题解 | #自动售卖机#

`timescale 1ns/1ns

module sale(
   input                clk   ,
   input                rst_n ,
   input                sel   ,//sel=0,5$dranks,sel=1,10&=$drinks
   input          [1:0] din   ,//din=1,input 5$,din=2,input 10$
 
   output   reg  [1:0] drinks_out,//drinks_out=1,output 5$ drinks,drinks_out=2,output 10$ drinks
   output	reg        change_out   
);

//首先要知道哪些情况要使用状态机
//购买B饮料（10块），先投5块之后，后面接着投5块，还是投10块
reg [1:0] state;

always@(posedge clk or negedge rst_n)
  if(!rst_n) 
    begin drinks_out <= 2'd0; change_out <= 1'd0; state <= 2'd0;end
  else case(sel)
    1'd0://购买A饮料，情况很简单
        case(din)
        2'd0:
            begin drinks_out <= 2'd0; change_out <= 1'd0; end
        2'd1:
            begin drinks_out <= 2'd1; change_out <= 1'd0; end
        2'd2:
            begin drinks_out <= 2'd1; change_out <= 1'd1; end
        default:begin drinks_out <= 2'd0; change_out <= 1'd0; end
        endcase
    1'd1:
        case(din)
        2'd0:
            begin drinks_out <= 2'd0; change_out <= 1'd0; end
        2'd1:
            begin 
                if(state==2'd0)
					//买B饮料，第一次投5块，state==1记录一下
                    begin drinks_out <= 2'd0; change_out <= 1'd0;state <= 2'd1;end
                else if(state==2'd1)
					//买B饮料，第二次投5块，state==0，返回初始状态
                    begin drinks_out <= 2'd2; change_out <= 1'd0;state <= 2'd0; end
            end
        2'd2:
            begin
                if(state==2'd0)
					//买B饮料，第一次投10块。输出后state==0，返回初始状态
                    begin drinks_out <= 2'd2; change_out <= 1'd0;state <= 2'd0; end
                else if(state==2'd1)
					//买B饮料，第一次投过5块后，再投10块。输出后state==0，返回初始状态
                    begin drinks_out <= 2'd2; change_out <= 1'd1;state <= 2'd0;end
            end
        default:begin drinks_out <= 2'd0; change_out <= 1'd0; end
        endcase
    default:begin drinks_out <= 2'd0; change_out <= 1'd0; end
    endcase

endmodule