用verilog实现特殊功能的触发器

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对输入上升沿捕获触发器

module top_module (
    input clk,
    input [7:0] in,
    output [7:0] pedge
);
    reg [7:0] in1;
    always@(posedge clk)begin
        in1<=in;
        pedge=in&~in1;
    end
endmodule

对时钟双边沿触发器
不可以用always@（posedge clk or negedge clk），因为该语句在FPGA中不可综合。
应运用a^ 0=a,a^a=0;

module top_module(
	input clk,
	input d,
	output q);
	
	reg p, n;
	
	// A positive-edge triggered flip-flop
    always @(posedge clk)
        p <= d ^ n;
        
    // A negative-edge triggered flip-flop
    always @(negedge clk)
        n <= d ^ p;
    
    // Why does this work? 
    // After posedge clk, p changes to d^n. Thus q = (p^n) = (d^n^n) = d.
    // After negedge clk, n changes to p^n. Thus q = (p^n) = (p^p^n) = d.
    // At each (positive or negative) clock edge, p and n FFs alternately
    // load a value that will cancel out the other and cause the new value of d to remain.
    assign q = p ^ n;
    
    
	// Can't synthesize this.
	/*always @(posedge clk, negedge clk) begin
		q <= d;
	end*/
    
    
endmodule

详情见 HDLbits flip-flop.