UART的回環(huán)實(shí)例代碼設(shè)計

1.UART

UART是異步串行通信口的總稱。它所包含的RS232RS449RS423等等是對應(yīng)各種異步串行通信口的接口標(biāo)準(zhǔn)和總線標(biāo)準(zhǔn)。他們規(guī)定了通信口的電氣特性、傳輸速率、連接特性和機(jī)械特性等一系列內(nèi)容，實(shí)際上屬于通信網(wǎng)絡(luò)的底層概念，與通信協(xié)議沒有直接關(guān)系。

幾個相關(guān)名詞的解釋：

·波特率：每秒鐘傳送的bit的個數(shù)。

·起始位：先發(fā)出一個邏輯0的信號，表示傳輸數(shù)據(jù)的開始。

·數(shù)據(jù)位：衡量通信中實(shí)際數(shù)據(jù)位的參數(shù)，標(biāo)準(zhǔn)數(shù)據(jù)位可以是5、7、8位，從最低位開始傳輸。

·奇偶校驗位：UART發(fā)送時，檢查發(fā)送數(shù)據(jù)中“1”的個數(shù)，自動在奇偶校驗位添加1/0，用于發(fā)送數(shù)據(jù)的校驗。

·停止位：一個數(shù)據(jù)的結(jié)束標(biāo)志，可以為1位、1.5位、2位的高電平。

·空閑位：處于邏輯1狀態(tài)，表示當(dāng)前線路上無數(shù)據(jù)傳輸。

·時序圖：

·發(fā)送數(shù)據(jù)過程：空閑狀態(tài)，線路處于高電平，當(dāng)收到發(fā)送數(shù)據(jù)指令后，拉低電平一個數(shù)據(jù)位的時間，接著數(shù)據(jù)按從低位到高位依次發(fā)送，數(shù)據(jù)發(fā)送完畢，接著發(fā)送奇偶校驗位和停止位（停止位為高電平），一幀數(shù)據(jù)發(fā)送結(jié)束。

·接收數(shù)據(jù)過程：空閑狀態(tài)，線路處于高電平，當(dāng)檢測到線路的下降沿，說明線路有數(shù)據(jù)傳輸，按照約定的波特率從低位到高位接收數(shù)據(jù)，數(shù)據(jù)接收完畢，接著接收并比較奇偶校驗位是否正確，如果正確則通知接收端設(shè)備準(zhǔn)備接收數(shù)據(jù)或存入緩存。

由于UART是異步傳輸，沒有同步傳輸時鐘。為保證數(shù)據(jù)傳輸?shù)恼_性，每個數(shù)據(jù)有16個時鐘采樣，取中間的采樣值，以保證不會誤碼或滑碼。

·設(shè)計實(shí)例：

下面是一個UART的回環(huán)實(shí)例代碼設(shè)計：

接收模塊uart_rx:

module uart_rx(    input rxd,    input clk,    output receive_ack,    output reg [7:0] data_i    );        parameter IDLE = 0;    parameter RECEIVE = 1;    parameter RECEIVE_END = 2;        reg [3:0] CS,NS;    reg [4:0] count;    reg [7:0] data_o_tmp;        always@(posedge clk)        CS <= NS;        always@(*) begin        NS <= CS;        case(CS)            IDLE:       if(!rxd) NS = RECEIVE;            RECEIVE:    if(count == 7) NS = RECEIVE_END;else NS = NS;            RECEIVE_END:NS = IDLE;            default:    NS = IDLE;        endcase    end        always@(posedge clk)        if(CS == RECEIVE)            count <= count + 1;        else if(CS == IDLE | CS == RECEIVE_END)            count <= 0;        always @(posedge clk)        if(CS == RECEIVE)begin            data_i[6:0] <= data_i[7:1];            data_i[7] <= rxd;        end            assign receive_ack = (CS == RECEIVE_END) ? 1 : 0;             endmodule

module uart_tx(    input [7:0] data_o,    input       clk,    input       receive_ack,    output reg  txd    );    parameter IDLE          = 0;    parameter SEND_START    = 1;    parameter SEND_DATA     = 2;    parameter SEND_END      = 3;        reg [3:0] CS,NS;    reg [4:0] count;    reg [7:0] data_o_tmp;        always @ (posedge clk)        CS <= NS;        always @ (*) begin        NS <= CS;        case(CS)            IDLE:       begin if(receive_ack) NS = SEND_START;  end            SEND_START: begin NS = SEND_DATA;                   end            SEND_DATA:  begin if(count == 7) NS = SEND_END;     end            SEND_END:   begin if(receive_ack) NS = SEND_START;  end            default:    NS = IDLE;        endcase    end        always @(posedge clk)        if(CS == SEND_START)            count <= count + 1;        else if(CS == IDLE | CS == SEND_END)            count <= 0;        else            count <= count;        always @(posedge clk)        if(CS == SEND_START)            data_o_tmp <= data_o;        else if(CS == SEND_DATA)            data_o_tmp[6:0] <= data_o_tmp[7:1];        always @(posedge clk)        if(CS == SEND_START)            txd <= 0;        else if(CS == SEND_DATA)            txd <= data_o_tmp;        else if(CS == SEND_END)            txd <= 1;            endmodulemodule uart_tx(    input [7:0] data_o,    input       clk,    input       receive_ack,    output reg  txd    );    parameter IDLE          = 0;    parameter SEND_START    = 1;    parameter SEND_DATA     = 2;    parameter SEND_END      = 3;        reg [3:0] CS,NS;    reg [4:0] count;    reg [7:0] data_o_tmp;        always @ (posedge clk)        CS <= NS;        always @ (*) begin        NS <= CS;        case(CS)            IDLE:       begin if(receive_ack) NS = SEND_START;  end            SEND_START: begin NS = SEND_DATA;                   end            SEND_DATA:  begin if(count == 7) NS = SEND_END;     end            SEND_END:   begin if(receive_ack) NS = SEND_START;  end            default:    NS = IDLE;        endcase    end        always @(posedge clk)        if(CS == SEND_START)            count <= count + 1;        else if(CS == IDLE | CS == SEND_END)            count <= 0;        else            count <= count;        always @(posedge clk)        if(CS == SEND_START)            data_o_tmp <= data_o;        else if(CS == SEND_DATA)            data_o_tmp[6:0] <= data_o_tmp[7:1];        always @(posedge clk)        if(CS == SEND_START)            txd <= 0;        else if(CS == SEND_DATA)            txd <= data_o_tmp;        else if(CS == SEND_END)            txd <= 1;            endmodule

