當(dāng)并行數(shù)據(jù)準(zhǔn)備好后，如果得到發(fā)送指令，則將數(shù)據(jù)按UART協(xié)議輸出，先輸出一個低電平的起始位，然后從低到高輸出8個數(shù)據(jù)位，接著是可選的奇偶校驗位，最后是高電平的停止位；

　　由于發(fā)送時鐘clk16x為波特率的16倍，因此對clk16x計數(shù)到16時，發(fā)送D0；計數(shù)到32時，發(fā)送D1……依此類推；

2.發(fā)送模塊代碼：
module tx_module(
input clk16x, /* transmit clock,16×115200 */
input rst_n, /* glabol reset signal */
input TransEn, /* transmit enable */
input [7:0] DataToTrans, /* Data prepared for transmitting */

output reg BufFull, /* Data buffer is full */
output reg tx /* serial data out */
);

/* capture the rising edge of TransEn */
reg TransEn_r;
wire pos_tri;
always@(posedge clk16x or negedge rst_n)
begin
if(!rst_n)
TransEn_r <= 1'b0;
else
TransEn_r <= TransEn;
end
assign pos_tri = ~TransEn_r & TransEn;

/*
* when the rising edge of DataEn comes up, load the Data to buffer
*/
reg [7:0] ShiftReg;
always@(posedge pos_tri or negedge rst_n)
begin
if(!rst_n)
ShiftReg <= 8'b0;
else
ShiftReg <= DataToTrans;
end
//----------------------------------------------
/* counter control */
reg cnt_en;
always@(posedge clk16x or negedge rst_n)
begin
if(!rst_n)
begin
cnt_en <= 1'b0;
BufFull <= 1'b0;
end
else if(pos_tri==1'b1)
begin
cnt_en <=1'b1;
BufFull <=1'b1;
end
else if(cnt==8'd160)
begin
cnt_en<=1'b0;
BufFull <=1'b0;
end
end

//---------------------------------------------
/* counter module */
reg [7:0] cnt;
always@(posedge clk16x or negedge rst_n)
begin
if(!rst_n)
cnt<=8'd0;
else if(cnt_en)
cnt<=cnt+1'b1;
else
cnt<=8'd0;
end

//---------------------------------------------
/* transmit module */

always@(posedge clk16x or negedge rst_n)
begin
if(!rst_n)
begin
tx <= 1'b1;
end
else if(cnt_en)
case(cnt)
8'd0 : tx <= 1'b0;
8'd16 : tx <= ShiftReg[0];
8'd32 : tx <= ShiftReg[1];
8'd48 : tx <= ShiftReg[2];
8'd64 : tx <= ShiftReg[3];
8'd80 : tx <= ShiftReg[4];
8'd96 : tx <= ShiftReg[5];
8'd112 : tx <= ShiftReg[6];
8'd128 : tx <= ShiftReg[7];
8'd144 : tx <= 1'b1;
endcase
else
tx <= 1'b1;
end

endmodule

測試結(jié)果：正確率100%

3.如需要校驗位或其它風(fēng)格的設(shè)計，可以參考Xilinx的發(fā)送模塊：
module txmit (din,tbre,tsre,rst,clk16x,wrn,sdo) ;

output tbre ; /*數(shù)據(jù)緩存區(qū)滿指示*/
output tsre ; /*移位寄存器空指示*/
output sdo ; /*串行數(shù)據(jù)發(fā)送端*/
input [7:0] din ;/*并行數(shù)據(jù)輸入*/
input rst ; /*復(fù)位，高電平有效*/
input clk16x ; /*發(fā)送時鐘*/
input wrn ; /*發(fā)送使能，低電平有效*/

reg sdo ;
reg tsre ;
reg tbre ;

reg[3:0] clkdiv ;
wire clk1x ;

reg [3:0] no_bits_sent ;

/*捕獲發(fā)送使能的下降沿*/
reg wrn1 ;
reg wrn2 ;
always @(posedge clk16x or posedge rst)
begin
if (rst)
begin
wrn1 <= 1'b1 ;
wrn2 <= 1'b1 ;
end
else
begin
wrn1 <= wrn ;
wrn2 <= wrn1 ;
end
end

/* 計數(shù)控制模塊 */

reg clk1x_enable ; /*發(fā)送時鐘允許信號*/
always @(posedge clk16x or posedge rst)
begin
if (rst)
begin
tbre <= 1'b0 ;
clk1x_enable <= 1'b0 ;
end
else if (!wrn1 && wrn2) /*如果捕獲到使能信號的下降沿，則開始計數(shù)*/
begin
clk1x_enable <= 1'b1 ;
tbre <= 1'b1 ; /*數(shù)據(jù)緩存區(qū)滿標(biāo)志置1*/
end
else if (no_bits_sent == 4'b0010)
tbre <= 1'b1 ;
else if (no_bits_sent == 4'b1101)
begin
clk1x_enable <= 1'b0 ;
tbre <= 1'b0 ;
end
end

/* 使能信號的下降沿，載入輸入數(shù)據(jù)到緩沖區(qū)*/
reg [7:0] tbr ;
always @(negedge wrn or posedge rst)
begin
if (rst)
tbr = 8'b0 ;
else
tbr = din ;
end

/*16分頻，產(chǎn)生發(fā)送時鐘*/
always @(posedge clk16x or posedge rst)
begin
if (rst)
clkdiv = 4'b0 ;
else if (clk1x_enable)
clkdiv = clkdiv + 1 ;
end

assign clk1x = clkdiv[3] ;

/* 計數(shù)模塊 */
always @(posedge clk1x or posedge rst or negedge clk1x_enable)
if (rst)
no_bits_sent = 4'b0000 ;
else if (!clk1x_enable)
no_bits_sent = 4'b0000 ;
else
no_bits_sent = no_bits_sent + 1 ;

/* 發(fā)送模塊 */
reg [7:0] tsr ; /*發(fā)送移位寄存器*/
reg parity ; /* 校驗位 */
always @(negedge clk1x or posedge rst)
if (rst)
begin
sdo <= 1'b1 ;
tsre <= 1'b1 ;
parity <= 1'b1 ;
tsr <= 8'b0 ;
end
else
begin
if (no_bits_sent == 4'b0001)
begin
tsr <= tbr ; //將緩沖區(qū)中的數(shù)據(jù)載入到移位寄存器
tsre <= 1'b0 ;
end
/*start bit*/
else if (no_bits_sent == 4'b0010)
begin
sdo <= 1'b0 ;
end
/*Data bits */
else if ((no_bits_sent >= 4'b0011) && (no_bits_sent <= 4'b1010))
begin
tsr[7:1] <= tsr[6:0] ;
sr[0] <= 1'b0 ;
sdo <= tsr[7] ;
parity <= parity ^ tsr[7] ;
end
/* 校驗位 */
else if (no_bits_sent == 4'b1011)
begin
sdo <= parity ;
end
else if (no_bits_sent == 4'b1100)
begin
sdo <= 1'b1 ;
tsre <= 1'b1 ;
end
end