Verilog Fundamentals

Jyotiprakash MishraJyotiprakash Mishra
13 min read

Module Declaration and Ports

Every Verilog design starts with a module. Think of it like a black box with inputs and outputs.

module my_module (
    input clk,           // Single bit input
    input rst,           // Single bit input  
    input [7:0] data_in, // 8-bit input bus
    output [7:0] data_out, // 8-bit output bus
    output valid         // Single bit output
);

// Module content goes here

endmodule

Key Points:

  • input = signals coming INTO your module

  • output = signals going OUT of your module

  • [7:0] = 8-bit wide signal (bit 7 down to bit 0)

  • All ports are implicitly wire type unless specified otherwise

Data Types: wire vs reg

wire

  • Represents physical connections (like actual wires)

  • Can't store values on their own

  • Must be continuously driven by something

  • Used for combinational logic outputs

wire [7:0] sum;        // 8-bit wire
wire carry_out;        // 1-bit wire
wire [15:0] product;   // 16-bit wire

reg

  • Represents storage elements (like flip-flops or latches)

  • Can hold values between clock edges

  • Used in sequential logic (inside always blocks)

  • Note: reg doesn't always mean a physical register!

reg [7:0] counter;     // 8-bit register
reg state;             // 1-bit register  
reg [31:0] memory [0:255]; // Array of 32-bit registers

assign Statement (Combinational Logic)

assign creates continuous assignments - like connecting wires together.

module adder_example (
    input [7:0] a, b,
    input cin,
    output [7:0] sum,
    output cout
);

// Simple assignments
assign sum = a + b + cin;
assign cout = (a + b + cin) > 8'hFF;

// Conditional assignment (multiplexer)
assign sum = (cin) ? a + b + 1 : a + b;

// Bitwise operations
assign result = a & b;      // AND
assign result = a | b;      // OR  
assign result = a ^ b;      // XOR
assign result = ~a;         // NOT
assign result = a << 2;     // Shift left by 2
assign result = a >> 1;     // Shift right by 1

endmodule

Key Points:

  • assign is for combinational logic only

  • Output changes immediately when inputs change

  • Think of it as permanent wire connections

always Blocks (Sequential and Combinational)

always blocks are where the action happens. They describe behavior.

Sequential Logic (Clocked)

// D Flip-Flop
always @(posedge clk) begin
    if (rst) begin
        q <= 1'b0;        // Reset
    end else begin
        q <= d;           // Normal operation
    end
end

// Counter with enable
always @(posedge clk) begin
    if (rst) begin
        counter <= 8'b0;
    end else if (enable) begin
        counter <= counter + 1;
    end
end

// Shift register
always @(posedge clk) begin
    shift_reg <= {shift_reg[6:0], serial_in}; // Shift left, new bit in
end

Combinational Logic (always_comb or always @(*))

// ALU example
always @(*) begin  // or always_comb in SystemVerilog
    case (operation)
        2'b00: result = a + b;      // ADD
        2'b01: result = a - b;      // SUB  
        2'b10: result = a & b;      // AND
        2'b11: result = a | b;      // OR
        default: result = 8'b0;
    endcase
end

// Priority encoder
always @(*) begin
    if (input_vec[7])      output_code = 3'd7;
    else if (input_vec[6]) output_code = 3'd6;
    else if (input_vec[5]) output_code = 3'd5;
    // ... etc
    else                   output_code = 3'd0;
end

Edge Detection: posedge, negedge

Controls when the always block executes.

always @(posedge clk) begin    // Rising edge of clock
    // This runs when clk goes from 0 → 1
end

always @(negedge clk) begin    // Falling edge of clock  
    // This runs when clk goes from 1 → 0
end

always @(posedge clk or posedge rst) begin  // Multiple edges
    if (rst) begin
        // Asynchronous reset
    end else begin
        // Normal clocked operation
    end
end

always @(*) begin              // Any input change
    // Combinational logic - runs whenever ANY input changes
end

Module Instantiation and Hierarchy

Modules are like LEGO blocks - you build complex designs by connecting simpler modules together.

