Verilog Tutorial

To deepen your understanding of Verilog and gain more insights into its usage, there is a wealth of resources available online. Below, you'll find a curated list of Verilog manuals, tutorials, videos, and articles to aid your learning journey:

Verilog Manuals and Official Documentation:

Verilog HDL Quick Reference Guide by Sutherland HDL

Articles and Blog Posts:

Books:

Digital Design , M. Morris Mano and Michael Ciletti , 6th Ed.
Digital Design , William J. Dally and R. Curtis Harting

These resources offer a well-rounded perspective on Verilog, from official documentation to tutorials and practical examples. You can explore them to deepen your knowledge, address specific topics, and gain hands-on experience with Verilog.

Before diving deeper into Verilog design, it's essential to understand some fundamental keywords and data structures. In this section, we'll explore key Verilog concepts and provide examples to illustrate their usage.

module:

module is a fundamental keyword in Verilog used to define hardware modules or blocks. Modules encapsulate specific functionality or components of a design. Example:
and_gate.v
```
// Code block 1
module and_gate (
    input wire A,
    input wire B,
    output wire Y
);
    assign Y = A & B;
endmodule
                                        
```
input and output:

input and output are used to declare ports in a Verilog module. input ports receive signals from external sources, while output ports transmit signals to other modules. Example:
or_gate.v
```
// Code block 2
module or_gate (
    input wire A,
    input wire B,
    output wire Y
);
    assign Y = A | B;
endmodule
                                        
```
assign:

The assign keyword is used to make continuous assignments in Verilog. It assigns a value to an output port based on combinational logic.

Example: assign Y = A & B;

always and begin/end:

always blocks define processes or events in Verilog. The begin/end block is used to encapsulate a series of statements to execute within the always block.

Example:


always @(posedge clk) begin 
    if (reset) begin 
        // Reset logic 
    end else begin 
        // Normal operation 
    end 
end

if/else:

if/else statements are used for conditional execution of code. They allow you to specify different actions based on conditions. Example:


always @(posedge clk) begin 
    if (reset) begin 
        // Reset logic 
    end else begin 
        // Normal operation 
    end 
end

Data Types (reg and wire):

reg represents registers and is used for storing values. wire represents wires and is used for connecting signals. Example:
```
reg [7:0] counter; 
wire result; 
                                        
```

These basic Verilog keywords and data structures provide the foundation for describing digital hardware. Understanding their usage and how to combine them is essential for creating more complex designs. In the examples provided, we've demonstrated the syntax and usage of these elements.

Combinational logic forms the foundation of digital circuit design, enabling you to create circuits where the output depends solely on the current inputs. In this section, we'll delve into combinational logic using Verilog and provide examples to illustrate various combinational circuits.

Basic Logic Gates:

Verilog allows you to model basic logic gates like AND, OR, and XOR gates. These gates are the building blocks of more complex digital circuits. Example: AND Gate
and_gate.v
```
module and_gate (
input wire A,
input wire B,
output wire Y
);
assign Y = A & B;
endmodule
                                        
```
Multiplexers (MUX):

Multiplexers are combinational circuits that select one of several inputs and route it to the output based on control signals. Example: 2-to-1 Multiplexer
mux_2to1.v
```
module mux_2to1 ( 
    input A, 
    input B, 
    input select, 
    output Y 
    ); 
    assign Y = (select) ? B : A; 
endmodule 
                                        
```

Demultiplexers (DeMUX):

Demultiplexers are combinational circuits that select one of several outputs and route it to the input based on control signals.

Example: 1-to-2 Demultiplexer

demux_1to2.v


module demux_1to2 ( 
    input A, 
    input select, 
    output Y0, 
    output Y1, 
); 
    assign y0 = (select) ? 1'b0 : a; // Output 0 when select is 1, else input a  
    
    assign y1 = (select) ? a : 1'b0; // Output 1 when select is 1, else 0  
    
endmodule

Decoders:

Decoders take an n-bit input and activate one of 2^n output lines based on the input value.

Example: 3-to-8 Decoder

decoder_3to8_1.v


module decoder_3to8_1 ( 
input [2:0] A, 
output [7:0] Y 
); 
assign Y = (A == 3'b000) ? 8'b00000001 : 
    (A == 3'b001) ? 8'b00000010 : 
    (A == 3'b010) ? 8'b00000100 : 
    (A == 3'b011) ? 8'b00001000 : 
    (A == 3'b100) ? 8'b00010000 : 
    (A == 3'b101) ? 8'b00100000 : 
    (A == 3'b110) ? 8'b01000000 : 
                    8'b10000000; 
endmodule

Example: 3-to-8 Decoder using case

decoder_3to8_2.v


module decoder_3to8_2 ( 
input [2:0] A, 
output [7:0] Y 
); 
always @(*) begin 
    case (A)
        3'b000: Y = 8'b00000001;
        3'b001: Y = 8'b00000010;
        3'b010: Y = 8'b00000100;
        3'b011: Y = 8'b00001000;
        3'b100: Y = 8'b00010000;
        3'b101: Y = 8'b00100000;
        3'b110: Y = 8'b01000000;
        3'b111: Y = 8'b10000000;
        default:Y = 8'b00000000;
    endcase
end
endmodule

Encoders:

Encoders take an 2^n - bit input and activate one of n output lines based on the input value.

