完成下列程序的空白,编程实现以下效果: 总共有8盏led灯,从每隔0.5秒点亮一盏,每盏灯点亮时,其前一盏灯熄灭,直至8盏灯都被点亮一次,然后从右往左每隔0.5秒点亮一盏,每盏灯点亮时,其前一盏灯熄灭,直至8盏灯都被点亮一次。然后重复以上动作(左右循环交替的跑马灯). module xunhuan_paoma( input clk, input rst, output reg [7:0] out ); _____ [24:0] cnt; reg direction; //direction为1表示从左往右,为0表示从右往左 always@(posedge clk or negedge rst) begin if(!rst) begin cnt<=25'd0; out<=8'h80; direction<=1; end else begin cnt<=cnt+25'd1; if((cnt==25'd24999999)&&(direction==1)) begin out<=________; cnt<=25'd0; if(out==_____) begin out<=8'h01; direction<=_______; end end if((cnt==25'd24999999)&&(direction==0)) begin out<=______; cnt<=25'd0; if(out==8'h80) begin out<=8'h80; direction<=~direction; end end end end endmodule