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1 //==========================================
2 // Function : Asynchronous FIFO (w/ 2 asynchronous clocks).
3 // Coder : Alex Claros F.
4 // Date : 15/May/2005.
5 // Notes : This implementation is based on the article
6 // 'Asynchronous FIFO in Virtex-II FPGAs'
7 // writen by Peter Alfke. This TechXclusive
8 // article can be downloaded from the
9 // Xilinx website. It has some minor modifications.
10 //=========================================
11
12 `timescale 1ns/1ps
13
14 module aFifo
15 #(parameter DATA_WIDTH = 8,
16 ADDRESS_WIDTH = 4,
17 FIFO_DEPTH = (1 << ADDRESS_WIDTH))
18 //Reading port
19 (output reg [DATA_WIDTH-1:0] Data_out,
20 output reg Empty_out,
21 input wire ReadEn_in,
22 input wire RClk,
23 //Writing port.
24 input wire [DATA_WIDTH-1:0] Data_in,
25 output reg Full_out,
26 input wire WriteEn_in,
27 input wire WClk,
28
29 input wire Clear_in);
30
31 // 32 reg [DATA_WIDTH-1:0] Mem [FIFO_DEPTH-1:0];
33 wire [ADDRESS_WIDTH-1:0] pNextWordToWrite, pNextWordToRead;
34 wire EqualAddresses;
35 wire NextWriteAddressEn, NextReadAddressEn;
36 wire Set_Status, Rst_Status;
37 reg Status;
38 wire PresetFull, PresetEmpty;
39
40 // 41 //Data ports logic:
42 //(Uses a dual-port RAM).
43 //'Data_out' logic:
44 always @ (posedge RClk)
45 if (ReadEn_in & ! Empty_out)
46 Data_out <= Mem[pNextWordToRead];
47
48 //'Data_in' logic:
49 always @ (posedge WClk)
50 if (WriteEn_in & ! Full_out)
51 Mem[pNextWordToWrite] <= Data_in;
52
53 //Fifo addresses support logic:
54 //'Next Addresses' enable logic:
55 assign NextWriteAddressEn = WriteEn_in & ~Full_out;
56 assign NextReadAddressEn = ReadEn_in & ~Empty_out;
57
58 //Addreses (Gray counters) logic:
59 GrayCounter GrayCounter_pWr
60 (.GrayCount_out(pNextWordToWrite),
61
62 .Enable_in(NextWriteAddressEn),
63 .Clear_in(Clear_in),
64
65 .Clk(WClk)
66 );
67
68 GrayCounter GrayCounter_pRd
69 (.GrayCount_out(pNextWordToRead),
70 .Enable_in(NextReadAddressEn),
71 .Clear_in(Clear_in),
72 .Clk(RClk)
73 );
74
75
76 //'EqualAddresses' logic:
77 assign EqualAddresses = (pNextWordToWrite == pNextWordToRead);
78
79 //'Quadrant selectors' logic:
80 assign Set_Status = (pNextWordToWrite[ADDRESS_WIDTH-2] ~^ pNextWordToRead[ADDRESS_WIDTH-1]) & 81 (pNextWordToWrite[ADDRESS_WIDTH-1] ^ pNextWordToRead[ADDRESS_WIDTH-2]);
82
83 assign Rst_Status = (pNextWordToWrite[ADDRESS_WIDTH-2] ^ pNextWordToRead[ADDRESS_WIDTH-1]) & 84 (pNextWordToWrite[ADDRESS_WIDTH-1] ~^ pNextWordToRead[ADDRESS_WIDTH-2]);
85
86 //'Status' latch logic:
87 always @ (Set_Status, Rst_Status, Clear_in) //D Latch w/ Asynchronous Clear & Preset.
88 if (Rst_Status | Clear_in)
89 Status = 0; //Going 'Empty'.
90 else if (Set_Status)
91 Status = 1; //Going 'Full'.
92
93 //'Full_out' logic for the writing port:
94 assign PresetFull = Status & EqualAddresses; //'Full' Fifo.
95
96 always @ (posedge WClk, posedge PresetFull) //D Flip-Flop w/ Asynchronous Preset.
97 if (PresetFull)
98 Full_out <= 1;
99 else
100 Full_out <= 0;
101
102 //'Empty_out' logic for the reading port:
103 assign PresetEmpty = ~Status & EqualAddresses; //'Empty' Fifo.
104
105 always @ (posedge RClk, posedge PresetEmpty) //D Flip-Flop w/ Asynchronous Preset.
106 if (PresetEmpty)
107 Empty_out <= 1;
108 else
109 Empty_out <= 0;
110
111 endmodule
You could download file aFifo.v here
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1 //==========================================
2 // Function : Code Gray counter.
3 // Coder : Alex Claros F.
4 // Date : 15/May/2005.
5 //=======================================
6
7 `timescale 1ns/1ps
8
9 module GrayCounter
10 #(parameter COUNTER_WIDTH = 4)
11
12 (output reg [COUNTER_WIDTH-1:0] GrayCount_out, //'Gray' code count output.
13
14 input wire Enable_in, //Count enable.
15 input wire Clear_in, //Count reset.
16
17 input wire Clk);
18
19 // 20 reg [COUNTER_WIDTH-1:0] BinaryCount;
21
22 // 23
24 always @ (posedge Clk)
25 if (Clear_in) begin
26 BinaryCount <= {COUNTER_WIDTH{1'b 0}} + 1; //Gray count begins @ '1' with
27 GrayCount_out <= {COUNTER_WIDTH{1'b 0}}; // first 'Enable_in'.
28 end
29 else if (Enable_in) begin
30 BinaryCount <= BinaryCount + 1;
31 GrayCount_out <= {BinaryCount[COUNTER_WIDTH-1],
32 BinaryCount[COUNTER_WIDTH-2:0] ^ BinaryCount[COUNTER_WIDTH-1:1]};
33 end
34
35 endmodule
You could download file GrayCounter.v here
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