Question

I have seen the following used to make state changes in Verilog modules:

state <= 2'b10;

state <= #1 IDLE;

Why is <= used and not just =? What is the purpose of using #1? Does it make a difference?

Here is some Verilog code for a FSM that shows the first one being used. Would it work the same if it was replaced with the second?

module fsm( clk, rst, inp, outp);

   input clk, rst, inp;
   output outp;

   reg [1:0] state;
   reg outp;

   always @( posedge clk, posedge rst )
   begin
   if( rst )
       state <= 2'b00;
   else
   begin
       case( state )
       2'b00:
       begin
            if( inp ) state <= 2'b01;
            else state <= 2'b10;
       end

       2'b01:
       begin
            if( inp ) state <= 2'b11;
            else state <= 2'b10;
       end

       2'b10:
       begin
            if( inp ) state <= 2'b01;
            else state <= 2'b11;
       end

       2'b11:
       begin
            if( inp ) state <= 2'b01;
            else state <= 2'b10;
       end
       endcase
   end
end

Answer 1

In a sequential logic always block like yours, it is better to use non-blocking assignments (<=) instead of blocking assignments (=). Simulations will be better representative of the actual resulting logic.

In pure RTL Verilog code, if you are using non-blocking assignments for all sequential logic, there should be no reason to use #1 delays.

I have also seen others use # delays like this. Sometimes it is due to mixing RTL and gate netlists in the same simulation. Other times it is done to compensate for poor modeling. You should avoid using delays in RTL code if you can.

See also:
Nonblocking Assignments in Verilog Synthesis, Coding Styles That Kill!

Also, it is better to use a parameter to name each of your states. It is much more meaningful if a state is named IDLE instead of 2'b10.