Lab Procedures & Experiments - 4 | Lab Module 3: CMOS Inverter Switching Characteristics & Delay Analysis | VLSI Design Lab
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4 - Lab Procedures & Experiments

Practice

Interactive Audio Lesson

Listen to a student-teacher conversation explaining the topic in a relatable way.

Transient Simulations

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0:00
Teacher
Teacher

Today, we're going to begin with transient simulations. Can anyone tell me what a transient simulation involves in the context of a CMOS inverter?

Student 1
Student 1

It’s about simulating how the inverter responds over time, right?

Teacher
Teacher

Exactly! We want to capture the dynamic behavior as the input changes. We create a schematic with NMOS and PMOS transistors and connect them to form our inverter. What parameters do we define for the input pulse?

Student 2
Student 2

We need the rise and fall times, pulse width, and period.

Teacher
Teacher

Correct! It’s important to have these settings right. Now, if we want a clearer visualization of the results, what should we do after running our simulation?

Student 3
Student 3

We should plot the input and output voltage waveforms together!

Teacher
Teacher

Great! Remember to take clear screenshots of these plots for your lab report. Key point here: capturing the waveform helps in understanding the switching characteristics of the inverter.

Propagation Delay Measurement

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0:00
Teacher
Teacher

Now let's discuss how to measure propagation delays. What specific delays are we interested in measuring?

Student 4
Student 4

We measure tpHL and tpLH, right?

Teacher
Teacher

That's correct! tpHL is from the input's rising edge to the output's falling edge, and tpLH is the reverse. How do we find the 50% VDD crossing points on our waveforms?

Student 1
Student 1

We could use waveform cursors to pinpoint where the waveforms cross 50% of VDD.

Teacher
Teacher

Exactly! Using cursors ensures accuracy in our measurements. Why is it important to average the two delays?

Student 2
Student 2

To get a more balanced measure of the total delay, right?

Teacher
Teacher

Exactly! Understanding these delays will help us analyze the dynamic performance of the inverter.

Effect of Load Capacitance

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0:00
Teacher
Teacher

Next, we're going to look at how varying load capacitance impacts our measured delays. What is our approach here?

Student 3
Student 3

We can set up a parametric sweep for different capacitance values!

Teacher
Teacher

Exactly! By sweeping values from 10 fF to 1 pF, we can observe trends in propagation delay. What trend do we expect to see?

Student 4
Student 4

As load capacitance increases, the propagation delay should also increase.

Teacher
Teacher

Precisely! This relationship is crucial for understanding how load impacts our circuit design. Make sure to document your findings thoroughly.

Transistor Sizing Effects

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0:00
Teacher
Teacher

Now let’s discuss transistor sizing. When we vary the W/L ratio of NMOS and PMOS, what are we trying to achieve?

Student 1
Student 1

We want to balance the rise and fall times for the inverter.

Teacher
Teacher

Correct! What is the significance of achieving balanced delays?

Student 2
Student 2

Balanced delays can improve the overall performance of the inverter, right?

Teacher
Teacher

Absolutely! It also results in less skew and better signal integrity. What’s a good method to find an optimal sizing ratio?

Student 3
Student 3

We should measure delays while adjusting W/L ratios and find a point where tpHL is approximately equal to tpLH.

Teacher
Teacher

Exactly! That's the key to optimizing our design. Great understanding!

Power Analysis

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0:00
Teacher
Teacher

Finally, let's look at power analysis. What do we mean by dynamic power?

Student 4
Student 4

It's the power consumed during switching, right?

Teacher
Teacher

Right! And how do we measure it?

Student 1
Student 1

We can plot the instantaneous power using the VDD and the current delivered.

Teacher
Teacher

Exactly! And what about static power?

Student 2
Student 2

It’s the power consumed when the inverter is not switching, right?

Teacher
Teacher

Correct! Understanding both power components is essential for optimizing inverter designs, focusing on both efficiency and performance. Remember to compare measured values with theoretical calculations!

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

This section outlines the lab procedures for analyzing the switching characteristics and delay optimization of CMOS inverters through dynamic performance assessments.

Standard

In the lab module, students will conduct experiments on CMOS inverters focusing on transient simulations, propagation delay measurements, effects of load capacitance and transistor sizing, and power analysis. Each experiment is designed to elucidate core concepts of CMOS circuit behavior and guide through practical application and analysis.

