// Metaprobe Stress Sensor
// Uses Ring Oscillators to measure physical "frustration" (jitter/noise)
// Derived from Lean: Semantics/Metaprobe.lean
// Target: Gowin GW1NR-9 (Tang Nano 9K)

`timescale 1ns / 1ps

module metaprobe_stress_sensor (
    input  wire        clk,      // Reference clock (e.g. 27MHz)
    input  wire        rst_n,
    output wire [31:0] stress    // Q16.16 stress signal
);

    // ═══════════════════════════════════════════════════════════════════════════
    // Ring Oscillator (RO) Array
    // ═══════════════════════════════════════════════════════════════════════════
    // We use a chain of inverters to create a free-running oscillator.
    // The frequency is sensitive to temperature and voltage noise.
    
    wire ro_out;
    wire [7:0] ro_chain;
    
    // Inverter chain (keep from being optimized away)
    (* LUT_TYPE="RO" *) assign ro_chain[0] = ~ro_chain[7];
    genvar i;
    generate
        for (i = 1; i < 8; i = i + 1) begin : gen_ro
            assign ro_chain[i] = ~ro_chain[i-1];
        end
    endgenerate
    
    assign ro_out = ro_chain[7];

    // ═══════════════════════════════════════════════════════════════════════════
    // Jitter Measurement (Frustration)
    // ═══════════════════════════════════════════════════════════════════════════
    // Compare RO cycles against ref clock to detect variance (stress).
    
    reg [15:0] ro_counter;
    reg [15:0] ref_counter;
    reg [15:0] last_ro_count;
    reg [15:0] jitter_accum;
    
    always @(posedge ro_out or negedge rst_n) begin
        if (!rst_n) ro_counter <= 16'd0;
        else ro_counter <= ro_counter + 1;
    end
    
    always @(posedge clk or negedge rst_n) begin
        if (!rst_n) begin
            ref_counter <= 16'd0;
            last_ro_count <= 16'd0;
            jitter_accum <= 16'd0;
        end else begin
            ref_counter <= ref_counter + 1;
            
            // Every 2^12 cycles of ref clock, sample RO counter
            if (ref_counter[11:0] == 12'hFFF) begin
                last_ro_count <= ro_counter;
                // Jitter = absolute difference from expected nominal count
                // (Nominal is arbitrary here, we care about change/variance)
                if (ro_counter > last_ro_count) 
                    jitter_accum <= ro_counter - last_ro_count;
                else
                    jitter_accum <= last_ro_count - ro_counter;
            end
        end
    end

    // ═══════════════════════════════════════════════════════════════════════════
    // Output Mapping
    // ═══════════════════════════════════════════════════════════════════════════
    // Map jitter to Q16.16 stress. 
    // baseline jitter = low stress. high variance = high stress.
    
    assign stress = {16'b0, jitter_accum}; // Simple mapping for PoC

endmodule