HSPICE Lab Worksheet
Lab Title: Static Noise Margin (SNM) Analysis of a 6T SRAM Cell
Objective:
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To analyze the stability of a 6T SRAM cell under different process corners.
To compute the Static Noise Margin (SNM) using the butterfly curve method.
To evaluate the impact of transistor sizing on SNM.
Background:
Static Noise Margin (SNM) is a key metric that quantifies the stability of an SRAM cell
by measuring the maximum tolerable noise voltage before data loss occurs. The SNM is
extracted from the butterfly curves obtained by sweeping the voltage at storage nodes
while maintaining bitline conditions.
Pre-Lab Questions:
1. Define Static Noise Margin (SNM) and explain its significance in SRAM
stability.
2. Why are different process corners (e.g., TT, FF, SS) analyzed in SRAM design?
3. How does transistor sizing (e.g., varying pull-up PMOS, pass-gate NMOS, and
pull-down NMOS ratios) affect the SNM?
4. Describe the basic structure of a 6T SRAM cell and its operation modes (Hold,
Read, Write).
Lab Exercises:
1. Setting Up the Simulation:
o Open the HSPICE environment and load the 6T SRAM netlist.
o Ensure that process models (e.g., 7nm_TT, 7nm_FF) are correctly
included.
2. Butterfly Curve Generation:
o Apply a DC sweep to the storage nodes.
o Extract the VTC (Voltage Transfer Characteristic) curves for both
inverters.
o Plot the butterfly curve.
3. SNM Extraction:
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Identify the maximum square that fits inside the butterfly curve.
Measure the SNM for each process corner.
4. Impact of Transistor Sizing:
o Simulate SNM for different transistor ratios (1:1:1, 1:2:2, etc.).
o Compare the SNM values.
Post-Lab Questions:
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What SNM values did you obtain for each process corner?
How does increasing the pull-down NMOS size affect the SNM? Why?
What is the impact of varying the pass-gate NMOS width on read stability?
How does SNM variation relate to SRAM yield and reliability in advanced
technology nodes?
5. If SNM is too low, what circuit design techniques can improve stability?
Submission Requirements:
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Attach HSPICE netlists used in the simulation.
Provide waveform plots for butterfly curves and SNM measurements.
Answer all pre-lab and post-lab questions in a structured report.
Grading Criteria:
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Correctness of simulation setup and execution (40%)
Accuracy of SNM extraction and analysis (30%)
Completeness and clarity of answers (20%)
Presentation of results (10%)
End of Worksheet