The-VLSI-Learning-Journey
Day 2: Understanding Logic Gates and Building with CMOS

Day 2: Understanding Logic Gates and Building with CMOS

Building the Foundation: Designing Logic Gates with CMOS Technology

Chandrakiran G's photo
Chandrakiran G
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3 min read

Welcome back, readers! It’s Day 2 of my VLSI design journey, and today, I focused on understanding the core building blocks of digital circuits: logic gates. Logic gates are fundamental to VLSI design, as they form the backbone of every digital system. Let’s dive into what I learned and built today!

What Are Logic Gates?

Logic gates are electronic circuits that perform logical operations on one or more binary inputs to produce a single binary output. These include basic gates like AND, OR, NOT, and more complex gates like NAND, NOR, XOR, and XNOR.

Logic Gates Using CMOS Technology

CMOS (Complementary Metal-Oxide-Semiconductor) technology allows us to design power-efficient and scalable logic gates. Each gate combines PMOS and NMOS transistors to achieve the desired logic.

Here’s a brief explanation of some basic gates:

  1. NOT Gate (Inverter)

    • Circuit: A single PMOS and NMOS transistor.

    • Operation: Inverts the input signal (0 → 1, 1 → 0).

    • CMOS Advantage: Highly efficient with minimal static power consumption.

  2. AND Gate

    • Circuit: Requires multiple CMOS transistors in series and parallel configurations.

    • Operation: Outputs HIGH only when both inputs are HIGH.

  3. OR Gate

    • Circuit: CMOS transistors are arranged to ensure output is HIGH when at least one input is HIGH.

    • Operation: Outputs HIGH if any input is HIGH.

  4. NAND and NOR Gates

    • Circuit: Built using CMOS in a configuration that complements AND and OR gates.

    • Operation: Widely used because they are functionally complete (any circuit can be built using only NAND or NOR gates).

Simulation with Cadence Virtuoso

Today, I designed basic CMOS logic gates (NOT, AND, OR, NAND) in Cadence Virtuoso. It was fascinating to see the theoretical designs come to life in the simulation environment. Key observations included:

  • Voltage Levels: Verified input-output transitions based on logic gate functionality.

  • Power Consumption: Analyzed the static and dynamic power during switching.

  • Propagation Delay: Measured the delay between input change and corresponding output change.

Key Challenges Faced

  1. Transistor Sizing: Adjusting PMOS and NMOS dimensions for better performance and balancing rise/fall times.

  2. Simulation Errors: Encountered errors while connecting multiple transistors, which required careful debugging.

What’s Next?

Tomorrow, I’ll explore how to design combinational circuits like adders and multiplexers using these logic gates. I’ll also start studying synthesis and how logic gates are translated into hardware.

Learning Resources for Today

Here are the resources I used to understand and design logic gates:

  1. Digital Design by M. Morris Mano

  2. Neso Academy - Logic Gates Tutorial

Conclusion

Understanding logic gates has been an exciting and essential step in my VLSI journey. These foundational components pave the way for designing more complex circuits and systems. Each simulation run reinforced my knowledge and helped me better appreciate the intricate world of CMOS technology.

Stay tuned for Day 3, where I’ll dive into combinational circuits and explore how these gates combine to perform more advanced tasks. Until then, happy designing! 😊

cmos