Basic Module Instantiation

// Define a simple adder module
module adder_4bit (
    input [3:0] a, b,
    input cin,
    output [3:0] sum,
    output cout
);
    assign {cout, sum} = a + b + cin;
endmodule

// Use the adder in a larger design
module calculator (
    input [3:0] x, y, z,
    output [3:0] result1, result2,
    output overflow1, overflow2
);

// Instantiate two adders
adder_4bit add1 (
    .a(x),           // Connect x to port a
    .b(y),           // Connect y to port b  
    .cin(1'b0),      // Tie cin to 0
    .sum(result1),   // Connect sum to result1
    .cout(overflow1) // Connect cout to overflow1
);

adder_4bit add2 (
    .a(result1),     // Chain the outputs
    .b(z),
    .cin(1'b0),
    .sum(result2),
    .cout(overflow2)
);

endmodule

Positional vs Named Port Connections

// POSITIONAL (order matters - error prone!)
adder_4bit add1 (x, y, 1'b0, result1, overflow1);

// NAMED (explicit - much better!)
adder_4bit add1 (
    .a(x),
    .b(y), 
    .cin(1'b0),
    .sum(result1),
    .cout(overflow1)
);

Generate Statements (Arrays of Modules)

module ripple_carry_adder_16bit (
    input [15:0] a, b,
    input cin,
    output [15:0] sum,
    output cout
);

wire [16:0] carry;  // Internal carry chain
assign carry[0] = cin;
assign cout = carry[16];

// Generate 16 full adders
genvar i;
generate
    for (i = 0; i < 16; i = i + 1) begin : adder_stage
        full_adder fa (
            .a(a[i]),
            .b(b[i]),
            .cin(carry[i]),
            .sum(sum[i]),
            .cout(carry[i+1])
        );
    end
endgenerate

endmodule

Control Structures

if-else Statements

// Simple if
always @(*) begin
    if (enable) begin
        output_data = input_data;
    end else begin
        output_data = 8'b0;
    end
end

// Nested if-else (priority encoder)
always @(*) begin
    if (priority[3]) begin
        grant = 4'b1000;
    end else if (priority[2]) begin
        grant = 4'b0100;
    end else if (priority[1]) begin
        grant = 4'b0010;
    end else if (priority[0]) begin
        grant = 4'b0001;
    end else begin
        grant = 4'b0000;
    end
end

// if in sequential logic
always @(posedge clk) begin
    if (rst) begin
        counter <= 0;
    end else if (load) begin
        counter <= load_value;
    end else if (enable) begin
        counter <= counter + 1;
    end
    // else counter keeps its value
end

case Statements

// Basic case (like switch statement)
always @(*) begin
    case (opcode)
        3'b000: result = a + b;      // ADD
        3'b001: result = a - b;      // SUB
        3'b010: result = a & b;      // AND
        3'b011: result = a | b;      // OR
        3'b100: result = a ^ b;      // XOR
        3'b101: result = ~a;         // NOT
        3'b110: result = a << 1;     // SHIFT LEFT
        3'b111: result = a >> 1;     // SHIFT RIGHT
        default: result = 8'b0;      // Always include default!
    endcase
end

// Case with don't cares (casez)
always @(*) begin
    casez (instruction[6:0])
        7'b0110011: instr_type = R_TYPE;    // R-type
        7'b0010011: instr_type = I_TYPE;    // I-type  
        7'b01100??: instr_type = B_TYPE;    // B-type (? = don't care)
        7'b???????:  instr_type = UNKNOWN;   // Catch-all
        default:    instr_type = UNKNOWN;
    endcase
end

// State machine with case
always @(posedge clk) begin
    if (rst) begin
        state <= IDLE;
    end else begin
        case (state)
            IDLE: begin
                if (start) state <= FETCH;
            end
            FETCH: begin
                state <= DECODE;
            end
            DECODE: begin
                state <= EXECUTE;
            end
            EXECUTE: begin
                if (done) state <= IDLE;
            end
            default: state <= IDLE;
        endcase
    end
end

Loops (for, while, repeat)

Important: Loops in Verilog create hardware, not software! Each loop iteration becomes parallel hardware.