Example: 2-to-4 Priority Encoder:

priority_encoder_2to4.v


module priority_encoder_2to4 ( 
    input wire [1:0] a,     // 2-bit Input 
    output wire [3:0] y     // 4-bit Output 
); 
    assign y[0] = (a[0] == 1'b1) ? 1'b1 : 1'b0; 
    assign y[1] = (a[1] == 1'b1) ? 1'b1 : (a[0] == 1'b1) ? 1'b0 : 1'b0; 
    assign y[2] = (a[1] == 1'b1 && a[0] == 1'b1) ? 1'b1 : 1'b0; 
    assign y[3] = (a[1] == 1'b0 && a[0] == 1'b0) ? 1'b1 : 1'b0; 
endmodule

Arithmetic Logic Units (ALU):

ALUs perform arithmetic and logic operations, such as addition, subtraction, AND, OR, and more, based on control signals.

Example: 4-bit ALU

alu_4bit.v


module alu_4bit ( 
    input [3:0] A, 
    input [3:0] B, 
    input [2:0] opcode, 
    output [3:0] Y 
); 
always @(*) begin 
    case (opcode) 
        3'b000: Y = A + B; // Add 
        3'b001: Y = A - B; // Subtract 
        3'b010: Y = A & B; // AND 
        3'b011: Y = A | B; // OR 
        // Add more operations as needed 
        default: Y = 4'bxxxx; // Output 'x' for unsupported opcode 
    endcase 
end 
endmodule

These examples provide a glimpse into the world of combinational logic in Verilog. Combinational circuits are essential for performing operations where the output depends only on the current input values. As you explore more complex designs, you'll build upon these basic building blocks to create intricate digital systems.

In this section, we will explore synchronous logic components that are integral to digital design. These components play a crucial role in sequential logic circuits and are widely used in various applications.

Flip-Flops (FFs):

Flip-flops are the fundamental building blocks of synchronous digital circuits. They are used for storing binary information and synchronizing signals with a clock. They are crucial for sequential logic circuits, where the output depends not only on the current input but also on the previous state. Flip-flops are synchronous devices, meaning they operate based on clock signals, ensuring precise timing and synchronization.

Types of Flip-Flops:
- D Flip-Flops: These are the simplest type of flip-flops and are often used for data storage and synchronization. A D flip-flop has a data input (D), a clock input (CLK), and an output (Q). It stores the value of D at the rising (or falling) edge of the clock.
- JK Flip-Flops: JK flip-flops offer more flexibility than D flip-flops. They have two inputs: J (Set) and K (Reset). Depending on the inputs and clock edge, JK flip-flops can toggle, set, or reset their outputs.
- T Flip-Flops: T flip-flops have a toggle input (T) that, when activated, toggles the output state. They are often used in frequency dividers and binary counters.
Clock Edge-Triggered Operation:

One of the defining characteristics of flip-flops is their clock edge-triggered operation. Flip-flops are sensitive to either the rising (positive) edge or falling (negative) edge of the clock signal, ensuring that changes in the input are captured precisely at the desired moment. This synchronization ensures the correct behavior of sequential circuits.

Applications:

Flip-flops have diverse applications in digital circuits:
- Registers: They are used to create registers for storing data temporarily.
- Memory Elements: In combination, flip-flops create memory elements, such as RAM cells.
- State Machines: Flip-flops are the core components of finite state machines (FSMs), used in control units, protocol implementations, and more.
- Clock Domain Crossing: In systems with multiple clock domains, flip-flops are employed for synchronization.
Understanding flip-flops is essential for digital designers, as they form the basis for more complex sequential circuits and provide the necessary building blocks for designing processors, controllers, and other digital systems.
Counters:

Counters are essential components for counting and sequencing operations in digital systems. They find applications in frequency division, event counting, and more.

Types of Counters:
- Up Counters: Describe up counters and their functionality.
- Down Counters: Explain down counters and how they operate.
Modulo-N Counters:

Modulo-N counters are a vital component in digital circuits designed for counting and sequencing operations. Unlike standard counters that continue counting indefinitely, modulo-N counters have the unique capability to count up to a specified value, N, before automatically resetting themselves to zero. This feature makes them exceptionally useful in scenarios where precise counting cycles are required. When the count reaches N, the counter resets, allowing for cyclic counting patterns.

