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Designing & Simulating Logic Gates in DSCH & Microwind – A Beginner’s Guide

Designing & Simulating Logic Gates in DSCH & Microwind – A Beginner’s Guide

Learn the fundamentals of digital logic design with DSCH and Microwind, simulate basic gates, and challenge yourself with a hands-on VLSI design task!

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Ayushi Lathiya
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6 min read

Table of contents

Introduction

In the field of VLSI (Very-Large-Scale Integration) design, digital circuits undergo a multi-step process before they can be physically fabricated. The initial stages involve designing and simulating circuits to ensure functionality, followed by the conversion of logical designs into physical layouts for CMOS implementation. This is where two powerful tools—DSCH (Digital Schematic Editor) and Microwind—play a critical role in the design workflow.

Getting Started with DSCH

  • What is DSCH?

    DSCH is a graphical tool that allows users to design digital circuits and simulate their behavior using logic signals. It is commonly used to verify Boolean functions, truth tables, and timing diagrams before proceeding to layout design.

  • Designing Basic Gates in DSCH

    In VLSI design, CMOS (Complementary Metal-Oxide-Semiconductor) technology is the preferred choice for implementing digital logic gates.

Why is CMOS Technology Preferred?
CMOS (Complementary Metal-Oxide-Semiconductor) technology is widely used in digital circuit design due to its low power consumption, as it only draws power during state transitions, unlike NMOS- or PMOS-only logic families that continuously dissipate energy. It also offers high noise immunity, making circuits more stable and resistant to electrical fluctuations. Additionally, CMOS is highly scalable, allowing for the miniaturization of transistors in modern VLSI systems, which is essential for integrating millions of logic gates in a single chip efficiently.
How are CMOS Logic Gates Built?
In CMOS (Complementary Metal-Oxide-Semiconductor) circuits, logic gates are built using two types of transistors: PMOS (P-type MOSFETs) and NMOS (N-type MOSFETs). These transistors work together to ensure low power consumption and efficient switching. PMOS transistors are used in the Pull-Up Network (PUN) and conduct when the input is LOW (0), meaning they allow current to flow and pull the output HIGH (1). NMOS transistors are used in the Pull-Down Network (PDN) and conduct when the input is HIGH (1), meaning they create a path to ground and pull the output LOW (0). By strategically arranging NMOS and PMOS transistors, we can implement different logic gates, ensuring they switch efficiently with minimal power loss.
Basic Design Implementations
In CMOS design, AND and OR gates are not implemented directly because NAND and NOR gates are more efficient in terms of transistor count, power consumption, and speed. NAND and NOR are known as universal gates since they can be used to build any logic function.

Let’s start by designing NOT and NAND gates using DSCH.

  • NOT GATE

  • Truth Table:

AY(NOT A)
01
10

  • A NOT gate (inverter) is made using one PMOS transistor and one NMOS transistor.

  • The PMOS is placed at the top and the NMOS at the bottom.

  • The source of the PMOS is connected to VDD, and the source of the NMOS is connected to GND.

  • The drains of both transistors are connected together to form the output. The gates of both transistors are tied together to form the input.

  • NAND GATE

  • Truth Table:

ABY (A NAND B)
001
011
101
110

NAND is implemented by connecting two NMOS structures in series and its complementary connections in the Pull Up Network [i.e. connecting PMOS structures in parallel].

  • Like the NOT gate implementation, provide a source and destination to the CMOS implementation.

  • NOTE: Basic implementation is different from optimised implementation, each has slight variations on the output.

Similarly you could try implementing NOR, OR, AND gates!

💡
Hover over the Resources section to find a GitHub repository containing the DSCH & MW implementations of all the gates!

Transitioning to Microwind

  • What is Microwind?

    Microwind is a VLSI layout editor used for designing and simulating CMOS circuits. It takes DSCH schematics and converts them into physical layouts using MOSFET technology.

  • Converting DSCH Design to Microwind

    1️⃣ In DSCH, go to File → Convert to Layout.
    2️⃣ Save the file as .mw and open it in Microwind.
    3️⃣ Microwind will automatically generate the CMOS layout for the circuit.

Showcasing Basic Logic Gates

The basic implementation is same but the layout is closer to the actual transistor-level design.

The software has simulation tools for timing analysis, voltage analysis, and checking the power usage of the layout.

How to Learn Microwind for VLSI Design?
To learn Microwind, start by understanding the basics of CMOS technology and VLSI design principles. Use tutorials and guides that explain transistor-level layout design. Practice designing simple circuits like logic gates (NOT, AND, OR) and move on to more complex systems. Explore Microwind's graphical user interface and experiment with the placement of components like PMOS and NMOS transistors. Gradually, try simulating your designs to analyze timing and power performance. Consistent practice and working through real-world examples will improve your proficiency.
What Are Design Rules and Why Are They Important in Layout Design?
Design rules are a set of guidelines that dictate the minimum size, spacing, and alignment of components in a layout. These rules ensure that the layout is manufacturable, functional, and free from errors like shorts or open circuits. Following design rules is crucial in VLSI layout design because they prevent manufacturing defects, optimize yield, and ensure electrical performance. They also help maintain reliability and consistency across different processes and technologies used in chip fabrication.

Let’s take an instance of NOT Gate in MW.

  • NOT GATE

This implementation is designed using Lambda Design Rules

This layout design is comprised of PMOS, NMOS along with the layer of Polysilicon as link input. The short drains of PMOS and NMOS provide the output. Since the entire plane is PMOS.

Simulation & Verification

Post-layout simulation in Microwind helps analyze delay, power, and transistor-level behavior. It provides insight into circuit performance in real-world conditions.

In the simulation settings, select the type of simulation you want to perform. For a NOT gate, the transient analysis is most useful, as it will allow you to see how the output changes over time when the input signal toggles.

The simulation of the NOT gate we implemented would be displayed as follows

The Voltage vs Time graph will show:

  • A square waveform for both input and output signals.

  • Opposite phases: When the input is at 0V, the output will be at VDD (high), and when the input is at VDD (high), the output will be at 0V (low).

💡
Hover over the Resources section to find a GitHub repository containing the DSCH & MW implementations of all the gates!

  • Analyze the Results:

    Check the timing diagram to confirm that the output exactly inverts the input signal, which is the expected behavior for a NOT gate.

    If any issues arise (e.g., delay, incorrect output), you can debug the layout or adjust the parameters in the simulation settings.

Resources

  1. DSCH & Microwind Official Guide - [ Read Guide ]

  2. Github Repo - [ View on Github ]

  3. Download the software and access support - [ Microwind Official Website ]

  4. Community-driven Q&A forum for troubleshooting and advice - [ Stack Overflow Microwind Tag ]

  5. Microwind Tutorials - [ YouTube ]

Have fun learning and designing :)

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