Detailed

Lab Procedures & Experiments

In this section, we explore the laboratory procedures and experiments essential for understanding the characteristics of CMOS inverters. The focus is on capturing dynamic performance and conducting delay analysis through six comprehensive experiments that allow students to analyze their findings thoroughly.

Lab Objectives

Students will achieve several objectives:
1. Transient Simulations: Setting up simulations for dynamic input/output waveform analysis.
2. Propagation Delays: Measuring delays and understanding the influence of load capacitance.
3. Transistor Effects: Analyzing how NMOS and PMOS sizing impacts delay.
4. Power Analysis: Calculating dynamic and static power dissipation.
5. Design Constraints: Applying iterative methods for inverter sizing per defined specifications.

Procedures

Each experiment is methodical, beginning with schematic setup, input signal design, load definitions, simulated runs, and culminating in detailed analysis, plotting, and documentation of results.
1. Experiment 1 focuses on basic transient responses.
2. Experiment 2 measures propagation delays accurately using waveform cursors and automated functions.
3. Experiment 3 examines the impact of load capacitance on delay.
4. Experiment 4 investigates transistor sizing effects on propagation delays and optimal design for balanced delays.
5. Experiment 5 introduces dynamic and static power measurements in the balanced inverter.
6. Experiment 6 details the iterative design process to meet specific delay targets.

This structured approach enables students to not only perform but also derive meaningful interpretations from their data, reinforcing their understanding of CMOS inverter behavior and circuit design.

Audio Book

Dive deep into the subject with an immersive audiobook experience.

Experiment 1: Basic CMOS Inverter Transient Response

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  1. Objective: Observe the fundamental transient behavior of a CMOS inverter.
  2. Procedure:
  3. Schematic Setup:
  4. Create a new schematic in your chosen simulator.
  5. Instantiate one NMOS and one PMOS transistor.
  6. Connect them to form a CMOS inverter: PMOS source to VDD, NMOS source to GND, gates connected for input, drains connected for output.
  7. Connect VDD and GND global nets.
  8. Transistor Sizing (Initial):
  9. Set initial W/L ratios. For this experiment, use:
    • NMOS: W=0.5μm, L=0.18μm (or your technology's minimum length).
    • PMOS: W=1.0μm, L=0.18μm (to achieve a common initial PMOS/NMOS width ratio of 2 for balanced delays).
  10. Input Signal Definition:
  11. Add a voltage source at the input (e.g., a VPULSE source in SPICE).
  12. Configure the pulse:
    • V1 =0V (Low voltage)
    • V2 =VDD (High voltage, use your technology's nominal VDD, e.g., 1.8V for 0.18 μm)
    • Tdelay =0s
    • Trise =1ns
    • Tfall =1ns
    • Tpulse =50ns (Pulse width)
    • Tperiod =100ns (Total period, ensures multiple cycles)
  13. Load Definition:
  14. Add a load capacitance (C) of 50 fF (femtoFarads) from the output node to GND. This represents the parasitic capacitance of interconnects and input capacitance of subsequent gates.
  15. Simulation Setup:
  16. Select "Transient Analysis."
  17. Set "Stop Time" to 200ns (to observe at least two full cycles).
  18. Set "Maximum Timestep" to 0.1ns (or smaller for finer resolution).
  19. Run Simulation: Execute the simulation.
  20. Waveform Analysis:
  21. Plot the input voltage and output voltage waveforms on the same graph.
  22. Capture a screenshot of the input and output waveforms, clearly showing at least one complete switching cycle.

Detailed Explanation

In Experiment 1, the main goal is to observe how a CMOS inverter behaves when subjected to varying input signals. To do this, students first set up a schematic that includes an NMOS and PMOS transistor connected to form a basic CMOS inverter. The inverter's performance relies on the appropriate sizing of the transistors and the chosen input signal. Once the schematic is ready, students configure the input pulse, which alternates between high and low values, and attach a load capacitance that simulates the effect of connecting to subsequent gates. The experiment culminates in executing a transient simulation, allowing students to visualize how the input affects the output by plotting the corresponding waveforms.

Examples & Analogies

Think of the CMOS inverter like a light switch in a circuit connected to a light bulb. When you flip the switch (input signal), it determines whether the light is on or off (output signal). In this experiment, we observe how quickly the light turns on and off when you flick the switch, noting how the amount of wiring (load capacitance) affects the delay in the light's response.

