[HDLBits] Lemmings4

See also: Lemmings1, Lemmings2, and Lemmings3.

Although Lemmings can walk, fall, and dig, Lemmings aren't invulnerable. If a Lemming falls for too long then hits the ground, it can splatter. In particular, if a Lemming falls for more than 20 clock cycles then hits the ground, it will splatter and cease walking, falling, or digging (all 4 outputs become 0), forever (Or until the FSM gets reset). There is no upper limit on how far a Lemming can fall before hitting the ground. Lemmings only splatter when hitting the ground; they do not splatter in mid-air.

Extend your finite state machine to model this behaviour.

Falling for 20 cycles is survivable:

module top_module(
    input clk,
    input areset,    // Freshly brainwashed Lemmings walk left.
    input bump_left,
    input bump_right,
    input ground,
    input dig,
    output walk_left,
    output walk_right,
    output aaah,
    output digging ); 
	parameter LEFT=0, RIGHT=1,FALLL=2,FALLR=3,DIGL=4,DIGR=5,DIE=6;
    parameter DIE_CLOCK=19;
    //fall分出左右更有利于编程，dig应该也是这样
    reg [2:0]state, next_state;
    reg [4:0]die_count;
    /*我原计划把这里die_count设置成足够大的位数（12位），让它一直掉也无所谓。后来还是报错。
    所以只好设置一个signal表示该死了，把die_count控制在最高19*/
    reg signal;
	//这里两位状态寄存器，必须记住！
    always @(*) begin
        // State transition logic
        case(state)
            LEFT:next_state<=ground?(dig?DIGL:(bump_left?RIGHT:LEFT)):FALLL;
            RIGHT:next_state<=ground?(dig?DIGR:(bump_right?LEFT:RIGHT)):FALLR;
            //这里要记得ground判定在前
            FALLL:next_state<=ground?(signal?DIE:LEFT):FALLL;
            FALLR:next_state<=ground?(signal?DIE:RIGHT):FALLR;
            DIGL:next_state<=ground?DIGL:FALLL;
            DIGR:next_state<=ground?DIGR:FALLR;
            DIE:next_state<=DIE;
        endcase
    end
    //这里把是否死亡的测试摘出来单独成一个always
    always@(posedge clk,posedge areset) begin
        if(areset) begin
            signal<=0;
            die_count<=0;
        end
        else if(state==FALLL ||state==FALLR) begin
            if(die_count==19)
                signal<=1;
            else
            	die_count<=die_count+1;
        end
        else begin
            die_count<=0;
            signal<=0;
        end
    end
    always @(posedge clk, posedge areset) begin
        // State flip-flops with asynchronous reset
        if(areset) begin
            state<=LEFT;
            //die_count<=0;
        end
        else begin
            state<=next_state;
            /*
            if(state==FALLL||state==FALLR)
                die_count<=die_count+1;
            else
                die_count<=0;
            */
        end
    end

    // Output logic
    assign walk_left = (state == LEFT);
    assign walk_right = (state == RIGHT);
    assign aaah=(state==FALLL||state==FALLR);
    assign digging=(state==DIGL||state==DIGR);
endmodule