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Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
8 years ago

Query regarding Simulation in Modelsim

Dear friends,

I have a query regarding simulation in Modelsim, I want to take a flop of a input signal to introduce a one clock cycle delay, but I didn't get one clock cycle delay in Modelsim.

So, my question is why I didn't get any delay? If any one have idea about this , please explain me.

I have attached image of simulation below and also write code for the same.

rtl:

`timescale 1ns/100ps

module flops(

input logic clk,

input logic reset_n,

input logic RxD_in

);

logic RxD_in_f1; //first flop of input signal

logic RxD_in_f2; //second flop of input signal

always_ff@(posedge clk or negedge reset_n)

begin

if ( ~reset_n )

begin

RxD_in_f1 <= 1'b0;

end

else

begin

RxD_in_f1 <= RxD_in;

end

end

always_ff@(posedge clk or negedge reset_n)

begin

if(~reset_n )

begin

RxD_in_f2 <= 1'b0;

end

else

begin

RxD_in_f2 <= RxD_in_f1;

end

end

endmodule

testbench:

`timescale 1ns/100ps

module flops_tb;

logic clk = 1'b1, reset = 1'b1;

logic RxD;

flops inst(

.clk(clk),

.reset_n(reset),

.RxD_in(RxD)

);

always# 1 clk <= ~clk;

always

begin

RxD = 1'b0;# 10

RxD = 1'b1;# 10

RxD = 1'b0;

end

endmodule

4 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    8 years ago

    That will be because you're using absolute time delays in the driver, and not synchonising with the clock. Hence the assignmenst to RxD happen just before the clock edge occurs, and it appears to be an immediate transfer. I suggest using @(posedge clk) in the driver: 

    
always
begin
RxD = 1'b0;
repeat(10) @(posedge clk);
RxD = 1'b1;
repeat(10) @(posedge clk);
end

    I removed the final assignment to 0 as this was ignored - it is an always block and loops forever.
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    8 years ago

    --- Quote Start ---

    "Hence the assignment to RxD happen just before the clock edge occurs, and it appears to be an immediate transfer."

    --- Quote End ---

    Thank you very much I try this ans it works proper.

    But I did'nt get your point mentioned in quotes. Can you please explani this, as we see in wave form RxD rises or falls with clock edge.

    I also wan to ask you that if we use different simulator then is it possible to get different outputs.
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    8 years ago

    The problem here is in the code, not in the simulator.

    Simulations are executed using "delta cycles" which are meant to be infinitely small divisions of time. Anyone signal can only have 1 value in any given delta cycle. So in your example, you have set the clock to change from 0 -> 1 after 2 ns. Your data changes from 0 -> 1 after 10 ns. At 10ns, the clock also changes from 0 to 1, giving a clock edge. But the edge is detected at the same time that RxD has also been set to 1, so it is detected as 1.

    so at 10 ns, what occurs is (when using your original code)

    delta 1: clk = 0, RxD = 0

    detla 2: clk = 1, RxD = 1 (clock edge detected, use the value of RxD now, which is 1)

    If you synchronise to the clock, the Assignment to RxD doesnt happen until 1 delta after the rising edge

    delta 1 : clk = 0, RxD = 0

    delta 2 : clk = 1, RxD = 0 (clock edge occurs, read value of RxD, schedule RxD to be set to 1

    delta 3 : clk = 1, RxD = 1
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    8 years ago

    Thank you very much sir for your warm reply.

    I got your point. :)