for Loops

// Parallel hardware - all operations happen simultaneously
always @(*) begin
    integer i;
    parity = 1'b0;
    for (i = 0; i < 8; i = i + 1) begin
        parity = parity ^ data[i];  // XOR all bits together
    end
end

// Array initialization
integer j;
always @(posedge clk) begin
    if (rst) begin
        for (j = 0; j < 16; j = j + 1) begin
            register_file[j] <= 32'b0;  // Clear all registers
        end
    end else if (write_enable) begin
        register_file[write_addr] <= write_data;
    end
end

// Generate-for (creates actual hardware instances)
generate
    for (genvar k = 0; k < 8; k = k + 1) begin : byte_lane
        assign byte_valid[k] = |data_bus[k*8 +: 8];  // OR all bits in byte
    end
endgenerate

while and repeat Loops

// while loop (rare in synthesizable code)
always @(*) begin
    temp = input_val;
    count = 0;
    while (temp != 0) begin
        count = count + 1;
        temp = temp >> 1;  // Count leading zeros
    end
end

// repeat loop (fixed number of iterations)  
always @(*) begin
    shifted_data = input_data;
    repeat (shift_amount) begin
        shifted_data = shifted_data << 1;
    end
end

Common Data Structures

Arrays and Memories

// Register array (like register file)
reg [31:0] registers [0:31];    // 32 registers, each 32-bits wide

// 2D array
reg [7:0] memory [0:255][0:3];  // 256 rows × 4 columns of bytes

// Accessing arrays
always @(posedge clk) begin
    if (write_enable) begin
        registers[write_addr] <= write_data;
    end
    read_data <= registers[read_addr];
end

// Initialize array
integer i;
initial begin
    for (i = 0; i < 32; i = i + 1) begin
        registers[i] = 32'b0;
    end
end

Packed vs Unpacked Arrays

// PACKED - contiguous bits, can be accessed as vector
reg [7:0] packed_array [0:15];     // 16 elements of 8 bits each
wire [127:0] flat_view = packed_array;  // Can treat as 128-bit vector

// UNPACKED - separate storage locations  
reg unpacked_array [0:15][7:0];    // Same data, different organization
// Cannot directly convert to single vector

Bit Selection and Slicing

wire [31:0] data = 32'hDEADBEEF;

// Bit selections
wire msb = data[31];           // Most significant bit
wire lsb = data[0];            // Least significant bit

// Bit slicing  
wire [7:0] byte3 = data[31:24]; // Upper byte
wire [7:0] byte0 = data[7:0];   // Lower byte
wire [15:0] upper = data[31:16]; // Upper 16 bits

// Variable bit selection (careful!)
wire selected_bit = data[bit_index];  // bit_index must be constant!

// Part-select with variable index
wire [7:0] byte_sel = data[byte_index*8 +: 8];  // +: means "8 bits starting at"
wire [7:0] byte_sel2 = data[byte_index*8 + 7 : byte_index*8];  // Same thing

Advanced Control Structures

Conditional Operator (Ternary)

// Simple mux
assign output = select ? input1 : input0;

// Nested conditionals  
assign priority_out = (high_pri) ? high_data :
                     (med_pri)  ? med_data :
                     (low_pri)  ? low_data : default_data;

// Inside always blocks
always @(*) begin
    next_state = (current_state == IDLE) ? 
                 (start ? ACTIVE : IDLE) :
                 (done ? IDLE : ACTIVE);
end

Functions and Tasks

// Function (combinational only, returns value)
function [7:0] count_ones;
    input [31:0] data;
    integer i;
    begin
        count_ones = 0;
        for (i = 0; i < 32; i = i + 1) begin
            count_ones = count_ones + data[i];
        end
    end
endfunction

// Task (can have delays, multiple outputs)
task write_memory;
    input [7:0] addr;
    input [31:0] data;
    begin
        @(posedge clk);
        mem_addr <= addr;
        mem_data <= data;
        mem_we <= 1'b1;
        @(posedge clk);
        mem_we <= 1'b0;
    end
endtask

