02.
August 2007
IBIS4.2 FOR
DDR2 TIMING ANALYSIS
September 11, 2007
Asian IBIS Summit, Beijing China
Guan Tao www.huawei.com
guantao@huawei.com
HUAWEI TECHNOLOGIES Co., Ltd.
Page 1
HUAWEI TECHNOLOGIES CO., LTD.
Outline
¾ New technologies have brought challenges to IBIS model
¾ DDR2 timing analysis
• The read cycle timing analysis
• The write cycle timing analysis
¾ IBIS4.2 model meets the challenges
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Challenges
¾ More SI and timing problems emerge as the rate of bus increases , these
problems can be eliminated by new techniques :
• ODT (On Die Termination);
• OCD (Off-chip Driver).
¾ Challenges of DDR2 modeling:
• How to model the ODT circuit;
• How to use the slew rate table.
The setup and holdup time are varying with the slope of the signal in
DDR2.
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DDR2 Timing Analysis Method
A proper method of a DDR2 timing analysis should contain the following essentials:
¾ Seeking for a balance between simulation efficiency and waveform quality.
¾ The parallel simulation and crosstalk can be processed by EDA platforms.
¾ The EDA platform can process the PRBS (pseudo-random bit sequence) code;
¾ Topology 、matching and transmitter/receiver buffer can be automatically analyzed
¾ The settle and switch delay between standard load and practical load can be
considered;
¾ Influence of voltage and temperature can be included in the simulation;
¾ The ODT circuit and slew rate table can be considered.
Current simulation method is not good enough to support the features listed here!
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Timing Relationships of DDR2
Three timing relationships in DDR2 timing analysis:
¾ ADDRESS、COMMAND and CK/CK#;
¾ DQ and DQS/DQS#;
¾ DQS and CK/CK#.
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Data Valid Window
The SSTL_18 is intended for DDR2 interface application.
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DDR2 Timing Analysis with IBIS 3.2
Following is an example of timing analysis of DQ and DQS. The
ADDRESS/COMMAND timing analysis is similar.
¾ The topology of DQ/DQS is point to point feature.
¾ The models of CONTROLLER and MEMORY are IBIS3.2 model.
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Read Cycle Timing Analysis (fast)
In fast mode, the simulated eye diagram of read cycle is as follow.
data valid window :1.255ns
waveform distortion: 0.245ns
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Read Cycle Timing Analysis (slow)
In slow mode, the simulated eye diagram of read cycle is as follow.
data valid window: 1.217ns
waveform distortion :0.283ns
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Read Cycle Timing Calculation
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Write Cycle Timing Analysis (fast)
In fast mode, the simulated eye diagram of write cycle is as follow.
data valid window :1.426ns
waveform distortion :0.074ns
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Write Cycle Timing Analysis (slow)
In slow mode, the simulated eye diagram of write cycle is as follow.
data valid window:1.271ns
waveform distortion: 0.229ns
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Write Cycle Timing Calculation
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Drawbacks
¾ Inefficient
The method of DDR2 timing analysis above is a time-consuming and
manual procedure. Simulations are run on each net to generate eye diagrams,
through which signal integrity and timing characteristics of an individual net
can be evaluated.
¾ Sometimes meaningless
The simulation is meaningless, when the margin is negative.
The DDR2 timing analysis is made under absolute worst-case conditions.
Owing to the worst-case prediction of all parameters applied to a single net,
the margin analysis method described above may predict a failure when in
fact the actual design is sound.
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Deficiencies of IBIS3.2
IBIS3.2 model has the following several deficiencies for timing analysis of
DDR2:
1. The IBIS3.2 model is not programmable, so that it is difficult to implement
a flexible simulation and process slew rate table automatically.
2. The IBIS3.2 model is not flexible enough to accommodate new techniques.
3. The IBIS3.2 model is a parameterized model, not able to process the
simulated waveforms.
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IBIS4.2 Model Meets the Challenges
Randy Wolff 、Gary L. Pratt. P.E and their collaborators solve these
problems in IBIS4.2 model. IBIS4.2 is an extension of IBIS3.2.
¾ IBIS4.2/AMS multiplexer permits the simulator to select between an IBIS
model with ODT and one without ODT;
¾ Supports seamless switch of read cycle and write cycle simulation;
¾ Through running a section of VHDL code, the simulator is able to measure
the slope of relevant signals;
¾ Automatically determines the corresponding value in the slew rate table;
It’s a good solution to problems we are facing now.
HUAWEI TECHNOLOGIES Co., Ltd. Page 16
IBIS 4.2 for Timing Analysis
Using the IBIS4.2 model ,we can make the following measurements automatically:
¾ Automatic overshoot area measurements
¾ Automatic setup and holdup timing measurements
¾ Slew-dependent timing calculation
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One Solution of ODT Circuit
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VHDL-AMS Code for Multiplexer
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Slew Rate Table Calculation
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Output Results
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Advantages
¾ Flexible
The IBIS4.2 model is programmable, so that the model is flexible and
appropriate to new technology.
¾ Real-time
The margin is properly measured on every edge in the method of timing
analysis, which takes the influence of slew rate table into consideration.
¾ Automatic
The procedure of timing analysis is automatic and prompt. The output
results are reasonable and simple.
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Requirements from System Vendors
The IBIS4.2 model is a good solution to the high rate bus of memory timing
analysis,
1. There is still few EDA vendors to fully support the IBIS4.2 model;
2. Due to no spice2AMS tool in place, the majority of IC vendors do not have a
schedule to provide the IBIS4.2 model;
3. The results are consistent , when we use the same IBIS model on different
platforms;
4. The IBIS model should be accurate enough to depict electrical characteristics of
chip properly;
5. Building the package model should be paid more attention to as the bus rate
increases;
The development of IBIS needs our efforts！
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