异步FIFO总结

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异步FIFO的基本概念

异步FIFO读写分别采用相互异步的不同时钟，使用异步FIFO可以在两个不同时钟系统之间快速而方便地传输实时数据

FIFO的常见参数

FIFO的宽度：即FIFO一次读写操作的数据位；

FIFO的深度：指的是FIFO可以存储多少个N位的数据（如果宽度为N）。

满标志：FIFO已满或将要满时由FIFO的状态电路送出的一个信号，以阻止FIFO的写操作继续向FIFO中写数据而造成溢出（overflow）。

空标志：FIFO已空或将要空时由FIFO的状态电路送出的一个信号，以阻止FIFO的读操作继续从FIFO中读出数据而造成无效数据的读出（underflow）。

读时钟：读操作所遵循的时钟，在每个时钟沿来临时读数据。

写时钟：写操作所遵循的时钟，在每个时钟沿来临时写数据。

异步FIFO的设计难点

如何同步异步信号，使触发器不产生亚稳态

如何正确地设计空、满以及几乎满等信号的控制电路

亚稳态问题的解决

对写地址/读地址采用格雷码

采用触发器来同步异步输入信号

空/满标志的产生

空/满标志产生的原则是：写满不溢出，读空不多读

FIFO的缓冲空间是有限的，在设计中需要考虑缓冲区的溢出和空情况。

缓冲区满，产生FULL信号，以阻塞数据继续向FIFO写入；

缓冲区空，产生EMPTY信号，以阻塞数据继续从FIFO读出。

方法：
1.采用握手协议

2.读时钟采样写指针，写时钟采样读指针，采样的写指针和读指针判断是否为EMPTY，采样的读指针和写指针判断是否为FULL

Gray code counter

FIFO 框图

VCS仿真结果

异步FIFO代码（verilog）

RTL描述

fifoif.v

module fifoif ( fifo_flush, data_out, full_out, empty_out, data_in, wren_in, wclk, wclr_in, rden_in, rclk, rclr_in, fifo_waddr, fifo_raddr, ram_douta, ram_dinb, ram_ada, ram_adb, ram_cena, ram_cenb, ram_clka, ram_clkb );parameter DSIZE = 32; parameter ASIZE = 7;input fifo_flush; output [DSIZE-1:0] data_out; output full_out; output empty_out; input [DSIZE-1:0] data_in; input wren_in, wclk, wclr_in; input rden_in, rclk, rclr_in; output [ASIZE-1:0] fifo_waddr, fifo_raddr;input [DSIZE-1:0] ram_douta; output [DSIZE-1:0] ram_dinb; output [ASIZE-1:0] ram_ada; output [ASIZE-1:0] ram_adb; output ram_cena; output ram_cenb; output ram_clka; output ram_clkb;wire [DSIZE-1:0] ram_douta; wire [DSIZE-1:0] ram_dinb; wire [ASIZE-1:0] ram_ada; wire [ASIZE-1:0] ram_adb; wire ram_cena; wire ram_cenb; wire ram_clka; wire ram_clkb;wire [DSIZE-1:0] data_out; wire [ASIZE-1:0] waddr; wire [ASIZE-1:0] raddr; wire [ASIZE:0] wptr; wire [ASIZE:0] rptr; wire [ASIZE:0] wq2_rptr; wire [ASIZE:0] rq2_wptr; wire [ASIZE-1:0] fifo_waddr = waddr; wire [ASIZE-1:0] fifo_raddr = raddr;sync_r2w #(ASIZE) u_sync_r2w( .wq2_rptr (wq2_rptr), .rptr (rptr), .wclk (wclk), .wrst_n (wclr_in), .wflush (fifo_flush));sync_w2r #(ASIZE) u_sync_w2r( .rq2_wptr (rq2_wptr), .wptr (wptr), .rclk (rclk), .rrst_n (rclr_in), .rflush (fifo_flush));fifomem #(DSIZE, ASIZE) u_fifomem( .rdata (data_out), .wdata (data_in), .waddr (waddr), .raddr (raddr), .wren (wren_in), .wclk (wclk), .rden (rden_in), .rclk (rclk), .ram_douta (ram_douta), .ram_dinb (ram_dinb), .ram_ada (ram_ada), .ram_adb (ram_adb), .ram_cena (ram_cena), .ram_cenb (ram_cenb), .CLKA (ram_clka), .CLKB (ram_clkb) );rptr_empty #(ASIZE) u_rptr_empty( .rempty (empty_out), .raddr (raddr), .rptr (rptr), .rq2_wptr (rq2_wptr), .rinc (rden_in), .rclk (rclk), .rrst_n (rclr_in), .rflush (fifo_flush) );wptr_full #(ASIZE) u_wptr_full( .wfull (full_out), .waddr (waddr), .wptr (wptr), .wq2_rptr (wq2_rptr), .winc (wren_in), .wclk (wclk), .wrst_n (wclr_in), .wflush (fifo_flush) );endmodule

