timestamp_to_serial.v

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`timescale 1ns/1ps

module  timestamp_to_serial(
    input                clk, // clock that drives time counters
    input                stb, // serialize and send timestamp message
    input         [31:0] sec,     // time seconds
    input         [19:0] usec,    // time microseconds
    output reg    [7:0] tdata    // byte-parallel timestamp data sent right after stb input
);
    reg  [2:0] cntr;
    reg        busy = 0;
    wire [2:0] cntr_w;
    assign cntr_w= stb? 3'b0 : cntr;
    always @ (posedge clk) begin
        if      (stb)          busy <= 1;
        else if (&cntr_w[2:1]) busy <= 0;
        if      (stb)   cntr <= 1;
        else if (busy)  cntr <= cntr + 1;
        
        case (cntr_w)
            3'h0:tdata <=  sec[ 7: 0];
            3'h1:tdata <=  sec[15: 8];
            3'h2:tdata <=  sec[23:16];
            3'h3:tdata <=  sec[31:24];
            3'h4:tdata <= usec[ 7: 0];
            3'h5:tdata <= usec[15: 8];
            3'h6:tdata <= {4'h0,usec[19:16]};
            3'h7:tdata <= 8'h0;
        endcase
    end

endmodule