CODEWITHSUNDEEP
verilog
JamJI
JamJI

Reputation: 87

What is the purpose ring counters?

Currently, I trying to review the ring counter in verilog. But I am curious about what is the purpose of ring counter? I didn't quite catch well. Does anyone know what is the purpose of the ring counters?

Upvotes: 2

Views: 596

Answers (1)

Morgan
Morgan

Reputation: 20514

With Ring counters four flip-flops can be used to create a four state FSM with no additional logic. A traditional state machine would require 2 flip-flops and next state logic. Wikipedia states that the Ring counter would be simpler and have a smaller synthesis area.

I would expect adder and next state logic required to be of a similar size to the two extra flip-flops used in the ring counter, there making a four state machine of either type roughly equivalent. The ring counter will grow quite quickly as well, each extra state requires an additional flip-flop. and states must always follow a linear pattern. Most FSMs have branches and react to inputs minimising the opportunities to use the ring counter.

The ring counter has the advantage that it state is one-hot. Which might simplify the output logic.

A ring counter FSM:

module ring_counter(
  input clk,
  input rst_n,

  //outputs ...
);
reg state0;
reg state1;
reg state2;
reg state3;

always @(posedge clk, negedge rst_n) begin
  if (~rst_n) begin
    state0 <= 1'b1;
    state1 <= 1'b0;
    state2 <= 1'b0;
    state3 <= 1'b0;
  end
  else begin
    state0 <= state3;
    state1 <= state0;
    state2 <= state1;
    state3 <= state2;
  end
end
endmodule

The overall state will start as 0001 and then rotate around 0010, 0100, 1000 and back to the beginning 0001.

In comparison a typical FSM could be:

reg [1:0] state;

always @(posedge clk, negedge rst_n) begin
  if (~rst_n) begin
    state <= 'b0;
   end
   else begin
     state <= state + 1;
   end
 end

Which will iterate through states 00 -> 01 -> 10 -> 11

Upvotes: 1

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