fifomem.v

module fifomem( rdata, wdata, waddr, raddr, wren, wclk, rden, rclk, //memory interface ram_douta, ram_dinb, ram_ada, ram_adb, ram_cena, ram_cenb, CLKA, CLKB ); parameter DATASIZE = 32; // Memory data word width parameter ADDRSIZE = 7; // Number of mem address bitsoutput [DATASIZE-1:0] rdata; input [DATASIZE-1:0] wdata; input [ADDRSIZE-1:0] waddr, raddr; input wren, wclk; input rden, rclk;input [DATASIZE-1:0] ram_douta; output [DATASIZE-1:0] ram_dinb; output [ADDRSIZE-1:0] ram_ada; output [ADDRSIZE-1:0] ram_adb; output ram_cena; output ram_cenb; output CLKA; output CLKB;wire ram_cena,ram_cenb; wire CLKA; wire CLKB; wire [DATASIZE-1:0] ram_dinb,ram_douta; wire [ADDRSIZE-1:0] ram_ada, ram_adb; wire [DATASIZE-1:0] rdata,wdata;assign ram_cena = ~rden; assign ram_ada = raddr; assign ram_cenb = ~wren; assign ram_adb = waddr; assign ram_dinb = wdata; assign CLKA = rclk; assign CLKB = wclk; assign rdata = ram_douta;endmodule

ram_dp.v

module ram_dp #( parameter DATA_WIDTH = 32, parameter ADDR_WIDTH = 7, parameter RAM_DEPTH = 1 << ADDR_WIDTH ) ( output reg [DATA_WIDTH-1:0] data_out , input [DATA_WIDTH-1:0] data_in , input [ADDR_WIDTH-1:0] addr_a , input [ADDR_WIDTH-1:0] addr_b , input web , //Write Enable/Read Enable,addr_b write enable, low active input clka , // write Clock Input input clkb , // read Clock Input input cena , //Chip Select input cenb //Chip Select ); //--------------Internal variables---------------- reg [DATA_WIDTH-1:0] mem [0:RAM_DEPTH-1]; reg oe_r;//--------------Code Starts Here------------------ // Memory Write Block // Write Operation : When web = 0, cenb = 1 always @ (posedge clkb) begin : MEM_WRITE if ((~cenb) && (~web) ) begin mem[addr_b] = data_in; end end// Memory Read Block // Read Operation : When web = 1, cena = 1 always @ (posedge clka) begin : MEM_READ if ((~cena) && web) begin data_out = mem[addr_a]; end endendmodule // End of Module ram_dp

sync_r2w.v

module sync_r2w #(parameter ADDRSIZE = 4) ( output reg [ADDRSIZE:0] wq2_rptr, input [ADDRSIZE:0] rptr, input wclk, input wrst_n, input wflush );reg [ADDRSIZE:0] wq1_rptr;always @(posedge wclk or negedge wrst_n) if (!wrst_n) {wq2_rptr,wq1_rptr} <= 0; else if(wflush) {wq2_rptr,wq1_rptr} <= 0; else {wq2_rptr,wq1_rptr} <= {wq1_rptr,rptr};endmodule

sync_w2r.v

module sync_w2r #(parameter ADDRSIZE = 4) ( output reg [ADDRSIZE:0] rq2_wptr, input [ADDRSIZE:0] wptr, input rclk, input rrst_n, input rflush );reg [ADDRSIZE:0] rq1_wptr;always @(posedge rclk or negedge rrst_n) if (!rrst_n) {rq2_wptr,rq1_wptr} <= 0; else if(rflush) {rq2_wptr,rq1_wptr} <= 0; else {rq2_wptr,rq1_wptr} <= {rq1_wptr,wptr};endmodule

