timestamp_fifo.v

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`timescale 1ns/1ps

module  timestamp_fifo(
//    input                rst,
    input                sclk,
    input                srst,    // @ posedge smclk - sync reset
    
    input                pre_stb, // marks pre-first input byte (s0,s1,s2,s3,u0,u1,u2,u3)
    input          [7:0] din,     // data in - valid for 8 cycles after pre_stb

    input                aclk,    // clock to synchronize "advance" commands
    input                arst,    // @ posedge aclk - sync reset
    
    input                advance, // @aclk advance registers
    
    input                rclk,    // output clock
    input                rrst,   // @ posedge rclk - sync reset
    input                rstb,    // @rclk, read start (data available next 8 cycles)
    output    reg [ 7:0] dout
);
    reg    [7:0] fifo_ram[0:15]; // 16x8 fifo
    reg    [3:0] wpntr;          // input fifo pointer
    reg          rcv;            // receive data
    reg    [3:0] rpntr;          // fifo read pointer
    reg    [1:0] advance_r;
    reg          snd;            // send data
    reg          snd_d;
    always @ (posedge sclk) begin
        if      (srst)        rcv <= 0; 
        else if (pre_stb)     rcv <= 1;
        else if (&wpntr[2:0]) rcv <= 0;
        
        if      (srst) wpntr <= 0;
        else if (!rcv) wpntr <= {wpntr[3],3'b0};
        else           wpntr <= wpntr + 1;
    end

    always @ (posedge sclk) begin
        if (rcv) fifo_ram[wpntr] <= din;
    end
    
    always @(posedge aclk) begin
        if (arst) advance_r <= 0;
        else      advance_r <= {advance_r[0], advance};
    end

    always @(posedge aclk) begin
        if (advance_r[0] && !advance_r[1]) rpntr[3] <= ~wpntr[3]; // previous value (now wpntr[3] is already inverted
    end
    
    always @(posedge rclk) begin
        if      (rrst)        snd <= 0; 
        else if (rstb)        snd <= 1;
        else if (&rpntr[2:1]) snd <= 0; // at count 6 
    
        snd_d <= snd;
        
        if      (rrst)          rpntr[2:0] <= 0;
        else if (!snd && !rstb) rpntr[2:0] <= 0;
        else                    rpntr[2:0] <= rpntr[2:0] + 1;
    end

    always @(posedge rclk) begin
        if (rstb || snd || snd_d) dout <= fifo_ram[rpntr];
    end
endmodule