Ring counter in Verilog

Introduction

A ring counter is a simple circuit that cycles a single '1' through a group of bits. It helps keep track of steps or states in a repeating pattern.

To create a simple sequence generator that repeats a pattern.

To control steps in a machine that moves in a circle or loop.

To divide a clock signal into equal parts for timing.

To cycle through LEDs one at a time in a repeating pattern.

Syntax

Verilog

module ring_counter(
    input clk,
    input reset,
    output reg [3:0] q
);

always @(posedge clk or posedge reset) begin
    if (reset)
        q <= 4'b0001;
    else
        q <= {q[0], q[3:1]};
end

endmodule

The module has a clock and reset input, and a 4-bit output.

On reset, the output sets to 0001, starting the ring.

Examples

This line shifts the bits to the right and wraps the rightmost bit to the left end, moving the '1' around.

Verilog

q <= {q[0], q[3:1]};

When reset is pressed, the counter starts fresh with only the first bit set.

Verilog

if (reset) q <= 4'b0001;

Sample Program

This program defines a 4-bit ring counter and a testbench that toggles the clock and releases reset. It prints the output bits over time to show the '1' moving around.

Verilog

module ring_counter(
    input clk,
    input reset,
    output reg [3:0] q
);

always @(posedge clk or posedge reset) begin
    if (reset)
        q <= 4'b0001;
    else
        q <= {q[0], q[3:1]};
end

endmodule

// Testbench to see the ring counter in action
module testbench();
    reg clk = 0;
    reg reset = 1;
    wire [3:0] q;

    ring_counter rc(.clk(clk), .reset(reset), .q(q));

    always #5 clk = ~clk; // Clock toggles every 5 time units

    initial begin
        $monitor($time, " q = %b", q);
        #10 reset = 0; // Release reset after 10 time units
        #50 $finish; // Stop simulation after 50 time units
    end
endmodule

Important Notes

The ring counter cycles through states with only one bit set at a time.

Make sure to reset the counter to start the ring properly.

The size of the ring counter depends on how many bits you use.

Summary

A ring counter moves a single '1' bit around a group of bits in a loop.

It is useful for simple step sequencing and timing tasks.

Resetting the counter sets the starting point for the cycle.