异步fifo设计

module afifo
#(parameter DATA_WIDTH = 64,
            FIFO_DEPTH = 8,
            AF_LEVEL   = (1<<FIFO_DEPTH - 10),
            AE_LEVEL   = 10
)
(
  input                         wclk          ,
  input                         wrst          ,
  input                         wen           ,
  input       [DATA_WIDTH-1:0]  wdata         ,
  output reg                    wfull         ,
  output reg                    walmost_full  ,

  input                         rclk          ,
  input                         rrst          ,
  input                         ren           ,
  output reg  [DATA_WIDTH-1:0]  rdata         ,
  output reg                    rempty        ,
  output reg                    ralmost_empty
);

wire                  fifo_rd;
wire                  fifo_wr;
wire [FIFO_DEPTH-1:0] raddr;
wire [FIFO_DEPTH-1:0] waddr;
reg  [DATA_WIDTH-1:0] memory [0:(1<<FIFO_DEPTH)-1];

wire                  wen_mask;
wire [FIFO_DEPTH:0]   wbinnext;
wire [FIFO_DEPTH:0]   wptrnext;
reg  [FIFO_DEPTH:0]   wbin;
reg  [FIFO_DEPTH:0]   wptr;
reg  [FIFO_DEPTH:0]   wptr_rsync1;
reg  [FIFO_DEPTH:0]   wptr_rsync2;
wire [FIFO_DEPTH:0]   wbin_rsync;

wire                  ren_mask;
wire [FIFO_DEPTH:0]   rbinnext;
wire [FIFO_DEPTH:0]   rptrnext;
reg  [FIFO_DEPTH:0]   rbin;
reg  [FIFO_DEPTH:0]   rptr;
reg  [FIFO_DEPTH:0]   rptr_wsync1;
reg  [FIFO_DEPTH:0]   rptr_wsync2;
wire [FIFO_DEPTH:0]   rbin_wsync;

genvar i;

// Dual port RAM

assign fifo_wr = wen_mask;
always @(posedge wclk) begin
  if(fifo_wr)
    memory[waddr] <= wdata;
end

assign fifo_rd = ren_mask;
always @(posedge rclk) begin
  if(fifo_rd)
    rdata <= memory[raddr];
end
//

// sync wptr to rclk domain

always @(posedge rclk or negedge rrst) begin
  if(rrst)
    {wptr_rsync2,wptr_rsync1} <= 0;
  else
    {wptr_rsync2,wptr_rsync1} <= {wptr_rsync1,wptr};
end

generate
  for(i=FIFO_DEPTH;i>=0;i=i-1) begin: wbin_rsync_gen
    assign wbin_rsync[i] = ^(<!--0-->,wptr_rsync2[FIFO_DEPTH:i]});
  end
endgenerate

// sync rptr to wclk domain

always @(posedge wclk or negedge wrst) begin
  if(wrst)
    {rptr_wsync2,rptr_wsync1} <= 0;
  else
    {rptr_wsync2,rptr_wsync1} <= {rptr_wsync1,rptr};
end

generate
  for(i=FIFO_DEPTH;i>=0;i=i-1) begin: rbin_wsync_gen
    assign rbin_wsync[i] = ^(<!--1-->,rptr_wsync2[FIFO_DEPTH:i]});
  end
endgenerate

// rd addr generate

always @(posedge rclk or negedge rrst) begin
  if(rrst)
    {rbin,rptr} <= 0;
  else
    {rbin,rptr} <= {rbinnext,rptrnext};
end
assign raddr    = rbin[FIFO_DEPTH-1:0];
assign ren_mask = ren & (~rempty);
assign rbinnext = ren_mask ? (rbin + 1'b1) : rbin;
assign rptrnext = (rbinnext>>1) ^ rbinnext;

// wr addr generate

always @(posedge wclk or negedge wrst) begin
  if(wrst)
    {wbin,wptr} <= 0;
  else
    {wbin,wptr} <= {wbinnext,wptrnext};
end
assign waddr = wbin[FIFO_DEPTH-1:0];
assign wen_mask = (wen & (~wfull));
assign wbinnext = wen_mask ? (wbin + 1'b1) : wbin;
assign wptrnext = (wbinnext>>1) ^ wbinnext;

//*************************************
// FIFO status
//*************************************
always @(posedge rclk or negedge rrst) begin
  if(rrst)
    rempty <= 1'b1;
  else
    rempty <= (rptrnext == wptr_rsync2);
end

always @(posedge rclk or negedge rrst) begin
  if(rrst)
    ralmost_empty <= 1'b1;
  else
    ralmost_empty <= (wbin_rsync - rbinnext <= AE_LEVEL);
end

always @(posedge wclk or negedge wrst) begin
  if(wrst)
    wfull <= 0;
  else
    wfull <= (wptrnext == {~rptr_wsync2[FIFO_DEPTH:FIFO_DEPTH-1],rptr_wsync2[FIFO_DEPTH-2:0]});
end

always @(posedge wclk or negedge wrst) begin
  if(wrst)
    walmost_full <= 1'b1;
  else
    walmost_full <= (wbinnext - rbin_wsync >= AF_LEVEL);
end

endmodule