Experiment 2: Measurement of Propagation Delays

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  1. Objective: Accurately measure tpHL, tpLH, and tp from the simulated waveforms.
  2. Procedure:
  3. Using Waveform Cursors:
  4. Navigate to your simulation results viewer.
  5. Place two cursors on the input waveform. Identify the time point where the input crosses 50% of VDD (for both rising and falling edges).
  6. Place two other cursors on the output waveform. Identify the corresponding time points where the output crosses 50% of VDD.
  7. For tpHL: Measure the time difference between the 50% point of the input rising edge and the 50% point of the output falling edge.
  8. For tpLH: Measure the time difference between the 50% point of the input falling edge and the 50% point of the output rising edge.
  9. Calculate tp = (tpHL + tpLH)/2.
  10. Using Automated Measurement Functions (If Available): Explore and utilize your simulator's built-in functions for delay measurement (e.g., MEASURE TRAN commands in SPICE, specific waveform calculator functions). This is generally more precise.
  11. Record Results: Create a table in your lab notebook or a spreadsheet to record the measured values of tpHL, tpLH, and tp.

Detailed Explanation

In Experiment 2, the purpose is to measure the propagation delays of a CMOS inverter, which includes both the high-to-low delay (tpHL) and the low-to-high delay (tpLH). By using waveform cursors on the simulation results, students identify when the input voltage reaches half of its maximum value (50% of VDD). This point marks the transition, allowing them to measure how long it takes for the output to respond to changes in the input. By averaging these delays, students can derive a clearer picture of the inverter's performance, which is crucial for designing effective digital circuits.

Examples & Analogies

Measuring propagation delays is similar to timing how quickly a runner starts moving after hearing a starting gun. The point where the runner crosses a certain mark (like halfway down the track) corresponds to how long it takes them to react and get going. Similarly, we measure the input signal's halfway point to determine how swiftly the output responds after changes occur.

Experiment 3: Impact of Load Capacitance on Delay

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  1. Objective: Investigate how varying the load capacitance affects the inverter's propagation delay.
  2. Procedure:
  3. Modify Existing Schematic: Use the inverter schematic from Experiment 1.
  4. Parametric Sweep Setup: Configure a parametric sweep for the load capacitance (C_load).
  5. Sweep Values: Vary C_load over a range (e.g., 10 fF, 20 fF, 50 fF, 100 fF, 200 fF, 500 fF, 1 pF).
  6. Run Sweep: Execute the transient simulation with the capacitance sweep.
  7. Data Collection: For each C_load value:
  8. Measure tpHL, tpLH, and tp.
  9. Record all measured values in a table.
  10. Plotting:
  11. Create a graph plotting tp (Y-axis) against C_load (X-axis).
  12. Capture a screenshot of this plot.
  13. Analysis: Observe and describe the relationship between tp and C_load.

Detailed Explanation

In Experiment 3, students will explore how varying the load capacitance connected to the output of the inverter impacts its propagation delay. By modifying the existing schematic and setting up a parametric sweep, they will test a series of load capacitance values. After running simulations for each capacitance setting, students will measure the propagation delays and compile the results into graphs. This will help illustrate the direct relationship between load capacitance and delay, which is vital in understanding circuit design considerations in real-world applications.

Examples & Analogies

This experiment can be likened to understanding how a heavier load on a bicycle affects its acceleration. Just as adding more weight (load capacitance) to the bike slows down how fast it can start moving (propagation delay), similarly, increasing the load capacitance on the inverter slows down its response time. By experimenting with different weights, we can see how much delay changes, guiding us on how best to design responsive systems.

