Timeouts in Verilog

Timeouts are important for digital designs that interact with external resources or that need to bound execution time. Implementing timeouts in Verilog requires careful consideration of clock cycles and state machines.

module timeouts(
    input wire clk,
    input wire reset,
    output reg [7:0] result
);

    // Define states
    localparam IDLE = 2'b00;
    localparam WAITING = 2'b01;
    localparam TIMEOUT = 2'b10;
    localparam DONE = 2'b11;

    reg [1:0] state;
    reg [31:0] counter;

    always @(posedge clk or posedge reset) begin
        if (reset) begin
            state <= IDLE;
            counter <= 0;
            result <= 8'b0;
        end else begin
            case (state)
                IDLE: begin
                    state <= WAITING;
                    counter <= 0;
                end
                WAITING: begin
                    if (counter == 100) begin  // Simulating a 2-second delay
                        state <= DONE;
                        result <= 8'h01;  // "result 1"
                    end else if (counter == 50) begin  // 1-second timeout
                        state <= TIMEOUT;
                        result <= 8'h02;  // "timeout 1"
                    end else begin
                        counter <= counter + 1;
                    end
                end
                TIMEOUT, DONE: begin
                    state <= IDLE;
                end
            endcase
        end
    end

endmodule

In this Verilog implementation, we use a state machine to simulate the concept of timeouts. The module has a clock input (clk), a reset input (reset), and an 8-bit output (result).

The state machine has four states: IDLE, WAITING, TIMEOUT, and DONE. We use a counter to simulate the passage of time. In this example, we’re assuming that 50 clock cycles represent 1 second, and 100 clock cycles represent 2 seconds.

The process starts in the IDLE state and immediately moves to the WAITING state. In the WAITING state, it increments the counter on each clock cycle. If the counter reaches 50 (simulating 1 second), it moves to the TIMEOUT state and sets the result to 0x02 (representing “timeout 1”). If the counter reaches 100 (simulating 2 seconds), it moves to the DONE state and sets the result to 0x01 (representing “result 1”).

This implementation simulates the behavior of the first select statement in the original code. To implement the second select statement with a longer timeout, you would need to extend this state machine or create a separate one with different timing parameters.

Note that in real-world applications, you would typically use more precise timing mechanisms, possibly involving a PLL (Phase-Locked Loop) or other clock management techniques to accurately represent seconds or milliseconds.

To test this module, you would need to create a testbench that provides clock and reset signals, and monitors the result output. The testbench would need to run for a sufficient number of clock cycles to observe both the timeout and successful completion scenarios.

module timeouts_tb;
    reg clk, reset;
    wire [7:0] result;

    timeouts uut (
        .clk(clk),
        .reset(reset),
        .result(result)
    );

    // Clock generation
    always #5 clk = ~clk;

    initial begin
        clk = 0;
        reset = 1;
        #10 reset = 0;
        
        #1000;  // Wait for 100 clock cycles
        $finish;
    end

    // Monitor results
    always @(result) begin
        case(result)
            8'h01: $display("Result: result 1");
            8'h02: $display("Result: timeout 1");
            default: $display("Result: %h", result);
        endcase
    end

endmodule

This testbench will run the simulation for 100 clock cycles, which should be enough to observe the timeout behavior. The actual output will depend on the exact timing and state transitions in your implementation.