The game Lemmings involves critters with fairly simple brains. So simple that we are going to model it using a finite state machine.

In the Lemmings' 2D world, Lemmings can be in one of two states: walking left or walking right. It will switch directions if it hits an obstacle. In particular, if a Lemming is bumped on the left, it will walk right. If it's bumped on the right, it will walk left. If it's bumped on both sides at the same time, it will still switch directions.

Implement a Moore state machine with two states, two inputs, and one output that models this behaviour.

module top_module(
    input clk,
    input areset,    // Freshly brainwashed Lemmings walk left.
    input bump_left,
    input bump_right,
    output walk_left,
    output walk_right);

我的设计

设计我的Lemmings：

module top_module(
    input clk,
    input areset,    // Freshly brainwashed Lemmings walk left.
    input bump_left,
    input bump_right,
    output walk_left,
    output walk_right); //  
 
    parameter LEFT=0, RIGHT=1;
    reg state, next_state;
 
    always @(*) begin
        // State transition logic
        case(state)
            LEFT: begin
                if(bump_left) next_state = RIGHT;
                else next_state = LEFT;
            end
            RIGHT: begin
                if(bump_right) next_state = LEFT;
                else next_state = RIGHT;
            end
            default: begin
               next_state = LEFT; 
            end
        endcase
    end
 
    always @(posedge clk, posedge areset) begin
        // State flip-flops with asynchronous reset
        if(areset) begin
            state <= LEFT;
        end
        else begin
           state <= next_state; 
        end
    end
    
    always@(*) begin
        case(state)
            LEFT: begin
                walk_left = 1; 
                walk_right = 0;
            end
            RIGHT: begin
                walk_right = 1;
                walk_left = 0;
            end
            default: begin // almost impossible
                walk_left = 0;
                walk_right = 0;
            end
        endcase
    end
 
    // Output logic
    // assign walk_left = (state == ...);
    // assign walk_right = (state == ...);
 
endmodule

我设计的Lemmings，就是两个状态，处于LEFT状态，则向左走，处于RIGHT状态，也就向右走，复位则向左走。

输出仅仅和当前处于那个状态有关，而状态的切换，也是看当前状态下有没有当前方向的阻碍，例如在LEFT状态下，存在bump_left，则下一个时钟状态切换到RIGHT，固然，Lemmings也就向右走。

测试成功。