp I-V Data
Ground Diode I-V
Measurement or Simulation Setup
I
+I
V
Tristate
Sweep V
–Vcc to 2Vcc
Output / Driver
Pull-up
Device
on
Pull-down
Device
off
I(V)
V(t)
I(V)
I(V)
V(t)
I(V)
Current is
negative below
Vss per
definition if I
flows
Generating pull up I-V Data
Pull-up I-V
Measurement or Simulation Setup
Driving
HIGH
18
I
+I
V
Vcc
Sweep V
–Vcc to 2Vcc
(P-channel
curve)
Output / Driver
Pull-up
Device
on
I(V)
V(t)
I(V)
V(t)
Pull-down
Device
off
I(V)
I(V)
Current is
negative below
Vcc per definition
if I flows.
It is desirable to
make the curve
referenced to
Vcc. Will explain
later
19
Generating Power Clamp I-V Data
Pull up diode I-V
Measurement or Simulation Setup
I
Power
Clamp
V
+I
Tristate
Sweep V
–Vcc to 2Vcc
Output / Driver
Pull-up
Device
on
Pull-down
Device
off
I(V)
V(t)
I(V)
I(V)
V(t)
I(V)
Current is
positive above
Vcc per
definition if I
flows
It is desirable to
make the curve
referenced to Vcc.
Will explain next
Double Counting Resolution
20
 Sometimes the clamp current is not zero in
the range of operation.
 Before use in IBIS the clamp current needs
to be subtracted.
 Below is an example for the ground clamp and
pull down data
I(V)
V(t)
I(V)
V(t)
I(V)
V(t)
I(V)
I(V)
V(t)
I(V)
Pull up
measurement
I(V)
I(V)
V(t)
I(V)
I(V)
I(V)
I
Power
Clamp
I
V
Vcc
I(V)
V(t)
Vcc
I
Vcc
Pull up
curve
I-V Curves in IBIS
21
 IBIS uses Vcc-referenced I-V curves for all devices


hooked to the power rail (pull-up and high-side diode).
This effectively shifts and flips the I-V curve.
Major reason is so same model can be used regardless
of power connection (independent of Vcc).
For example, a 5-V and 3.3-V part can use the same model.
Measured Curve
I
IBIS Curve
I
V
V
Vcc
Vcc
Driving
HIGH
Vcc
Pull-up
Pull-up
Sweep V
–Vcc to 2Vcc
+I
I
Power
Clamp
Power
Clamp
V
I
V
Simple model of High/Low drive
I-V
I(V)
V(t)
I(V)
V(t)
I-V
Controls V(t)
for High Curve
Controls V(t)
for Low Curve
 The high and low switches are ideally
complementary
They switch in opposite senses simultaneously
 Real devices have slightly different switching
characteristics.
22
How to Generate the V-t Data
Pull-up V-t
Measurement or Simulation Setup
V
23
V
VCC
VOH
VCC
VOH
Driver
+
RLOAD
(typically 50 ohms)
t
Pull-down V-t
Measurement or Simulation Setup
t
V
V
VCC
VCC
Vcc
+
RLOAD
(typically 50 ohms)
VOL
Driver
VOL
t
t
 4 V-t curves are required
2 for each switch for high and low switching
 Accuracy is improved if Rload is within 20% of the usage model
load
Why Four V-t Curves?
24
 It is important for the V-t curves to be time-correlated.
 The four V-t curves describe the relative switching
times of the pull-up and pull-down devices.
NMOS is
completely OFF
PMOS is
completely ON
PMOS begins
turning OFF
NMOS begins
turning ON
VCC
VOH
All V-t curve measurements
or simulations are started
at time zero.
VOL
NMOS begins
turning OFF
PMOS begins
turning ON
PMOS is
completely OFF
NMOS is
completely ON
More on IBIS transition time
 Two ways to synchronize switch
Build delay into curves
Use version 3.1 Scheduled drivers
 Make sure the total transition time to
settling is shorter that half the period.
Start of bit time
25
PVT Corners
 PVT = Process, Voltage, Temperature
 Models in the past have historically been built at the
“corners.” All buffer characteristics are considered
dependent parameters with respect to PVT.
Fast Corner = Fast process, high voltage, low temp.
Slow Corner = Slow process, low voltage, high temp.
 These can be entered into an IBIS model in the “min” and
“max” columns.
Fast/strong in the max column
Slow/weak in the min column
 In recent generations we have found that just providing fast
and slow corners does not adequately cover all effects. In
these cases other model types can be given (e.g., “max
ringback” model).
 Compensated buffers explode the combination of required
buffer corners.
They use extra circuits to counteract (compensate) PVT effects
This makes PVT and buffer characteristics independent
parameters.
26
“Envelope” or “Spec” Models
27
 Historically, we have repeatedly predicted buffer
strength and edge rates incorrectly.
Buffer strengths are often weaker in silicon.
Edge rates are often slower in silicon.
 One approach that can be used is to create
“envelope” or “spec” models. For example:
I
V
Envelope.
All measured curves should
fall within these specs.
Strong
Weak
V
Key point!!!:
These spec curves can be
given to I/O designers to
describe required buffer
behavior.
t
28
Issues with spec curve models
I
V
Envelope.
All measured curves should
fall within these specs.
Strong
Instantaneously
a short
Weak
V
Instantaneously
an open
t
Non-monotonic
 These are legal according to the spec.
 Sometimes more qualification is
required.
Example: Create CMOS Model
 Given:
Vcc = 2.0 V
Measurement threshold = 1 V; VIL = 0.8 V; VIH = 1.2 V
NMOS RON = 10 ohms
PMOS RON = 10 ohms
All edge rates are ramps of 2 V/ns
Capacitance at the die pad of the buffer = 2.5 pF
Clamps are 1 ohms and start 0.6V above and below rails
PMOS starts turning on 100 ps after NMOS starts turning
off (rising edge)
NMOS starts turning on 100 ps after PMOS starts turning
off (falling edge)
 Will use Mentor Graphic Visual IBIS editor in
example
http://www.mentor.com/hyperlynx/visibis.cfm
29
Example: Header information
30
Package definition and pin allocation
mysimple_buffer
signal001
12mohms
2pF
2nH
31
32
Model statement
 Notice the name “special_IO” is assign to our single pin before.
 Many pins and models can specified for single component
mysimple_buffer
signal001
12mohms
2pF
2nH
2.5pF
33
I-V curves
 Construct in this