// Using function and task
always @(*) begin
    ones_count = count_ones(input_vector);
end

always @(posedge clk) begin
    if (write_request) begin
        write_memory(address, write_data);
    end
end

Parameters and Localparam

// Module parameters (configurable)
module fifo #(
    parameter WIDTH = 8,
    parameter DEPTH = 16
) (
    input clk, rst,
    input [WIDTH-1:0] din,
    input push, pop,
    output [WIDTH-1:0] dout,
    output full, empty
);

localparam ADDR_BITS = $clog2(DEPTH);  // Local parameter

reg [WIDTH-1:0] memory [0:DEPTH-1];
reg [ADDR_BITS:0] wr_ptr, rd_ptr;

// Implementation...

endmodule

// Instantiate with different parameters
fifo #(.WIDTH(32), .DEPTH(64)) data_fifo (...);
fifo #(.WIDTH(16), .DEPTH(8))  cmd_fifo (...);

Complete Examples

Simple Counter

module counter (
    input clk,
    input rst,
    input enable,
    output reg [7:0] count
);

always @(posedge clk) begin
    if (rst) begin
        count <= 8'b0;           // Use <= for sequential
    end else if (enable) begin
        count <= count + 1;
    end
end

endmodule

State Machine

module fsm (
    input clk, rst,
    input start, done,
    output reg busy,
    output reg [1:0] state
);

parameter IDLE = 2'b00;
parameter WORKING = 2'b01; 
parameter FINISH = 2'b10;

reg [1:0] next_state;

// State register
always @(posedge clk) begin
    if (rst) begin
        state <= IDLE;
    end else begin
        state <= next_state;
    end
end

// Next state logic  
always @(*) begin
    case (state)
        IDLE: begin
            if (start) next_state = WORKING;
            else       next_state = IDLE;
        end
        WORKING: begin
            if (done)  next_state = FINISH;
            else       next_state = WORKING;
        end
        FINISH: begin
            next_state = IDLE;
        end
        default: next_state = IDLE;
    endcase
end

// Output logic
always @(*) begin
    busy = (state == WORKING);
end

endmodule

Memory Module

module simple_memory (
    input clk,
    input [7:0] addr,
    input [31:0] data_in,
    input we,                    // Write enable
    output reg [31:0] data_out
);

reg [31:0] memory [0:255];      // 256 words of 32-bit memory

always @(posedge clk) begin
    if (we) begin
        memory[addr] <= data_in;  // Write
    end
    data_out <= memory[addr];     // Read (always happens)
end

endmodule

Real-World Example: UART Transmitter

module uart_tx #(
    parameter CLOCK_FREQ = 50_000_000,
    parameter BAUD_RATE = 115200
) (
    input clk, rst,
    input [7:0] data,
    input send,
    output reg tx,
    output reg busy
);

localparam CLKS_PER_BIT = CLOCK_FREQ / BAUD_RATE;
localparam COUNTER_BITS = $clog2(CLKS_PER_BIT);

// State machine states
localparam IDLE      = 3'b000;
localparam START_BIT = 3'b001;
localparam DATA_BITS = 3'b010;
localparam STOP_BIT  = 3'b011;

reg [2:0] state;
reg [COUNTER_BITS-1:0] clk_counter;
reg [2:0] bit_counter;
reg [7:0] tx_data;

always @(posedge clk) begin
    if (rst) begin
        state <= IDLE;
        tx <= 1'b1;
        busy <= 1'b0;
        clk_counter <= 0;
        bit_counter <= 0;
    end else begin
        case (state)
            IDLE: begin
                tx <= 1'b1;
                busy <= 1'b0;
                clk_counter <= 0;
                bit_counter <= 0;

                if (send) begin
                    tx_data <= data;
                    state <= START_BIT;
                    busy <= 1'b1;
                end
            end

            START_BIT: begin
                tx <= 1'b0;  // Start bit is 0

                if (clk_counter < CLKS_PER_BIT - 1) begin
                    clk_counter <= clk_counter + 1;
                end else begin
                    clk_counter <= 0;
                    state <= DATA_BITS;
                end
            end

