FSM Example: Moore Machine - 3.6.1 | 3. Verilog-Based RTL Design | SOC Design 1: Design & Verification
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3.6.1 - FSM Example: Moore Machine

Practice

Interactive Audio Lesson

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Introduction to Moore Machines

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0:00
Teacher
Teacher

Let's start by discussing the core idea of a Moore machine. Can anyone tell me how a Moore machine differs from a Mealy machine?

Student 1
Student 1

In a Moore machine, the output depends only on the state, while in a Mealy machine, it depends on the state and inputs.

Teacher
Teacher

Exactly! This distinction is crucial. Since the outputs depend solely on the state, we can simplify our design. Now, let's move on to our specific implementation in Verilog.

Student 2
Student 2

What kind of states do we define in our example?

Teacher
Teacher

Great question! We define parameters for three states: IDLE, RUN, and DONE. This structure helps manage the state transitions clearly. Remember the acronym IRD: Idle, Run, Done for our states!

State Encoding and Transition Logic

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Teacher
Teacher

Now, let’s examine how we encode these states in Verilog. How might we write a module to handle state transitions?

Student 3
Student 3

We should define the module with inputs like clock and reset, and then use the case statement to handle transitions.

Teacher
Teacher

Exactly. In our `always` block, we’ll switch states based on the current state and inputs. How do we ensure the state machine operates reliably during a reset?

Student 4
Student 4

We would set the state to IDLE when the reset signal is high.

Teacher
Teacher

Correct! Ensuring the state resets properly prevents undefined behavior. Let's now write down the code for state transitions.

Edge-Sensitivity in State Machines

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Teacher
Teacher

A critical aspect in digital logic is edge sensitivity. Can anyone explain why it's important in our state machine design?

Student 1
Student 1

It helps trigger state changes precisely when we want them, like on the positive edge of a clock.

Teacher
Teacher

Exactly! This ensures that state transitions occur in a controlled manner, preventing glitches. Now let’s consider what happens if we don’t handle edges properly.

Student 2
Student 2

Uh-oh, we might get multiple transitions or unexpected states!

Teacher
Teacher

Right! This could cause our design to behave erratically. It’s essential to handle edge detection carefully.

Summary of Moore Machine Implementation

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Teacher
Teacher

Let’s summarize our discussion on the Moore machine. What are the key elements we covered?

Student 3
Student 3

We discussed state encoding, transition logic, and the importance of edge sensitivity!

Teacher
Teacher

Perfect! Remember, these principles apply to many digital designs. Being able to implement a Moore machine effectively is a valuable skill in Verilog design.

Student 4
Student 4

I feel much more confident about finite state machines now!

Teacher
Teacher

That’s fantastic to hear! Keep practicing these concepts, and you'll master them in no time!

Introduction & Overview

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

Quick Overview

This section introduces the design of a Moore state machine using Verilog, highlighting state definitions, transitions, and the overall structure.

Standard

The section describes a Moore machine model implemented in Verilog, explaining the use of parameters for state encoding and the behavior of state transitions based on input signals. It illustrates how state changes occur in response to clock edges or resets.

Detailed

FSM Example: Moore Machine

In this section, we delve into the design of a Moore state machine using Verilog. A Moore machine is a type of finite state machine where the outputs depend only on the current state, not on the input signals. This is distinct from a Mealy machine, where outputs can change according to both the current state and the input values. We will explore how to implement a Moore machine structure in Verilog by defining states as parameters and managing transitions in an always block.

Key Points Covered:

  1. Module Definition: The Verilog module is defined with inputs for clock, reset, and start signals, and the output is a register that represents the current state.
  2. State Encoding: We use parameters to define three states: IDLE, RUN, and DONE. This method enhances the clarity of state management in the design.
  3. State Transitions: Transitions occur within an always block that responds to the positive edge of the clock and the reset signal, showcasing the importance of edge sensitivity in digital design.
  4. Default State Handling: The default case ensures that the state machine transitions to a known state, improving reliability.

The understanding of Moore machines is crucial in various digital applications, especially for control units in processors, as they provide a structured approach to handling states in logic design.

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Audio Book

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Module Definition

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module fsm (
input wire clk,
input wire reset,
input wire start,
output reg [1:0] state
);

Detailed Explanation

The module 'fsm' is defined as a hardware component in Verilog. It begins with the 'module' keyword, followed by the name of the module. Inside the parentheses, you see the input and output declarations. 'clk', 'reset', and 'start' are inputs that will affect the FSM's operation, while 'state' is the output that indicates the current state of the FSM.
- Chunk Title: State Encoding
- Chunk Text: ```verilog
parameter IDLE = 2'b00, RUN = 2'b01, DONE = 2'b10;

- **Detailed Explanation:** In this line, parameters are defined for the states of the FSM. 'IDLE', 'RUN', and 'DONE' are associated with binary values. Each state is assigned a unique binary code: IDLE is '00', RUN is '01', and DONE is '10'. This is crucial for the FSM as it helps in managing the transitions and understanding its current operation.
- **Chunk Title:** State Transitions
- **Chunk Text:** ```verilog
always @(posedge clk or posedge reset) begin
if (reset)
state <= IDLE;
else begin
case (state)
IDLE: if (start) state <= RUN;
RUN: state <= DONE;
DONE: state <= IDLE;
default: state <= IDLE;
endcase
end
end
  • Detailed Explanation: This part of the code is responsible for determining how the FSM transitions between different states based on specific conditions. It monitors the clock and reset signals. If 'reset' is triggered, the state returns to IDLE. If the FSM is in IDLE and the 'start' signal is activated, it moves to the RUN state. After RUN, the FSM proceeds to the DONE state, which then loops back to IDLE.
  • Chunk Title: Edge-Sensitive Operations
  • Chunk Text: ```verilog
    // Edge-sensitive operations are used in the always block to trigger state changes based on the clock signal.

Examples & Analogies

No real-life example available.

Definitions & Key Concepts

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

Key Concepts

  • State Encoding: The practice of assigning unique binary values to each state.

  • State Transition: The process of changing states based on input signals or conditions.

  • Edge-Sensitivity: The behavior of the state machine as a response to clock signal edges.

  • Moore Output: The output that results solely from the current state, not directly from the input.

Examples & Real-Life Applications

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

Examples

  • Example of a Moore machine with states IDLE, RUN, DONE illustrated in Verilog code.

  • A state machine diagram depicting transitions between IDLE, RUN, and DONE states based on certain inputs.

Memory Aids

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

🎡 Rhymes Time

  • In Moore we explore, states defined at the core, outputs stay the same, as we play the state game.

πŸ“– Fascinating Stories

  • Once there was a state machine named Moore who lived in the land of Logic. He only followed his friends' signals, known as states, to show outputs, no matter what the inputs said!

🧠 Other Memory Gems

  • Remember IDLE, RUN, DONE - 'I’m Really Done' as we transition through these states.

🎯 Super Acronyms

Use the acronym IRD for the state transitions

  • Idle
  • Run
  • Done.

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Moore Machine

    Definition:

    A type of finite state machine where the outputs depend only on the current state.

  • Term: State Encoding

    Definition:

    The process of defining unique binary codes for each state in a state machine.

  • Term: EdgeSensitivity

    Definition:

    The property of a circuit to respond to changes in signals at specific points in time, such as rising or falling edges.

  • Term: Parameters

    Definition:

    Variables that hold constant values in Verilog, often used for state names.