johnson counter

A Johnson counter is a digital circuit with a series of flip flops connected in a feedback manner.

The circuit is a special type of shift register where the last flip flop's complement output is fed back to the first flip flop's input. This is almost similar to the ring counter with a few extra advantages.

The Johnson counter's main advantage is that it only needs half the number of flip-flops compared to the standard ring counter, and then it's modulo number is halved. So an n-stage Johnson counter will circulate a single data bit, giving a sequence of 2n different states and can therefore be considered a mod-2n counter.

Verilog code: 

module johnson_counter(clk,reset,count);

  //Johnson counter can be implement for any number of bits by changing Width Parameter.Initially it was 4.

parameter WIDTH=4;

  //inputs

input clk,reset;

  //outputs

output reg [WIDTH-1:0] count;

always@(posedge clk)

begin

if(reset)

count={~count[0],count[WIDTH-1:1]};

else 

count=1;

end

endmodule

Test bench code: 

module johnson_counter_tb();

parameter WIDTH=4;

reg clk,reset;

wire [WIDTH-1:0] count;

johnson_counter dut(clk,reset,count);

always #10 clk=~clk;

initial

begin

reset=0;

clk=0;

 $monitor ("T=%0t out=%b", $time, count);

 #50 reset=1;

 end

 endmodule

#verilog #rtlcoding #Johnsoncounter