Build a 4-digit BCD (binary-coded decimal) counter. Each decimal digit is encoded using 4 bits: q[3:0] is the ones digit, q[7:4] is the tens digit, etc. For digits [3:1], also output an enable signal indicating when each of the upper three digits should be incremented.文章来源：https://www.toymoban.com/news/detail-744818.html
You may want to instantiate or modify some one-digit decade counters.文章来源地址https://www.toymoban.com/news/detail-744818.html
module top_module (
input clk,
input reset, // Synchronous active-high reset
output [3:1] ena,
output [15:0] q);
wire ena0;
always@(posedge clk) begin
if(reset)
ena0=1;
end
bcd1 counter1(clk,reset,ena0,q[3:0]);
assign ena[1]=(q[3:0]==4'h9);
bcd1 counter2(clk,reset,ena[1],q[7:4]);
assign ena[2]=(q[7:0]==8'h99);//这里必须是8‘h99，因为是16进制的99才进位
bcd1 counter3(clk,reset,ena[2],q[11:8]);
assign ena[3]=(q[11:0]==12'h999);
bcd1 counter4(clk,reset,ena[3],q[15:12]);
endmodule
module bcd1 (
input clk,
input reset, // Synchronous active-high reset
input ena,//表示下一级传来的标志位，若标志位为1说明这一级该+1了
output [3:0] q);
always@(posedge clk) begin
if(reset)
q<=4'b0;
else if(ena) begin
if(q==9)
q<=0;
else
q<=q+1;
end
else;
end
endmodule
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