example with a spread
sheet
Break session to IBIS
Edit to view I/V curves
Assignment: Use this
example and change the
pull and pull down curves
to 15 ohms. Check with
Visual IBIS. Correct VT
waveforms.
The 4 V-t waveforms w/ spec 100ps delay
34
Match V-t and I-Curves
 The intersection of the load line of the
fixture (specified in the waveform
section) and a corresponding I-V curve
determines the Voh and Voh that
should to be used in the respective V-t
Vdd
Vdd
section
Pull down
I
Fixture load
line
More on
load lines
later
Vol
R_fixture
Vdd
Vdd
V-t
35
End and Ramp
 The ramp is specified but the simulator
tool can determine whether to use the
ramp or the V-t data
 The End statement is require
 The IBIS 3.1 and 2.1 are spec are actually
readable IBIS code and can be view with an
IBIS editor.
36
GTL+ on die termination
 Recall that a GTL buffer contains pull-down



transistors only
No switched PMOS
Many of Intel’s processors and chipsets have
started to include termination devices inside the
I/O buffer.
This eliminates the stub on the PWB to connect to
the termination resistance
Vcc
On- or off-die
resistor for pull-up
and termination
37
On-die Termination
38
 One way to include on-die termination is to use

superposition and add the termination currents to
the diode currents in the clamp sections.
The clamps are always active in an IBIS model,
regardless of whether the buffer is driving or
receiving. Since the termination is always active,
also, this scheme works well.
I
I
V
Vcc
On-die
Pull-up
Resistor
+
Power Clamp + On-die term.
(Put full curve into power clamp
section of IBIS model.)
Power
Clamp
Vcc
I
V
V
Vcc
Package Modeling in IBIS
 Three ways to model packages in IBIS:
Lumped R, L, C values in IBIS file
Package models
EBD (Electrical Board Description)
 Package models and EBDs follow this convention:

[Len=l R=r L=l C=c]
Examples:
Lumped resistor: Len=0 R=50 L=0 C=0
Capacitor package: Len=0 R=[ESR] L=[ESL] C=1uF
Package trace: Len=1.234 R=0 L=10E-9 C=2E-12
39
Example: VOL Calculation – Resistor Load Line
 The I-V for the resistor load is below
Vcc = 2V
50 ohms
RLoad
I
Vcc
RLOAD
Pull-down
I-V curve
50 ohm load line
Zero
Voltage
Load line
Slope = -1/RLOAD
V
VOL
Vcc
Zero Current
40
Example: VOL Calculation - buffer
 Now create the NMOS I-V curve for load line
analysis below:
~10ohms
~10 
I-V
I
Vcc
RLOAD
Pull-down
I-V curve
V
VOL
Vcc
41
42
Example: VOL Calculation
 Using the intersection of the NMOS I-V curve and

load line, calculate VOL:
The Vol should correspond the Vol in the V-t
waveforms
Vcc = 2V
50 ohms
~10ohms
~10 
I-V
50 ohms
I
Vcc
RLOAD
Pull-down
I-V curve
Sanity check and solution:
Vcc = 2V
Zero
Voltage
50 ohms
Load line
Slope = -1/RLOAD
VOL = 0.33 V
50 ohm load line
V
VOL
Vcc
Zero Current
10 ohms
Example: Calculate VOH
43
 calculate VOH from the intersection of PMOS I-V