Experiment 4: Impact of Transistor Sizing (W/L) on Delay

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  1. Objective: Analyze how adjusting the W/L ratios of NMOS and PMOS transistors influences propagation delays and how to achieve balanced delays.
  2. Procedure:
  3. Reset Load: Set C_load back to 50 fF (or a fixed reasonable value).
  4. Part A: Varying NMOS Width (W_N) while keeping PMOS fixed:
  5. Keep PMOS: W=1.0μm, L=0.18μm.
  6. Vary NMOS WN: 0.25μm, 0.5μm, 1.0μm, 2.0μm, 4.0μm. Keep LN =0.18μm.
  7. For each WN, measure tpHL, tpLH, and tp. Record in a table.
  8. Part B: Varying PMOS Width (W_P) while keeping NMOS fixed:
  9. Keep NMOS: W=0.5μm, L=0.18μm.
  10. Vary PMOS WP: 0.5μm, 1.0μm, 2.0μm, 4.0μm, 8.0μm. Keep LP =0.18μm.
  11. For each WP, measure tpHL, tpLH, and tp. Record in a table.
  12. Part C: Achieving Balanced Delays:
  13. Based on your findings from Part A and B, determine an optimal PMOS W/L to NMOS W/L ratio (β ratio) that results in tpHL ≈ tpLH.
  14. Set the transistor sizes accordingly (e.g., if you found β≈2, set WN =0.5μm, WP =1.0μm).
  15. Measure tpHL, tpLH, and tp for this "balanced" inverter.
  16. Plotting:
  17. Create plots showing tpHL, tpLH, and tp vs. NMOS Width (from Part A) and PMOS Width (from Part B).
  18. Capture screenshots of these plots.
  19. Analysis: Discuss the individual and combined effects of WN and WP on delays. Explain why a specific β ratio is often chosen.

Detailed Explanation

In this Experiment 4, students delve into how the width-to-length ratios (W/L) of NMOS and PMOS transistors impact the propagation delay of the inverter. By systematically adjusting one transistor type while keeping the other constant, students can isolate the effects of transistor sizing on performance. Recording their results allows them to identify a balanced ratio that optimizes performance between the two types of transistors, achieving a delay that's acceptable for many digital applications. This balance is crucial since discrepancies can lead to inefficient circuit designs and performance issues.

Examples & Analogies

Adjusting the W/L ratio is like changing the size of a water faucet to regulate flow. A wider faucet (larger W) allows more water to flow quickly, akin to a faster signal. However, if only one faucet is wide while the other remains narrow (unbalanced ratios), the overall system doesn’t function efficiently—just like in circuits, where imbalanced transistors lead to delays.

Experiment 5: Introduction to Power Analysis

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  1. Objective: Measure and understand dynamic and static power dissipation in the CMOS inverter.
  2. Procedure:
  3. Use Balanced Inverter: Use the balanced inverter from Experiment 4, Part C, with C_load = 50 fF.
  4. Part A: Dynamic Power Measurement:
  5. Set input T_period to 100ns (equivalent to fclock = 10 MHz).
  6. Run transient simulation.
  7. Measure Dynamic Power:
    • In your simulator, plot the instantaneous power delivered by the VDD source (P(t)=VDD⋅ IDD(t)).
    • Use the average measurement function over several full cycles (e.g., from 50ns to 150ns to avoid initial transients) to find the average dynamic power.
    • Record this value.
    • Verify with Formula: Calculate Pdynamic = αCload VDD^2 fclock. Assume α=1 (since the inverter switches every cycle). Compare with your measured value.
    • Effect of Frequency: Repeat the dynamic power measurement for T_period = 200ns (fclock = 5 MHz) and T_period = 50ns (fclock = 20 MHz). Record and discuss the trend.
  8. Part B: Static Power Measurement:
  9. Method 1 (DC Operating Point):
    • Change the input voltage source to a DC voltage source, setting its value to 0V (logic LOW) and run a DC operating point analysis.
    • Measure the IDDQ (quiescent supply current) from the VDD source. Calculate Pstatic = VDD ⋅ IDDQ.
    • Repeat for input voltage set to VDD (logic HIGH). You should observe very low current in both cases for an ideal CMOS inverter (some leakage will be present in real models).
  10. Method 2 (Long Transient):
    • Set the input pulse to stay at 0V for a very long duration (e.g., Tpulse = 500ns, Tperiod also very long).
    • Run a transient simulation for a duration where the output is stable (e.g., 500ns).
    • Measure the average supply current (IDD) from the VDD source during the stable period.
    • Calculate Pstatic = VDD ⋅ IDD.
    • Record Results: Note down the static power measured by both methods.

Detailed Explanation

Experiment 5 focuses on analyzing the power dissipation characteristics of a CMOS inverter. Students will differentiate between dynamic and static power, learning how dynamic power depends on switching activity, while static power is associated with leakage current when the circuit is idle. By assessing both dynamic and static power, students gain insights into the energy efficiency of their designs, which is crucial in modern electronics. They will conduct simulations based on established methods and formulas to verify their findings, cementing their understanding of power management in circuit design.

