Verilog Johnson Counter

Introduction

A Johnson counter is a digital circuit that counts in a cyclic manner. It is also known as a 'twisted ring counter' or 'walking ring counter'. In a Johnson counter, the output of the last flip-flop is connected to the input of the first flip-flop, creating a continuous loop. This allows the counter to count up and down.

Implementation

A Johnson counter can be implemented using D flip-flops and XOR gates. The number of flip-flops required depends on the number of bits in the counter. For example, a 4-bit Johnson counter would require 4 D flip-flops and 3 XOR gates.

Verilog Code for a 4-bit Johnson Counter

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

  assign q[0] = ~q[3] ^ q[2];
  assign q[1] = ~q[0] ^ q[3];
  assign q[2] = ~q[1] ^ q[0];
  assign q[3] = ~q[2] ^ q[1];

  always @(posedge clk, negedge reset) begin
    if (reset == 0) begin
      q <= 4'b0000;
    end else begin
      q <= {q[2:0], q[3]};
    end
  end

endmodule

Explanation

The Verilog code defines a module called 'johnson_counter' with clock input 'clk', reset input 'reset', and output 'q' with 4 bits.

The assign statements define the logic for the XOR gates that generate the next state of the counter. The outputs of the XOR gates are connected to the input of the D flip-flops.

The always block is triggered on the positive edge of the clock signal and the negative edge of the reset signal. When the reset signal is low (active low), the output of the counter is set to 0000. Otherwise, the output of the counter is shifted to the left by one bit, with the most significant bit being fed back to the least significant bit.

Conclusion

The Johnson counter is a simple and versatile counter that can be used in various applications. It can be used to generate sequences, control LEDs or displays, and as a frequency divider. The Verilog code provided in this article is an implementation of a 4-bit Johnson counter using D flip-flops and XOR gates.