rptr_empty.v

module rptr_empty #(parameter ADDRSIZE = 4) ( output reg rempty, output [ADDRSIZE-1:0] raddr, output reg [ADDRSIZE :0] rptr, input [ADDRSIZE :0] rq2_wptr, input rinc, input rclk, input rrst_n, input rflush );reg [ADDRSIZE:0] rbin; wire [ADDRSIZE:0] rgraynext, rbinnext;//------------------- // GRAYSTYLE2 pointer //------------------- always @(posedge rclk or negedge rrst_n) if (!rrst_n) {rbin, rptr} <= 0; else if(rflush) {rbin, rptr} <= 0; else {rbin, rptr} <= {rbinnext, rgraynext};// Memory read-address pointer (okay to use binary to address memory) assign raddr = rbin[ADDRSIZE-1:0];assign rbinnext = rbin + (rinc & ~rempty); assign rgraynext = (rbinnext>>1) ^ rbinnext;//--------------------------------------------------------------- // FIFO empty when the next rptr == synchronized wptr or on reset //--------------------------------------------------------------- wire rempty_val = (rgraynext == rq2_wptr);always @(posedge rclk or negedge rrst_n) if (!rrst_n) rempty <= 1'b1; else rempty <= rempty_val;endmodule

wptr_full.v

module wptr_full #(parameter ADDRSIZE = 4) ( output reg wfull, output [ADDRSIZE-1:0] waddr, output reg [ADDRSIZE :0] wptr, input [ADDRSIZE :0] wq2_rptr, input winc, input wclk, input wrst_n, input wflush );reg [ADDRSIZE:0] wbin; wire [ADDRSIZE:0] wgraynext, wbinnext;// GRAYSTYLE2 pointer always @(posedge wclk or negedge wrst_n) if (!wrst_n) {wbin, wptr} <= 0; else if(wflush) {wbin, wptr} <= 0; else {wbin, wptr} <= {wbinnext, wgraynext};// Memory write-address pointer (okay to use binary to address memory) assign waddr = wbin[ADDRSIZE-1:0];assign wbinnext = wbin + (winc & ~wfull); assign wgraynext = (wbinnext>>1) ^ wbinnext;//------------------------------------------------------------------ // Simplified version of the three necessary full-tests: // assign wfull_val=((wgnext[ADDRSIZE] !=wq2_rptr[ADDRSIZE] ) && // (wgnext[ADDRSIZE-1] !=wq2_rptr[ADDRSIZE-1]) && // (wgnext[ADDRSIZE-2:0]==wq2_rptr[ADDRSIZE-2:0])); //------------------------------------------------------------------wire wfull_val = (wgraynext=={~wq2_rptr[ADDRSIZE:ADDRSIZE-1], wq2_rptr[ADDRSIZE-2:0]});always @(posedge wclk or negedge wrst_n) if (!wrst_n) wfull <= 1'b0; else wfull <= wfull_val;endmodule