Applications:

Modulo-N counters find extensive applications in various fields, including frequency division, event counting, digital timers, and clock dividers, where precise control over counting ranges is essential. Counters are versatile digital components that play a crucial role in a wide range of applications, demonstrating their importance in digital design. One common application is in clock dividers, where counters are used to divide the frequency of an input clock signal by a specific factor. This division process is essential in microprocessor design, ensuring that various components of a CPU operate at different clock speeds. Counters are also the heart of digital timers, used to measure time intervals with precision. Whether it's in a stopwatch, a countdown timer, or any timekeeping device, counters keep track of elapsed time by incrementing their counts. Their ability to count and sequence events accurately makes counters indispensable in creating time-based functions in digital systems.
3. Memories:

Memory elements are pivotal for data storage in digital systems.

Memory Types:

Static RAM (SRAM) and Dynamic RAM (DRAM)

Memory Interfaces:

We will discuss about the interface of a memory with an example in Section 4.
Finite State Machines (FSMs):

Finite State Machines are critical for controlling sequential logic and managing complex operations.

Synchronous FSMs:

Synchronous Finite State Machines (FSMs) represent a fundamental concept in digital logic and control systems. These FSMs are designed to synchronize their state transitions with clock signals, ensuring precise timing and coordination in digital circuits. The design and operation of synchronous FSMs revolve around the concept of clock-driven state changes. Each state in the machine corresponds to a unique condition or operation within the digital system. When the clock signal's edge triggers a state transition, the FSM moves to the next state based on its current state and the inputs it receives. This synchronization mechanism guarantees that state changes occur at well-defined moments in time, allowing for predictable and reliable behavior in digital systems. Synchronous FSMs are widely used in various applications, including digital controllers, protocol implementations, and complex control units, where precise timing and ordered state transitions are paramount for successful operation.

State Diagrams:

Before writing your verilog code make sure you have a good representaion of your FSM in a state diagram. It will help during the implementation.

Applications:

Synchronous Finite State Machines (FSMs) find diverse and crucial applications across various domains of digital design and control systems. One prominent application lies in digital controllers, where FSMs are used to manage and orchestrate complex processes. These controllers govern the behavior of systems like industrial automation, robotics, and embedded systems, ensuring precise control and real-time responsiveness. Additionally, synchronous FSMs play a pivotal role in protocol implementations, where they dictate the communication sequence between devices or networks. They enable error detection, data packet routing, and the establishment of secure connections. Beyond these applications, synchronous FSMs are employed in digital design scenarios that require precise timing, such as clock domain crossing, state machines for video processing, and even in creating specialized arithmetic units. Their versatility and ability to synchronize state transitions with clock signals make synchronous FSMs indispensable in modern digital systems, powering innovations in technology and automation.

By understanding these advanced synchronous logic components, you'll gain valuable insights into the inner workings of digital systems and be well-equipped to design and implement complex digital circuits.

Asynchronous logic plays a critical role in digital circuit design, particularly when dealing with events that don't rely on a clock signal for synchronization. In this section, we'll explore asynchronous logic concepts in Verilog and provide examples to illustrate their usage.

Latches:

Latches are simple asynchronous storage elements that store data when enabled. They are level-sensitive and can lead to timing hazards if not used carefully.
Flip-Flops

Flip-flops are edge-triggered storage elements that are commonly used in synchronous designs to store data on clock edges. However, they can also be used asynchronously.
Metastability:

Metastability is a phenomenon where a flip-flop's output becomes unpredictable when it samples an input that changes near the clock edge. Proper synchronization techniques, like double synchronization with flip-flops, are used to mitigate metastability.
Asynchronous Input Handling:

In Verilog, asynchronous inputs, such as button presses or external signals, can be used to trigger specific actions or affect circuit behavior.

Asynchronous logic is crucial when dealing with external events that do not align with the clock signal, and understanding its behavior is essential for robust digital designs.

3.1 Verilog Resources

Verilog Manuals and Official Documentation:

Articles and Blog Posts:

Books:

3.2 Verilog Syntax

module:

input and output:

assign:

always and begin/end:

if/else:

Data Types (reg and wire):

3.3 Combinational Logic

Basic Logic Gates:

Multiplexers (MUX):

Demultiplexers (DeMUX):

Decoders:

Encoders:

Arithmetic Logic Units (ALU):

3.4 Synchronous Sequential Logic

Flip-Flops (FFs):

Types of Flip-Flops:

Clock Edge-Triggered Operation:

Applications:

Counters:

Types of Counters:

Modulo-N Counters:

Applications:

3. Memories:

Memory Types:

Memory Interfaces:

Finite State Machines (FSMs):

Synchronous FSMs:

State Diagrams:

Applications:

3.5 Asynchronous Sequential Logic

Latches:

Flip-Flops

Metastability:

Asynchronous Input Handling: