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verilogflip-flopnand2tetrisedaplayground
greut
greut

Reputation: 4363

DFF in verilog with a delay

I'm trying to implement the nand2tetris project in verilog and am hitting a wall using icarus verilog. In the book they implement the DFF as so, q(t) = d(t-1). The output at the current time is the input at the previous posedge clk. Here is the DFF I realized.

module Dff (
    output reg q,
    input data, clk
);

    reg p;
    reg o;

    always @(posedge clk) begin
        o <= data;
        q <= p;
    end

    always @(negedge clk) begin
        p <= o;
    end

endmodule

This DFF seems to work just fine when I test it directly. But when I reused it to create a Bit (a memory cell), it gets crazy. Interestingly, the craziness is different using Icarus Verilog or EDAPlayground (which uses VCS).

module Mux (out, a, b, sel);
    input a, b;
    input sel;
    output reg out;

    assign out = ~sel ? a : b;
endmodule

module Bit (
    output out,
    input data, load, clk
);

    Mux m0(in, out, data, load);
    Dff d0(out, in, clk);
endmodule

Icarus Verilog output

 data | load | clk | out
------+------+-----+-----
    0 |    1 |   1 |   x
    0 |    1 |   0 |   x
    1 |    1 |   1 |   x
    1 |    1 |   0 |   x
    0 |    1 |   1 |   1
    0 |    1 |   0 |   1
    0 |    0 |   1 |   0
    0 |    0 |   0 |   0
    1 |    1 |   1 |   0
    1 |    1 |   0 |   0
    0 |    0 |   1 |   0   # !?!?!
    0 |    0 |   0 |   0   # it should be 1 here.

EDAPlayground output

 data | load | clk | out
------+------+-----+-----
    0 |    1 |   1 |   x
    0 |    1 |   0 |   x
    1 |    1 |   1 |   x
    1 |    1 |   0 |   x
    0 |    1 |   1 |   1
    0 |    1 |   0 |   1
    0 |    0 |   1 |   0
    0 |    0 |   0 |   0
    1 |    1 |   1 |   1  # !?!?!
    1 |    1 |   0 |   1  # it should be 0 here.
    0 |    0 |   1 |   1
    0 |    0 |   0 |   1

The code is testable on EDAPlayground.

Upvotes: 1

Views: 3059

Answers (1)

greut
greut

Reputation: 4363

Thanks to Uun and Greg, here is how I fixed the issue. The code is on EDA playground

A DFF can be simpler.

module Dff (
    output reg q,
    input data, clk
);
    always @(posedge clk) begin
        q <= data;
    end
endmodule

Then the tests are the culprits. Blocking assignments are causing confusion. A simple way to fix that is to change all the = by <= like that:

module Bit_tb;
    reg clk = 0;
    reg data = 0;
    reg load = 1;
    wire out;

    initial begin
        #2
        data <= 1;

        #2
        load <= 0;  // load = 0; /!\ would be blocking before
        data <= 0;  // data = 0; doing this line. /!\

        #2
        $finish;
    end
endmodule

Upvotes: 2

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