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这个设计实现什么功能,明天要考试了,各位大哥帮帮我吧

SUBDESIGN debounce
(
clk :input;
key_pressed:input;
pulse:output;
)
variable
count_reg[7..0]:dff;
at_zero:node;
begin
count_reg[].clk=clk;
count_reg[].prn=key_pressed;
count_reg[].d=count_reg[].q-(0,!at_zero);

% emit single pulse when counter reach 1.%
pulse =count_reg[].q==h"01";

% don't let counter decrement below zero.%
at_zero=count_reg[].q==h"00";
end;