Electrical – D flip flop in verilog

flipflopverilog

When i tried to code the below flip flop, the program failed. I'm using altera .

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module DFF_SYNC_RESET(Din, nRst, Dout, Clk);
output Dout;
input nRst,Clk,Din;

wire D;
assign D= Din & nRst;

always@(posedge Clk)
 begin 
  Dout<= D;
end

endmodule

Can you please tell my failure in the code?
enter image description here

Edit: After i encountered to another problem which is the same as in this page, the problem was solved

https://stackoverflow.com/questions/25832326/altera-quartus-error-12007-top-level-design-entity-alt-ex-1-is-undefined

Best Answer

In Verilog, if you assign to something in a procedural block (a block between begin and end), you need to make it a regsiter type. If you assign to something in a continual assignment (either an assign statement or the output of an instance of a module) you need to make it a wire type.

So you need to declare (You already edited this change into your question)

wire D;

on line 5, and

output reg Dout;

on line 2.

Also, D is somewhat superfluous. You could just have

output reg Dout
...
always @(posedge clk) begin
    Dout <= Din & nRst;
end

Even better, you should not use logic to simulate control features on flip-flops if your architecture makes full-featured flip-flops available. To code a DFF with syncronous reset for synthesis you would rather have

always @(posedge clk) begin
    if ~nRst 
         Dout <= 1'b0;
    else
         Dout <= Din;
end

This will be more likely to use the built in features of the flip-flops in your architecture, resulting in faster and more resource-efficient implementation.

The error you showed doesn't seem to relate to how you defined your DFF_SYNC_RESET module. More likely it has to do with what files you are including in the synthesis or where you instantiated it, either in your testbench or in a higher-level module.

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