//可逆计数器
module cy4(input PE,Up_down,En,CP,
           input[3:0] D,
		   output reg[3:0] Q
		  );
integer direction;//设置计数方向
always @(posedge CP)
begin
 if(Up_down)
  direction <= 1;//实现递增计数
 else
  direction <= -1;//实现递减计数
 if(PE)
  Q <= D;
 else if(En)
  Q <= Q + direction;//实现加1或减1计数功能
 else
  Q <= Q;//输出保持不变
end
endmodule