Subversion Repositories vg_z80_sbc

//==========================================
// Function : Asynchronous FIFO (w/ 2 asynchronous clocks).
// Coder    : Alex Claros F.
// Date     : 15/May/2005.
// Notes    : This implementation is based on the article 
//            'Asynchronous FIFO in Virtex-II FPGAs'
//            writen by Peter Alfke. This TechXclusive 
//            article can be downloaded from the
//            Xilinx website. It has some minor modifications.
//=========================================
 
`timescale 1ns / 1ps
 
module async_fifo
  #(parameter    DATA_WIDTH    = 8,
                 ADDRESS_WIDTH = 4,
                 FIFO_DEPTH    = (1 << ADDRESS_WIDTH))
     //Reading port
    (output wire [DATA_WIDTH-1:0]        Data_out, 
     output reg                          Empty_out,
     input wire                          ReadEn_in,
     input wire                          RClk,        
     //Writing port.	 
     input wire  [DATA_WIDTH-1:0]        Data_in,  
     output reg                          Full_out,
     input wire                          WriteEn_in,
     input wire                          WClk,
 
     input wire                          Clear_in);
 
    /////Internal connections & variables//////
    reg   [DATA_WIDTH-1:0]              Mem [FIFO_DEPTH-1:0];
    wire  [ADDRESS_WIDTH-1:0]           pNextWordToWrite, pNextWordToRead;
    wire                                EqualAddresses;
    wire                                NextWriteAddressEn, NextReadAddressEn;
    wire                                Set_Status, Rst_Status;
    reg                                 Status;
    wire                                PresetFull, PresetEmpty;
 
    //////////////Code///////////////
    //Data ports logic:
    //(Uses a dual-port RAM).
    //'Data_out' logic:
    assign  Data_out = Mem[pNextWordToRead];
//    always @ (posedge RClk)
//        if (!PresetEmpty)
//            Data_out <= Mem[pNextWordToRead];
//        if (ReadEn_in & !Empty_out)
 
    //'Data_in' logic:
    always @ (posedge WClk)
        if (WriteEn_in & !Full_out)
            Mem[pNextWordToWrite] <= Data_in;
 
    //Fifo addresses support logic: 
    //'Next Addresses' enable logic:
    assign NextWriteAddressEn = WriteEn_in & ~Full_out;
    assign NextReadAddressEn  = ReadEn_in  & ~Empty_out;
 
    //Addreses (Gray counters) logic:
    GrayCounter #(
		.COUNTER_WIDTH( ADDRESS_WIDTH )
    ) GrayCounter_pWr (
        .GrayCount_out(pNextWordToWrite),
        .Enable_in(NextWriteAddressEn),
        .Clear_in(Clear_in),
 
        .Clk(WClk)
       );
 
    GrayCounter #(
		.COUNTER_WIDTH( ADDRESS_WIDTH )
    ) GrayCounter_pRd (
        .GrayCount_out(pNextWordToRead),
        .Enable_in(NextReadAddressEn),
        .Clear_in(Clear_in),
        .Clk(RClk)
       );
 
 
    //'EqualAddresses' logic:
    assign EqualAddresses = (pNextWordToWrite == pNextWordToRead);
 
    //'Quadrant selectors' logic:
    assign Set_Status = (pNextWordToWrite[ADDRESS_WIDTH-2] ~^ pNextWordToRead[ADDRESS_WIDTH-1]) &
                         (pNextWordToWrite[ADDRESS_WIDTH-1] ^  pNextWordToRead[ADDRESS_WIDTH-2]);
 
    assign Rst_Status = (pNextWordToWrite[ADDRESS_WIDTH-2] ^  pNextWordToRead[ADDRESS_WIDTH-1]) &
                         (pNextWordToWrite[ADDRESS_WIDTH-1] ~^ pNextWordToRead[ADDRESS_WIDTH-2]);
 
    //'Status' latch logic:
    always @ (Set_Status, Rst_Status, Clear_in) //D Latch w/ Asynchronous Clear & Preset.
        if (Rst_Status | Clear_in)
            Status = 0;  //Going 'Empty'.
        else if (Set_Status)
            Status = 1;  //Going 'Full'.
 
    //'Full_out' logic for the writing port:
    assign PresetFull = Status & EqualAddresses;  //'Full' Fifo.
 
    always @ (posedge WClk, posedge PresetFull) //D Flip-Flop w/ Asynchronous Preset.
        if (PresetFull)
            Full_out <= 1;
        else
            Full_out <= 0;
 
    //'Empty_out' logic for the reading port:
    assign PresetEmpty = ~Status & EqualAddresses;  //'Empty' Fifo.
 
    always @ (posedge RClk, posedge PresetEmpty)  //D Flip-Flop w/ Asynchronous Preset.
        if (PresetEmpty)
            Empty_out <= 1;
        else
            Empty_out <= 0;
 
endmodule