Examples & Analogies

Power analysis in this context is similar to monitoring the energy use of your home. Dynamic power is like the energy consumed when you turn on lights (the amount fluctuates with usage), while static power is similar to energy lost when devices are plugged in but not active (vampire energy). Realizing how to manage both helps you make informed decisions on energy efficiency and savings.

Experiment 6: Designing an Inverter for Specific Delay Constraints

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  1. Objective: Apply iterative design principles to meet a target propagation delay.
  2. Procedure:
  3. Design Specification:
  4. Your goal is to design an inverter with an average propagation delay (tp) of approximately 25 ps (picoseconds) while driving a load capacitance of 100 fF.
  5. Use VDD from your technology model.
  6. Maintain the PMOS/NMOS width ratio for balanced delays found in Experiment 4, Part C.
  7. Iterative Design Steps:
  8. Initial Guess: Start with the "balanced" inverter from Experiment 4.
  9. Simulate: Run a transient simulation with the 100 fF load.
  10. Measure: Determine the current tp.
  11. Adjust:
    • If tp is too high (slower than 25 ps), incrementally increase both NMOS and PMOS widths (maintaining your chosen β ratio).
    • If tp is too low (faster than 25 ps) or you want to minimize area/power, incrementally decrease both widths.
  12. Repeat: Continue simulating, measuring, and adjusting until you achieve a tp close to 25 ps (e.g., within +/- 5%).
  13. Record Final Design:
  14. Note down the final W/L ratios of your NMOS and PMOS transistors.
  15. Record the final measured tpHL, tpLH, and tp.
  16. Waveform Capture: Take a screenshot of the input and output waveforms for your final optimized design, highlighting the measured delay.
  17. Power Calculation: Calculate the dynamic and static power dissipation for your final optimized inverter.

Detailed Explanation

In Experiment 6, students engage in iterative design, improving the performance of a CMOS inverter by targeting a specific propagation delay. They start with a balanced design and analyze its performance against the set goal of achieving a propagation delay of 25 ps. By adjusting the transistor sizes methodically and retesting, students experience firsthand how careful tweaking can enhance circuit performance. This process highlights the importance of balancing speed, power consumption, and area in modern digital designs.

Examples & Analogies

This design process is akin to adjusting a recipe to achieve the perfect dish. Initially, you follow a balanced recipe (like the balanced inverter) but find you want it a little spicier (target delay). By incrementally adding seasonings (adjusting transistor sizes) and tasting it each time (simulating and measuring), you refine the recipe to your preference. Just as with food, achieving the right combination involves a process of trial and error, guided by results from each iteration.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Transient Simulation: Analyzing dynamic behavior of a CMOS inverter.

  • Propagation Delay: Critical measurement in circuit design representing switching times.

  • Load Capacitance: Significant factor affecting switching delays.

  • Transistor Sizing: Adjusting W/L ratios to optimize inverter performance.

  • Power Analysis: Understanding dynamic vs static power dissipation.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • Setting up a CMOS inverter circuit in simulation software to observe its transient response.

  • Measuring propagation delays using waveform analysis tools and comparing them with theoretical values.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • CMOS inverts with a twist, output low when input’s a fist.

📖 Fascinating Stories

  • Imagine a seesaw where one end goes up when the other goes down; that’s how a CMOS inverter works with its inputs and outputs.

🧠 Other Memory Gems

  • Delays are measured as TP - Time Parts: to remember tpHL and tpLH easily.

🎯 Super Acronyms

POWER for remembering Dynamic and Static Power

  • P: = Power
  • O: = Output
  • W: = Watts
  • E: = Energy
  • R: = Resistance.

Flash Cards

Review key concepts with flashcards.

Glossary of Terms

Review the Definitions for terms.

  • Term: Propagation Delay (tpHL, tpLH)

    Definition:

    The time it takes for the output to transition from high to low (tpHL) or low to high (tpLH) in response to input signal changes.

  • Term: Load Capacitance (C_load)

    Definition:

    The capacitance connected at the output of the CMOS inverter, affecting its response time.

  • Term: W/L Ratio

    Definition:

    The width to length ratio of the transistors, crucial for determining drive strength and performance characteristics.

  • Term: Dynamic Power

    Definition:

    Power dissipated in a CMOS inverter during the switching activities, proportional to the frequency of operation and load capacitance.

  • Term: Static Power

    Definition:

    Power consumed in a CMOS inverter when it is not actively switching, often due to leakage currents.