curve and the resistor load line:
The Voh should correspond to the Voh in the V-T
waveforms
Vcc = 2V
~10 
I-V
~10ohms
65 ohms
30 ohms
VOH
V
VCC
Example: VOH = 1.5 V
Needs to agree with V-T data
30 ohm load line terminated
to ground this time)
I
Using IBIS Models in HSPICE
 Use the IBIS file presented earlier (10 ohm
up down resistor.
 Compare to
0-2V
.33ns r/f full
transition time
10 
Using prior HSPICE example and MYBUF
subciruit library and switch case with alters.
 New net list name: testckt_ibis.sp
44
45
Recall HSPICE Block Diagram
Printed Wiring
Board
package
package
Data generator
Buffers
Receiver
Create three libraries for MYBUF
 ‘driver’ – source/resistor model
 ‘driver_ibis’ – 10 ohm CMOS IBIS model
using ramp data
 ‘driver_ibis_two’ - 10 ohm CMOS IBIS model
2 V-t curves for rising and falling edges. (4
total)
 Good example to show how to use libraries.
In some cases we start with a behavioral model
move to a transistor model to fine tune the
buffer design and solutions space.
This modularity enables this migration path with
minimal impact to the system model.
46
The three alters produces .tr0, .tr1, .tr2
 Before the end statement insert the
alter statements
 Adjust the pulse source to .333 ns
47
Resistor Source Library
 Use delay to synchronize cases
 We will force IBIS to start on the 50%
point in the bit drive waveform
48
49
HSPICE IBIS example
 This is a simple





example. Many more
controls are possible
Buffer=2 tells hspice
to use an output
buffer model
Ramp_fwf and
ramp_rwf = 0 means
use the ramp
Ramp_fwf and
ramp_rwf = 2 means
use the 2 V-t curves
for each edge
The edges are scaled
by 1/10 also to match
the resistor/source
What does NINT do?
Results: first glance seem not bad
50
Closer look at rising wave
Ramp is
slightly
distorted
51
52
Closer look at falling edge
Ramp
produces
unexpected
results
Additional IBIS Modeling Information
 IBIS files can be tuned to produce
desired performance
 Simulator may vary on how the IBIS
files are used. Especially when the used
far away from the specified loads.
53
Bergeron Diagrams – Intro.
 A Bergeron diagram is another way of analyzing a transmission
line. It is useful to analyze:
Reflections from non-linear drivers or loads
Usage is in industry is low – Can do same with equations and
simulators.
 First example – analyze a low-to-high transition:
 Process
1.
2.
3.
4.
5.
6.
7.
Draw all I-V curves of transmitter and receiver
Transmission lines are load lines of 1/Zo or -1/Zo depending on
direction of wave.
Start at initial condition. For this case, it is 0V, 0A and move on
the transmission line slope to intersection of load.
Determine intersection V and I.
Create equation for transmission line with -1/Zo slope at the
intersection
Bounce back and forth using the parallel transmission line load
curves and the receiver load which is a 0v horizontal line for this
case and repeat until stable.
For this case, voltage on the load line is for Tx and a 0v is for Tx
54
Simple Bergeron Bounce Diagram Example
Why I-V’s work?
I
Initial Voltage Tx
Vs/R
Vs
R Pull-up
I-V curve
~R=10 
I-V
Zo=50 ohm
(open)
Open
load
line
V
1) Slope
of 1/Z0
t=0
2) Slope
of -1/Z0
V at Rx
55
Determine Initial Voltage
56
Bergeron Analysis
Vs  1
let
I
I
V
R

Vs
R
or
I
R  10 Zo  50
V  0  .1  2
f( V) Source Resistor Load Line ( More on f(V) later)
V
Zo
First Forward Wave Transmission Line Load Curve
0.12
0.096
V
Zo
0.072
 V Vs
 0.048
R R
Zo
Zo  R
0.024
0
Vs  0.833
Initial wave looks like the
voltage divider we expect
0
0.4
0.8
1.2
1.6
2
V
The intersection is where source resistor load line and
transmission line forward wave is
Vs
V
V Vs
V
Zo
Solve for V

R

Zo
at Tx
Zo
R
R
Determine first voltage step at Rx
Now the wave continues with as slope for -1/Zo from this point
The next task is to determine the equation of this line which has
the form
V
I mV  b -->
I
b
Zo
solve for b and substitute V and I
We can find b because we know one V,I point
Vs
b 2
Zo
Vs
R  Zo
Given
V
Vs and
I
Zo  R
Zo  R
V
Vs
I
 2
Zo
R  Zo
The open circuit receiver load line is horizontal line at
0 amps. This where the next wave reflects from. So
lets solve for V in the above for where I=0
0.12
V
Zo
0.096
 V Vs

