HDLBits-Lemmings4

题目：

思路：

在原有的状态机中，向FALL状态跳转前，开启一个计时器；在FALL状态中当输入ground拉高时，对计时值进行判断。

verilog代码实现：

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 ); 
    
    wire [2:0] state;
    reg  [2:0] state_next;
    assign state = state_next;
    parameter LEFT = 0;
    parameter RIGHT = 1;
    parameter FALL_L = 2;
    parameter FALL_R = 3;
    parameter DIG_L = 4;
    parameter DIG_R = 5;
    parameter SPLAT = 6;
    
    reg [7:0] falling_cnt = 8'd0;
    reg cnt_valid = 1'b0;
    always @(posedge clk) begin
        if (cnt_valid) begin
            falling_cnt <= falling_cnt + 1;
        end else begin
            falling_cnt <= 8'd0;
        end
    end
    
    always @(posedge clk or posedge areset) begin
        if (areset) begin
        	state_next <= LEFT;
        end else begin
            case (state) 
                LEFT: begin 
                    if (!ground) begin
                        state_next <= FALL_L;
                        cnt_valid <= 1'b1; end
                    else if (dig) state_next <= DIG_L;
                    else if (bump_left) state_next <= RIGHT;
                    else state_next <= LEFT; end
                RIGHT: begin
                    if (!ground) begin 
                        state_next <= FALL_R;
                        cnt_valid <= 1'b1; end
                    else if (dig) state_next <= DIG_R;
                    else if (bump_right) state_next <= LEFT;
                    else state_next <= RIGHT; end
                FALL_L: begin
                    if (ground) begin
                        cnt_valid <= 1'b0;
                        if (falling_cnt >= 8'd20) state_next <= SPLAT;
                        else state_next <= LEFT; end
                    else state_next <= FALL_L; end
                FALL_R: begin
                    if (ground) begin
                        cnt_valid <= 1'b0;
                        if (falling_cnt >= 8'd20) state_next <= SPLAT;
                        else state_next <= RIGHT; end
                    else state_next <= FALL_R; end
                DIG_L: begin
                    if (ground) state_next <= DIG_L;
                    else state_next <= FALL_L; end
                DIG_R: begin
                    if (ground) state_next <= DIG_R;
                    else state_next <= FALL_R; end
                SPLAT: begin
                    state_next <= SPLAT; end
            endcase
        end
    end

    always @ * begin
        walk_left = (state != SPLAT & state == LEFT) ? 1'b1 : 1'b0;
        walk_right = (state != SPLAT & state == RIGHT) ? 1'b1 : 1'b0;
        aaah = (state != SPLAT & (state == FALL_L | state == FALL_R)) ? 1'b1 : 1'b0;
        digging = (state != SPLAT & (state == DIG_L | state == DIG_R)) ? 1'b1 : 1'b0;
    end


endmodule