module johnson(clk,rst_n,sw1_n,sw2_n,sw3_n,led);
input clk; //时钟信号，50MHz
input rst_n; //复位信号，低电平有效
input sw1_n,sw2_n,sw3_n; //3个独立按键，低表示按下
output[3:0] led; //LED控制，1--灭，0--亮
reg led_dir; //1'b1--left,1'b0--right
reg led_on; //1'b0--off, 1'b1--on
reg[23:0] cnt24;
always @(posedge clk or negedge rst_n)
if(!rst_n) cnt24 <=24'd0;
else cnt24 <=cnt24+1'b1;
reg[3:0] led_r;
always @(posedge clk or negedge rst_n)
if(!rst_n) led_r <= 4'b0001;
else if(cnt24 == 24'hffffff&&led_on) begin
if(led_dir) led_r <= {led_r[2:0],led_r[3]}; //left
else led_r <= {led_r[0],led_r[3:1]}; //right
end
assign led = led_r;
reg[2:0] key_rst;
always@(posedge clk or negedge rst_n)
if(!rst_n) key_rst <= 3'b111;
*************else key_rst <= {sw3_n,sw2_n,sw1_n};
reg[2:0] key_rst_r; //每个时钟的上升沿把key_rst锁存到key_rst_r中
always@(posedge clk or negedge rst_n)
if(!rst_n) key_rst<=3'b111;
else key_rst_r <= key_rst;
//当寄存器key_rst由1变为0时，led_an值变为高，维持一个时钟周期
wire[2:0] key_an =key_rst_r & (~key_rst);
reg[19:0] cnt; //计数寄存器
always@(posedge clk or negedge rst_n)
if(!rst_n) cnt<=20'd0; //异步复位
else if(key_an) cnt<=20'd0; // 在key_an高脉冲时候复位
else cnt<=cnt+1'b1;
reg[2:0] low_sw;
always@(posedge clk or negedge rst_n)
if(!rst_n) low_sw<=3'b111;
else if (cnt == 20'hfffff) //满20ms，将按键值锁存到寄存器low_sw中，cnt==20'hfffff
************** low_sw <= {sw3_n,sw2_n,sw1_n};
reg[2:0] low_sw_r;
always@(posedge clk or negedge rst_n)
if(!rst_n) low_sw<=3'b111;
else low_sw_r<=low_sw;
//当寄存器low_sw由1变为0时，led_ctrl值变为高，维持一个时钟周期
wire[2:0] led_ctrl = low_sw_r[2:0] & (~low_sw[2:0]);
always@(posedge clk or negedge rst_n)
if(!rst_n) begin
led_on=1'd0;
led_dir=1'd1;
end
else begin //进行按键选择
if(led_ctrl[0]) led_on <= ~led_on;
if(led_ctrl[1]) led_dir <= 1'd0;
if(led_ctrl[2]) led_dir <= 1'd1;
end
endmodule
其中前面加星号的地方给我报错。。怎么解决