(* fsm_safe_state = “default_state“ *)

FSM安全状态设计与应用

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**一、(* fsm_safe_state = "default_state" *)**

这个属性是用于确保有限状态机（FSM）在异常情况下能够恢复到安全状态的重要指令。

1、作用：

指定当状态机发生异常或未定义行为时，强制恢复到指定的安全状态。

2、语法：

(* fsm_safe_state = "state_name" *)

二、使用示例

1、基础用法：

module safe_fsm (
    input wire clk,
    input wire rst_n,
    input wire trigger,
    output reg [1:0] status
);

// 状态定义
localparam IDLE    = 2'b00;
localparam WORKING = 2'b01;
localparam DONE    = 2'b10;
// 注意：2'b11 是未定义状态

(* fsm_safe_state = "IDLE" *)
reg [1:0] current_state, next_state;

// 状态寄存器
always @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
        current_state <= IDLE;
    end else begin
        current_state <= next_state;
    end
end

// 状态转移逻辑
always @(*) begin
    case (current_state)
        IDLE: begin
            status = 2'b00;
            next_state = trigger ? WORKING : IDLE;
        end
        WORKING: begin
            status = 2'b01;
            next_state = DONE;
        end
        DONE: begin
            status = 2'b10;
            next_state = IDLE;
        end
        default: begin
            status = 2'b00;
            next_state = IDLE;  // 安全恢复
        end
    endcase
end

endmodule

2、更复杂的安全状态机：

module traffic_light_fsm (
    input wire clk,
    input wire rst_n,
    input wire emergency,
    output reg [2:0] light  // [red, yellow, green]
);

// 状态定义
localparam RED    = 3'b001;
localparam YELLOW = 3'b010;
localparam GREEN  = 3'b100;
localparam ALL_RED = 3'b001;  // 安全状态

(* fsm_safe_state = "ALL_RED" *)
reg [2:0] state, next_state;

always @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
        state <= RED;
    end else begin
        state <= next_state;
    end
end

always @(*) begin
    case (state)
        RED: begin
            light = 3'b001;
            next_state = emergency ? RED : GREEN;
        end
        GREEN: begin
            light = 3'b100;
            next_state = emergency ? RED : YELLOW;
        end
        YELLOW: begin
            light = 3'b010;
            next_state = emergency ? RED : RED;
        end
        default: begin
            light = 3'b001;  // 强制红灯
            next_state = RED;
        end
    endcase
end

endmodule

三、综合效果

1、硬件实现

工具会生成额外的逻辑来检测非法状态并恢复：

状态寄存器编码验证
自动恢复电路
安全状态强制逻辑

2.、可靠性提升

抗辐照能力：防止宇宙射线等导致的位翻转
电源噪声免疫：电压波动时的状态保护
时序违例恢复：时钟偏移时的安全恢复

四、最佳实践

1、选择合适的安全状态

// 好的安全状态：停止、空闲、复位状态
(* fsm_safe_state = "IDLE" *)
(* fsm_safe_state = "STOP" *)
(* fsm_safe_state = "SAFE_MODE" *)

// 避免的安全状态：可能引起危险操作的状态
// (* fsm_safe_state = "MOTOR_ON" *)  // 危险！
// (* fsm_safe_state = "HEATER_ON" *) // 危险！

2、结合其他安全属性

(* fsm_safe_state = "IDLE" *)
(* fsm_encoding = "safe" *)           // 使用安全编码
(* fsm_extract = "yes" *)             // 明确提取为FSM
reg [2:0] state;

3、完整的安全FSM模板

module safe_fsm_template (
    input wire clk,
    input wire rst_n
);

localparam SAFE_STATE = 3'b000;
localparam STATE_A = 3'b001;
localparam STATE_B = 3'b010;
localparam STATE_C = 3'b100;

(* fsm_safe_state = "SAFE_STATE" *)
(* fsm_encoding = "gray" *)  // 使用格雷码减少位翻转影响
reg [2:0] current_state;

always @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
        current_state <= SAFE_STATE;
    end else begin
        case (current_state)
            SAFE_STATE: current_state <= STATE_A;
            STATE_A:    current_state <= STATE_B;
            STATE_B:    current_state <= STATE_C;
            STATE_C:    current_state <= SAFE_STATE;
            default:    current_state <= SAFE_STATE;  // 硬件安全网
        endcase
    end
end

endmodule