module clk_div(    input clk,    output reg clk_out    );
    parameter baud_rata = 9600;parameterdiv_num='d125_000_000/baud_rata;//分頻數(shù)等于時鐘頻率除以想要得到的波特率    reg [15:0] num;
    always @(posedge clk) begin        if(num == div_num) begin            num <= 0;            clk_out <= 1;        end        else begin            num <= num + 1;            clk_out <= 0;        end    end
endmodule

頂層文件uart_top:

module uart_top(    input clk,inputrxd,outputtxd    );
    wire clk_9600;    wire receive_ack;    wire [7:0] data;
    uart_tx uart_tx    (        .clk        (clk_9600),        .txd        (txd),        .data_o     (data),        .receive_ack(receive_ack)    );
    uart_rx uart_rx    (        .clk        (clk_9600),        .rxd        (rxd),        .data_i     (data),        .receive_ack(receive_ack)    );
    clk_div clk_div    (        .clk        (clk),        .clk_out    (clk_9600)    );
endmodule

2.PS/2 PS/2是一種雙向同步串行通信協(xié)議。接口是一種6針的連接口，但只有四個引腳是有意義的，分別是Clock(時鐘)、Data(數(shù)據(jù))、VCC和GND。其中時鐘和數(shù)據(jù)引腳是雙向的。PS/2常用于連接某些輸入設(shè)備，例如鼠標(biāo)、鍵盤等。通信的兩端通過時鐘來同步，通過數(shù)據(jù)引腳來交換數(shù)據(jù)。任何一方想要抑制另外一方的通信，只需要將時鐘引腳拉低即可。 如果是PC和PS/2鍵盤之間通信，PC必須做主機(jī)，即PC可以抑制鍵盤發(fā)送數(shù)據(jù)，而鍵盤不能抑制PC發(fā)送數(shù)據(jù)。 PS/2的每一位數(shù)據(jù)幀包含11-12位，具體含義如下：

·PS/2的時序圖：

由設(shè)備產(chǎn)生時鐘和數(shù)據(jù)，主機(jī)根據(jù)時鐘來讀取數(shù)據(jù)。以FPGA和PS/2鍵盤為例，鍵盤產(chǎn)生時鐘和數(shù)據(jù)，F(xiàn)PGA只需要讀數(shù)據(jù)。當(dāng)時鐘下降沿時，F(xiàn)PGA記錄數(shù)據(jù)信號。 ·設(shè)計實(shí)例： 主機(jī)為FPGA，根據(jù)PS/2的時序，得到鍵盤的按鍵值。雖然在時序圖中，主機(jī)是在時鐘下降沿讀取數(shù)據(jù)，但實(shí)際上要為了排除噪聲干擾，需要在FPGA端對信號進(jìn)行濾波。下面給出設(shè)計代碼。

module ps2_keyboard(    input clk,    input clr,    input PS2C,       //ps2 clk in    input PS2D,       //ps2 data in        output [15:0] xkey);reg         PS2CF;reg         PS2DF;reg [7:0]   ps2c_filter;reg [7:0]   ps2d_filter;reg [10:0]  shift1;reg [10:0]  shift2;
assign xkey = { shift2[8:1], shift1[8:1] };always @(posedge clk or posedge clr) begin    if (clr) begin        ps2c_filter <= 11'b0;        ps2d_filter <= 11'b0;        PS2CF <= 1;        PS2DF <= 1;    end    else begin        ps2c_filter[7] <= PS2C;        ps2c_filter[6:0] <= ps2c_filter[7:1];        ps2d_filter[7] <= PS2D;        ps2d_filter[6:0] <= ps2d_filter[7:1];        if(ps2c_filter == 8'b1111_1111)            PS2CF <= 1;                         //去時鐘毛刺        else if(ps2c_filter == 8'b0000_0000)            PS2CF <= 0;        if(ps2d_filter == 8'b1111_1111)            PS2DF <= 1;                         //去數(shù)據(jù)毛刺        else if(ps2d_filter == 8'b0000_0000)            PS2DF <= 0;    endend
always @(negedge PS2CF or posedge clr) begin    if (clr) begin        shift1 <= 11'b0;        shift2 <= 11'b0;    end    else begin        shift1 <= {PS2DF, shift1[10:1]};        shift2 <= {shift1[0], shift2[10:1]};     endend
endmodule

原文標(biāo)題：常用通信協(xié)議總結(jié)及FPGA實(shí)現(xiàn)（上）

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