            DATA_BITS: begin
                tx <= tx_data[bit_counter];

                if (clk_counter < CLKS_PER_BIT - 1) begin
                    clk_counter <= clk_counter + 1;
                end else begin
                    clk_counter <= 0;

                    if (bit_counter < 7) begin
                        bit_counter <= bit_counter + 1;
                    end else begin
                        bit_counter <= 0;
                        state <= STOP_BIT;
                    end
                end
            end

            STOP_BIT: begin
                tx <= 1'b1;  // Stop bit is 1

                if (clk_counter < CLKS_PER_BIT - 1) begin
                    clk_counter <= clk_counter + 1;
                end else begin
                    state <= IDLE;
                end
            end

            default: state <= IDLE;
        endcase
    end
end

endmodule

Key Rules and Best Practices

Blocking vs Non-Blocking Assignments

// BLOCKING (=) - happens immediately
always @(*) begin
    temp = a + b;      // This happens first
    result = temp + c; // This uses the NEW value of temp
end

// NON-BLOCKING (<=) - happens at end of time step  
always @(posedge clk) begin
    temp <= a + b;      // Both assignments happen
    result <= temp + c; // simultaneously using OLD values
end

Rule of Thumb:

  • Use <= for sequential logic (flip-flops, registers)

  • Use = for combinational logic (inside always @(*))

Common Patterns

// Reset pattern
always @(posedge clk) begin
    if (rst) begin
        // Reset all registers to known values
        counter <= 0;
        state <= IDLE;
        valid <= 0;
    end else begin
        // Normal operation
    end
end

// Enable pattern
always @(posedge clk) begin
    if (rst) begin
        data <= 0;
    end else if (enable) begin
        data <= new_data;  // Only update when enabled
    end
    // else data keeps its old value
end

// Mux pattern
assign output = (select) ? input1 : input0;

// Decoder pattern  
always @(*) begin
    outputs = 8'b0;           // Default all off
    outputs[address] = 1'b1;  // Turn on selected bit
end

Summary

  • wire: For connections and combinational outputs

  • reg: For storage and variables in always blocks

  • assign: Continuous combinational logic

  • always @(posedge clk): Sequential logic (flip-flops)

  • always @(*): Combinational logic

  • <=: Non-blocking (use for sequential)

  • \=: Blocking (use for combinational)

  • Modules: Building blocks - connect them to make complex designs

  • if/case: Control flow - creates multiplexers and decoders in hardware

  • for loops: Create parallel hardware, not sequential execution

  • Functions/tasks: Reusable code blocks

  • Parameters: Make modules configurable

The key is understanding when things happen:

  • assign and always @(*) react immediately to input changes

  • always @(posedge clk) only updates on clock edges

  • Loops unroll into parallel hardware

  • Each module instance is separate physical hardware

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Written by

Jyotiprakash Mishra
Jyotiprakash Mishra

I am Jyotiprakash, a deeply driven computer systems engineer, software developer, teacher, and philosopher. With a decade of professional experience, I have contributed to various cutting-edge software products in network security, mobile apps, and healthcare software at renowned companies like Oracle, Yahoo, and Epic. My academic journey has taken me to prestigious institutions such as the University of Wisconsin-Madison and BITS Pilani in India, where I consistently ranked among the top of my class. At my core, I am a computer enthusiast with a profound interest in understanding the intricacies of computer programming. My skills are not limited to application programming in Java; I have also delved deeply into computer hardware, learning about various architectures, low-level assembly programming, Linux kernel implementation, and writing device drivers. The contributions of Linus Torvalds, Ken Thompson, and Dennis Ritchie—who revolutionized the computer industry—inspire me. I believe that real contributions to computer science are made by mastering all levels of abstraction and understanding systems inside out. In addition to my professional pursuits, I am passionate about teaching and sharing knowledge. I have spent two years as a teaching assistant at UW Madison, where I taught complex concepts in operating systems, computer graphics, and data structures to both graduate and undergraduate students. Currently, I am an assistant professor at KIIT, Bhubaneswar, where I continue to teach computer science to undergraduate and graduate students. I am also working on writing a few free books on systems programming, as I believe in freely sharing knowledge to empower others.