Verilog Blocking vs Non-Blocking Assignments

Blocking (=) and non-blocking (<=) assignments are the concept that separates engineers who understand Verilog from those who merely write it. Choosing wrong does not usually produce a syntax error — it produces a design that simulates one way and synthesises another.

Blocking =: evaluate and update immediately

A blocking assignment completes before the next statement runs, exactly like a software assignment. Inside a clocked block this creates an unintended chain — the updated value is used by the very next line in the same tick.

Non-blocking <=: sample now, update at tick end

All non-blocking right-hand sides are evaluated first, using the old values; the left-hand sides are updated together at the end of the time step. That is precisely how a bank of real flip-flops behaves on a clock edge. The shift register below makes the difference visible:

module shift_compare;
  reg clk;
  reg a, b, c;          // non-blocking chain  (correct shift register)
  reg x, y, z;          // blocking chain      (collapses!)
  reg din;

  always @(posedge clk) begin
    // non-blocking: b gets OLD a, c gets OLD b -> proper 3-stage shift
    a <= din;  b <= a;  c <= b;
    // blocking: x=din, then y=x(=din), then z=y(=din) -> all equal din
    x = din;   y = x;   z = y;
  end

  always #5 clk = ~clk;
  initial begin
    clk = 0; din = 0; a=0;b=0;c=0;x=0;y=0;z=0;
    #7 din = 1;              // one pulse into both chains
    #10 din = 0;
    #40 $finish;
  end
  always @(posedge clk)
    $display("t=%0t | NB a,b,c=%b%b%b | B x,y,z=%b%b%b", $time, a,b,c, x,y,z);
endmodule

t=5 | NB a,b,c=000 | B x,y,z=000
t=15 | NB a,b,c=000 | B x,y,z=111
t=25 | NB a,b,c=100 | B x,y,z=111
t=35 | NB a,b,c=010 | B x,y,z=000
t=45 | NB a,b,c=001 | B x,y,z=000
t=55 | NB a,b,c=000 | B x,y,z=000

Read the log: the non-blocking chain a,b,c moves the pulse one stage per clock, a true shift register. The blocking chain x,y,z collapses — all three take din on the same edge, inferring a single flip-flop instead of three.

The rules that keep you safe