why does my output signal have 2 clock cycles delay?

Question

For each bit in a 32-bit vector, capture when the input signal changes from 1 in one clock cycle to 0 the next. "Capture" means that the output will remain 1 until the register is reset (synchronous reset). Each output bit behaves like a SR flip-flop: The output bit should be set (to 1) the cycle after a 1 to 0 transition occurs. The output bit should be reset (to 0) at the positive clock edge when reset is high. If both of the above events occur at the same time, reset has precedence. In the last 4 cycles of the example waveform below, the 'reset' event occurs one cycle earlier than the 'set' event, so there is no conflict here. In the example waveform below, reset, in1 and out1 are shown again separately for clarity.

my code:

module top_module (
  input clk,
  input reset,
  input [31:0] in,
  output [31:0] out );
  integer i;
  reg [31:0] in_del;
  reg [31:0] out_del;
  always @ (posedge clk)
      begin
          in_del<=in;
          out_del<=~in & in_del;
          if (reset)
              out=0;
          else
              begin
                  for (i=0; i<32;i=i+1) begin
                      if (out_del[i])
                          out[i]=out_del[i];
                  end

              end
      end
endmodule

my output

in_del and out_del are registers assigned-to in an always @ posedge block, so they will each encounter one clock cycle of delay. You could consider using the blocking assignment statement out_del = ~in & in_del instead and it might work — nanofarad
Are you aware that this code cannot be synthesized, contains semantic errors (cannot be compiled) and violates industry practice for use of blocking/non-blocking assignments? What is the purpose of it? — Serge

Serge Serge · Accepted Answer · 2020-03-20T01:25:04

First about your code.

it cannot be compiled. The out must be a reg in order to be assignable within the always block.
using non-blocking assignment in out_del <= in & in_del will cause a one-cycle delay for the if (out_del) comparison. Non-blocking assignments schedule lhs assignment after the block gets evaluated. The rule of thumb is to always use blocking assignments for intermediate signals in the sequential block.
because of the above and because of the in & in_del, this cannot be synthesized, or at least it cannot be synthesized correctly.
you violate industry practices by using the blocking assignment on the out signal. The rule of thumb is to always use non-blocking assignments for the outputs of the sequential blocks.
the code just does not work :-(

If I understood your requirement correctly the following code does it:

module top_module (
  input clk,
  input reset,
  input [31:0] in,
  output reg [31:0] out );
  reg lock;
  always @ (posedge clk)
      begin
        if (reset) begin
              lock <= 0;
              out <= 0;
          end
          else if (lock == 0)
              begin
                out <= in;
                lock <= 1;
              end
      end
endmodule

Just use the lock signal to allow updated. And yes, here is a simple test bench to check it:

module real_top();

    reg clk, reset;
    reg [31:0] in;
    reg [31:0] out;

    top_module tm(clk, reset, in, out);

    initial begin
        clk = 0;
        forever #5 clk = ~clk;
    end

    integer i;
    initial begin 
      in = 0;
      reset = 1;
      #7 reset = 0;
      for (i = 1; i < 5; i++) begin
        #10 in = i;
        #10 reset = 1;
        #10 reset = 0;
      end
      $finish;
    end

    initial 
      $monitor(clk, reset, in, out);
endmodule

why does my output signal have 2 clock cycles delay?

1 Answers