Verilog HDL: Instantiating a DFFE

This example describes how to generate a D flipflop with enable (DFFE) behaviorally with asynchronous preset and reset signals. Both the preset and reset signals are active low, controlling the output of the DFFE whenever either signal goes low.

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module dffeveri (q, d, clk, ena, rsn, prn);

// port declaration

input   d, clk, ena, rsn, prn;
output  q;
reg     q;

always @ (posedge clk or negedge rsn or negedge prn) begin

//asynchronous active-low preset
    if (~prn)
        if (rsn)
            q = 1'b1;
            q = 1'bx;

//asynchronous active-low reset
     else if (~rsn)
        q = 1'b0;

     else if (ena)
        q = d;


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