testbench

fifo_tb.v

module fifo_tb;reg fifo_flush; wire [31:0] data_out; wire full_out; wire empty_out; wire [31:0] data_in; wire wren_in; wire wclk; wire wr_rst_n; wire rd_rst_n; wire rden_in; wire rclk; wire [6:0] fifo_waddr; wire [6:0] fifo_raddr;// memory interface signal wire [31:0] if_ram_douta; wire [31:0] if_ram_dinb; wire [6:0] if_ram_ada; wire [6:0] if_ram_adb; wire if_ram_cena; wire if_ram_cenb; wire if_ram_clka; wire if_ram_clkb;//****************************************************************** // generate clk //****************************************************************** clock #(.CLK_FREQ(100.0)) u_clock_wr ( .clk ( wclk ) );clock #(.CLK_FREQ(70.0)) u_clock_rd ( .clk ( rclk ) );assign if_ram_clka = rclk; assign if_ram_clkb = wclk;//****************************************************************** // generate read and write data //******************************************************************fifo_data #(.DATA_WIDTH(8)) u_fifo_data ( .wr_rst_n ( wr_rst_n ), .wr_clk ( wclk ), .wr_en ( wren_in ), .wr_data ( data_in ), .wr_full ( full_out ), .rd_rst_n ( rd_rst_n ), .rd_clk ( rclk ), .rd_en ( rden_in ), .rd_empty ( empty_out ) );//****************************************************************** // creat FSDB //****************************************************************** initial begin $fsdbDumpfile("tb.fsdb"); $fsdbDumpvars(); end//=================================================== // u_fifoif from rtl/fifoif.v //===================================================fifoif u_fifoif ( .fifo_flush ( 1'b0 ), .data_out ( data_out ), .full_out ( full_out ), .empty_out ( empty_out ), .data_in ( data_in ), .wren_in ( wren_in ), .wclk ( wclk ), .wclr_in ( wr_rst_n ), .rden_in ( rden_in ), .rclk ( rclk ), .rclr_in ( rd_rst_n ), .fifo_waddr ( fifo_waddr ), .fifo_raddr ( fifo_raddr ), // Memory interface .ram_douta ( if_ram_douta ), .ram_dinb ( if_ram_dinb ), .ram_ada ( if_ram_ada ), .ram_adb ( if_ram_adb ), .ram_cena ( if_ram_cena ), .ram_cenb ( if_ram_cenb ), .ram_clka ( if_ram_clka ), .ram_clkb ( if_ram_clkb ) );ram_dp u_ram_dp( .data_out (if_ram_douta[31:0]), //A data output, 32 bits .data_in (if_ram_dinb[31:0]), //B data input , 32 bits .addr_a (if_ram_ada[6:0]), //A adress, 7 bits .addr_b (if_ram_adb[6:0]), //B adress, 7 bits .web (if_ram_cenb), //Write Enable/Read Enable,addr_b write enable, low active .clka (if_ram_clka), // write Clock Input .clkb (if_ram_clkb), // read Clock Input .cena (if_ram_cena), //Chip Select ,low active .cenb (if_ram_cenb) //Chip Select ,low active ); endmodule

clock.v

module clock #(parameter CLK_FREQ = 100.0) //MHz ( output reg clk ); localparam CLK_CYCLE = 1000.0 / CLK_FREQ; initial begin clk = 0; forever #(CLK_CYCLE/2) clk = ~clk; endendmodule

fifo_data.v

module fifo_data #(parameter DATA_WIDTH = 8) ( output reg wr_rst_n, input wr_clk, output reg wr_en, output reg [DATA_WIDTH-1:0] wr_data, input wr_full, output reg rd_rst_n, input rd_clk, output reg rd_en, input rd_empty ); reg normal_wr; reg normal_rd; initial begin wr_rst_n = 1'b0; rd_rst_n = 1'b0; normal_wr = 1'b0; normal_rd = 1'b0; #492; wr_rst_n = 1'b1; rd_rst_n = 1'b1; #100; //only write FIFO normal_rd = 1'b0; normal_wr = 1'b1; repeat(500) @(negedge wr_clk); //only read FIFO normal_wr = 1'b0; normal_rd = 1'b1; repeat(500) @(negedge rd_clk); //read and write FIFO normal_rd = 1'b0; normal_wr = 1'b0; normal_wr = 1'b1; normal_rd = 1'b1; repeat(1000) @(negedge wr_clk); normal_wr = 1'b0; normal_rd = 1'b0; repeat(50) @(negedge rd_clk); $finish; end //****************************************************************** // write FIFO data generate //****************************************************************** always @(negedge wr_clk or negedge wr_rst_n) begin if(wr_rst_n == 1'b0) begin wr_en <= 1'b0; wr_data <= {(DATA_WIDTH){1'b0}}; end else if(normal_wr == 1'b1) begin if(wr_full == 1'b0) begin wr_en <= 1'b1; wr_data <= {$random%((1 << (DATA_WIDTH-1)))}; //wr_data <= $random; end else begin wr_en <= 1'b0; wr_data <= {(DATA_WIDTH){1'b0}}; end end else begin wr_en <= 1'b0; wr_data <= {(DATA_WIDTH){1'b0}}; end end //****************************************************************** // read FIFO data generate //****************************************************************** always @(negedge wr_clk or negedge wr_rst_n) begin if(wr_rst_n == 1'b0) rd_en <= 1'b0; else if(normal_rd == 1'b1) begin if(rd_empty == 1'b0) rd_en <= 1'b1; else rd_en <= 1'b0; end else rd_en <= 1'b0; endendmodule

参考资料

[0].Simulation and Synthesis Techniques for Asynchronous FIFO Design

[1].Memories

[2].异步FIFO的FPGA实现

[3].异步FIFO的设计

[4].基于FPGA的异步FIFO设计

[5].基于FPGA的异步FIFO验证

正文

异步FIFO总结