画状态机，接收1，2，5分钱的卖报机，每份报纸5分钱

module maib(
input clk,
input rst_n,
input [2:0]din,  //ABC [0]:1分钱       [1]：2分钱    [2]：5分钱
output reg [1:0]dout  //    [0]:输出报纸一份 [1]：找钱1分
);  
parameter idle=7‘b000_0001; //空闲状态
parameter st1 =7‘b000_0010; //一共一分钱
parameter st2 =7‘b000_0100; //一共两分钱
parameter st3 =7‘b000_1000; //一共三分钱
parameter st4 =7‘b001_0000; //一共四分钱
parameter st5 =7‘b010_0000; //一共五分钱
parameter st6 =7‘b100_0000; // 六分钱了

reg [6:0]cur_state;  //当前状态与下一状态
reg [6:0]nxt_state;  //当前状态与下一状态
//在每个时钟的上升沿切换状态
always@(posedge clk or negedge rst_n)
if(~rst_n)
 cur_state<=idle;
else 
 cur_state<=nxt_state;  //切换到下一个应该到达的状态
 
//组合逻辑切换状态
always@(din)
begin
case(cur_state)
idle:
 begin
  if(din==3‘b001)
   nxt_state<=st1;  //有一分钱的状态
  else if(din==3‘b010)
   nxt_state<=st2;  //有两分钱的状态
  else if(din==3‘b100)
   nxt_state<=st5;  //有五分钱的状态    
  else
    nxt_state<=idle;
 end 
st1:   //有一分钱了
 begin
  if(din==3‘b001)
   nxt_state<=st2;  //有两分钱的状态
  else if(din==3‘b010)
   nxt_state<=st3;  //有三分钱的状态
  else if(din==3‘b000)
   nxt_state<=st1;   //在有一分钱的状态保持
 end 
st2:   //有两分钱了
 begin
  if(din==3‘b001)
   nxt_state<=st3;  //有三分钱的状态
  else if(din==3‘b010)
   nxt_state<=st4;  //有四分钱的状态
  else if(din==3‘b000)
   nxt_state<=st2;   //在有两分钱的状态保持
 end 
st3:  //有三分钱了
 begin
  if(din==3‘b001)
   nxt_state<=st4;  //有四分钱的状态
  else if(din==3‘b010)
   nxt_state<=st5;  //有五分钱的状态
  else if(din==3‘b000)
   nxt_state<=st3;   //在有三分钱的状态保持
 end 
st4:  //有四分钱了
 begin
  if(din==3‘b001)
   nxt_state<=st5;  //有五分钱的状态
  else if(din==3‘b010)
   nxt_state<=st6;  //有六分钱的状态
  else if(din==3‘b000)
   nxt_state<=st4;   //在有两分钱的状态保持
 end 
st5:
 begin
 nxt_state<=idle;
 end 
st6:
 begin
 nxt_state<=idle;
 end 
default:;
endcase  
end  

always@(posedge clk )
if(nxt_state==st5)
 dout<=2‘b01;    //不找钱
else if(nxt_state==st6)
 dout<=2‘b11;  //找钱1分
endmodule