R R
0.072
V
2
0.048
V
 Zo
2 
Vs
RZo
0.024
0
0
0.4
0.8
1.2
V
1.6
2
2
Vs
R  Zo
Vs
R  Zo
Zo
at Rx
Zo  1.667
57
58
Find next voltage at Tx again
Now the wave follows the 1/Zo I=mV+b and we solve for b again from above
2 
b
Vs
and
R  Zo
This line intersects the Tx load line
V Vs
V Vs
so
I


R
R
R
R
Vs 
V
3 R  Zo
( R  Zo)
2
Zo I
V
I
Vs 
Zo
V
Zo
 2 
 2 
Vs
R  Zo
Vs
R  Zo
Zo  R
( R  Zo)
at Tx
2
0.08
V
Zo
0.06
0.04
 V Vs

R R
V
 Zo
V
Zo
2 
 2 
0.02
Vs
Vs 
0
( R  Zo)
RZo 0.02
Vs 0.04
RZo 0.06
0.08
0.1
3 R  Zo
0
0.4
0.8
1.2
V
1.6
2
2
Zo  1.111
59
Find voltage at Rx again
The reflected wave follows a 1/-Zo line. Again the task is to find b. But since we know
a V and I above this is easy
V
I
Zo
b
when I=0
b
R
4 Vs 
( R  Zo)
V
R
4 Vs 
( R  Zo)
2
Then
2
Zo
I
4 Vs R
I
V
Zo
R
 4 Vs 
( R  Zo)
Zo  1
( R  Zo)
2
2
at Rx
0.08
V
Zo
0.06
 V Vs

R R
0.04
0.02
V
 Zo
V
Zo
2 
 2 
Vs
RZo
0
Vs
4 Vs 
R
( R  Zo)
0.02
2
Zo  0.556
RZo
0.04
V
 Zo
4  Vs 
R
( RZo)
2 0.06
0.08
0.1
0
0.4
0.8
1.2
V
1.6
2
And so on....
60
The non-linear case
Bergeron Analysis For Non-Linear I/V
let Vs  1
R  20 Zo  10
V  0  .01  2
5


2
  V 



 2
Vs 
Source I-V curve)
Ifct ( V)  

R 
 R
I
V
Zo
First Forward Wave Transmission Line Load Curve
0.12
0.096
V
Zo
0.072
Ifct ( V)0.048
0.024
0
0
0.4
0.8
1.2
V
Given
I0
2
V0
Zo
1.6
5
I0
 V0 


 2   Vs
R
R
2
61
Use MathCad Solve blocks at Tx
 4.8548445530883573148 10-2 
 I1 
 2
   Find ( I0  V0)  

 V1 
 .48548445530883573148 
 I1 
 0.049 
 

V1
0.485
  

at Tx
need to choose correct solution, look at graph
to pick
Given next line is
Given
I1
V1
b
Zo
b1  0.097
-2
b1  Find ( b)  9.7096891061767146296 10
62
First Step at the Rx
I2  0
at the axis
Given
V2
I2
Zo
 b1
V2  Find ( V2)  .97096891061767146296
0.12
0.096
V
V2  0.971
Zo
at Rx
0.072
Ifct ( V)
V
 Zo
0.048
b1
0.024
0
0
0.4
0.8
1.2
1.6
2
V
I3  0
Reflected line
Given
I2
I3
V2
Zo
V3
Zo
 b2
 b2
-2
b2  Find ( b2)  9.7096891061767146296 10
63
Assignment:
.12
V
Zo
0.12
Solve for next voltage
and current at Rx
0.096
Ifct ( V) 0.072
V
b1
 Zo
0.048
V
b2
Zo
0.024
0
0
0
0
0.4
0.8
1.2
V
1.6
2
2
Example: Under-damped Case with Diode
64
 Multiple I/V curves can be overlaid to estimate
performance
In this case an ideal diode’s I-V characteristics gives a feel
for what to expect
I
20 ohms
60 ohms
Vcc = 2V
Pull-up
I-V curve
2V
1V
1/Z0
Diode
I-V curve
-1/Z0
V
Vcc
t=0
TD
2TD
3TD
4TD
5TD
6TD
Linear vs. Non-linear
65
 The accuracy of a linear approximation can be
determined with a Bergeron diagram:
Voltages from the
reflections are close to
linear approximation
I
PMOS curve
NMOS curve
Voltages from the
reflections are NOT close
to linear approximation
1/Zo
V
I
1/Zo
V
Summary: We now understand









What is a model?
Importance of accurate models
Types of buffer models
IBIS and the portions of an IBIS model
How model data is generated
How to calculate VOL and VOH from a model
On-die termination
Package modeling in IBIS
Bergeron diagrams
66