You're familiar with flip-flops that are triggered on the positive edge of the clock, or negative edge of the clock. A dual-edge triggered flip-flop is triggered on both edges of the clock. However, FPGAs don't have dual-edge triggered flip-flops, and always @(posedge clk or negedge clk) is not accepted as a legal sensitivity list.
Build a circuit that functionally behaves like a dual-edge triggered flip-flop:
clkdq
(Note: It's not necessarily perfectly equivalent: The output of flip-flops have no glitches, but a larger combinational circuit that emulates this behaviour might. But we'll ignore this detail here.)
module top_module (
input clk,
input d,
output q
);
/*以下这种是不对的
always@(posedge clk) q<=d;
always@(negedge clk) q<=d;
*/
reg qpos,qneg;
always@(posedge clk) qpos<=d;
always@(negedge clk) qneg<=d;
assign q=clk ? qpos:qneg;
endmodule
同一个值不能被赋两次
