【AXIS】AXI-Stream FIFO设计实现（四）——异步时钟

  前文介绍了几种同步时钟情况下的AXI Stream FIFO实现方式，一般来说，FIFO也需要承担异步时钟域模块间数据传输的功能，本文介绍异步AXIS FIFO的实现方式。

  如前文所说，AXI-Stream FIFO十分类似于FWFT异步FIFO，推荐参考前文FWFT异步FIFO的实现方式【FIFO】Standard / FWFT FIFO设计实现（二）——异步时钟，也可以参考同步AXIS FIFO的实现方式【AXIS】AXI-Stream FIFO设计实现（一）——基本模式。

  对于已经实现好的FWFT FIFO，只进行些许调整，将其写FIFO信号wr_en替换为s_axis_tvalid，将FIFO写满信号wfull替换为_{s_axis_tready，将读FIFO信号rd_en替换为m_axis_tvalid，将FIFO读空信号rempty替换为}m_axis_tready。

`timescale 1ns / 1ps

module async_axis_fifo_tb(
    );
    
    localparam TDATA_WIDTH = 4;

    bit m_clk;
    bit s_clk;

    bit   [TDATA_WIDTH - 1 : 0]   m_axis_tdata;
    bit                           m_axis_tvalid; // wr_en
    bit                           m_axis_tready; // ~wfull

    bit   [TDATA_WIDTH - 1 : 0]   s_axis_tdata;  
    bit                           s_axis_tvalid; // ~rempty
    bit                           s_axis_tready;

    always #5 m_clk = ~m_clk;
    always #7 s_clk = ~s_clk;
    
    logic [TDATA_WIDTH - 1 : 0] send_queue[$], recv_queue[$];

    always_ff @(posedge m_clk) begin
        if (~m_axis_tvalid) begin
            `ifdef FLOW
                m_axis_tvalid <= 1;
            `else
                m_axis_tvalid <= $random();
            `endif
            m_axis_tdata <= $random();
        end else if (m_axis_tvalid & m_axis_tready) begin
            `ifdef FLOW
                m_axis_tvalid <= 1;
            `else
                m_axis_tvalid <= $random();
            `endif
            m_axis_tdata <= $random();
        end
    end
    
    initial begin 
        forever begin
            if (m_axis_tvalid & m_axis_tready) begin
                send_queue.push_back(m_axis_tdata);
            end

            @(posedge m_clk);
        end
    end

    initial begin
        forever begin

            if (s_axis_tvalid & s_axis_tready) begin
                recv_queue.push_back(s_axis_tdata);
            end

            if (send_queue.size != 0 && recv_queue.size != 0) begin
                if (send_queue[0] == recv_queue[0]) begin
                    send_queue.pop_front();
                    recv_queue.pop_front();
                end else begin
                    $error();
                    $stop();
                end
            end

            @(posedge s_clk);
        end
    end

    always_ff @(posedge s_clk) begin
        `ifdef FLOW
            s_axis_tready <= 1;
        `else
            s_axis_tready <= $random();
        `endif
    end

    logic wfull, rempty;
    assign m_axis_tready = ~wfull;
    assign s_axis_tvalid = ~rempty;
    async_fifo_huge #(
        .TDATA_WIDTH    (TDATA_WIDTH    ),
         .FIFO_DEPTH     (4) // 2 ** n
    ) async_fifo_huge_inst(
        .m_clk(m_clk),
        .s_clk(s_clk),
    
        .m_axis_tdata(m_axis_tdata),
        .wr_en(m_axis_tvalid), 
        .wfull(wfull),
       
        .s_axis_tdata(s_axis_tdata),  
        .rempty(rempty), 
        .rd_en(s_axis_tready)
    );
endmodule

  下图展示了通过调整利用FWFT FIFO作为AXI Stream FIFO进行数据传输的仿真波形，其中上半部分为FWFT FIFO，下半部分为AXIS Stream输入激励。