Synchronous Counter

As we have discussed the asynchronous counter, output of one flip-flop stage is connected to the input clock pin of the next stage that introduces propagation delay. In a synchronous counter, all flip-flops within the counter are clocked together by the same clock which makes it faster when compared with an asynchronous counter. It is known as a parallel counter.

Block Diagram

4-bit Up-Down Synchronous Counter

module synchronous_counter #(parameter SIZE=4)(
  input clk, rst_n,
  input up,
  output reg [3:0] cnt
);

  always@(posedge clk) begin
    if(!rst_n) begin
      cnt <= 4'h0;
    end
    else begin
      if(up) cnt <= cnt + 1'b1;
      else cnt <= cnt - 1'b1;
    end
  end
endmodule

Testbench Code

module tb;
  reg clk, rst_n;
  reg up;
  wire [3:0] cnt;
  synchronous_counter(clk, rst_n, up, cnt);
  
  initial begin
    clk = 0; rst_n = 0; 
    up = 1;
    #4; rst_n = 1;
    #80;
    rst_n = 0;
    #4; rst_n = 1; up = 0;
    #50;
    $finish;
  end
  always #2 clk = ~clk;

  initial begin
    $dumpfile("dump.vcd"); $dumpvars;
  end
endmodule

Waveform

Difference between synchronous and asynchronous counter

Asynchronous Counter	Synchronous Counter
In asynchronous counter, the output of one flip flop stage is driven to the clock input of the next stage.	In the synchronous counter, the same clock is driven to all flip-flop stages
Slower as compared to the synchronous counter	Faster as compared to the asynchronous counter.
It is also known as serial counter	It is also known as parallel counter
High propagation delay	Propagation delay is less compared to the asynchronous counter.