// ---------------------------------------------------------------------------
// sample_sync.sv  —  QX-250 FPGA block B5 (sample-sync & timestamp)
// FPGA-QX250-001 · traces REQ-CTRL-004, functional F7
//
// The single timebase for the sense/control chain. All ticks are clock-ENABLES
// off clk_sys (single-clock design — see B8), never separate clocks:
//   * us_tick       — 1 MHz, drives the free-running microsecond `timestamp`
//   * imu_tick      — IMU acquisition rate (default 8 kHz); starts a B3 burst
//   * baro_tick     — barometer rate (default 50 Hz); starts a B4 read
//   * imu_sample_ts — `timestamp` latched at imu_tick, so software knows exactly
//                     WHEN the sample was taken → time-coherent sensor fusion.
//
// Divisors are parameters (defaults sized for 48 MHz) so the testbench can shrink
// them for fast simulation without changing the logic.
// ---------------------------------------------------------------------------
`timescale 1ns/1ps
`default_nettype none

module sample_sync #(
    parameter int US_DIV   = 48,       // clk_sys/1MHz   (48 MHz → 48)
    parameter int IMU_DIV  = 6000,     // clk_sys/8kHz   (48 MHz → 6000)
    parameter int BARO_DIV = 960000    // clk_sys/50Hz   (48 MHz → 960000)
) (
    input  wire        clk,
    input  wire        rst_n,
    output reg         us_tick,
    output reg [31:0]  timestamp,       // microseconds since reset (wraps ~71 min)
    output reg         imu_tick,
    output reg         baro_tick,
    output reg [31:0]  imu_sample_ts
);
    reg [31:0] uc, ic, bc;

    always_ff @(posedge clk or negedge rst_n) begin
        if (!rst_n) begin
            uc <= 0; ic <= 0; bc <= 0;
            us_tick <= 1'b0; imu_tick <= 1'b0; baro_tick <= 1'b0;
            timestamp <= 32'd0; imu_sample_ts <= 32'd0;
        end else begin
            us_tick <= 1'b0; imu_tick <= 1'b0; baro_tick <= 1'b0;

            // 1 MHz microsecond timebase
            if (uc == US_DIV - 1) begin
                uc        <= 0;
                us_tick   <= 1'b1;
                timestamp <= timestamp + 32'd1;
            end else uc <= uc + 1;

            // IMU acquisition tick (+ timestamp latch for this sample)
            if (ic == IMU_DIV - 1) begin
                ic            <= 0;
                imu_tick      <= 1'b1;
                imu_sample_ts <= timestamp;
            end else ic <= ic + 1;

            // Barometer acquisition tick
            if (bc == BARO_DIV - 1) begin
                bc        <= 0;
                baro_tick <= 1'b1;
            end else bc <= bc + 1;
        end
    end
endmodule

`default_nettype wire