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在verilog程序設(shè)計(jì)中，我們往往要對(duì)一個(gè)頻率進(jìn)行任意分頻，而且占空比也有一定的要求這樣的話，對(duì)于程序有一定的要求，現(xiàn)在我在前人經(jīng)驗(yàn)的基礎(chǔ)上做一個(gè)簡單的總結(jié)，實(shí)現(xiàn)對(duì)一個(gè)頻率的任意占空比的任意分頻。

比如：我們FPGA系統(tǒng)時(shí)鐘是50M Hz，而我們要產(chǎn)生的頻率是880Hz，那么，我們需要對(duì)系統(tǒng)時(shí)鐘進(jìn)行分頻。我們很容易想到用計(jì)數(shù)的方式來分頻：

50000000/880 = 56818

這個(gè)數(shù)字不是2的整冪次方，那么怎么辦呢？我們可以設(shè)定一個(gè)參數(shù)，讓它到56818的時(shí)候重新計(jì)數(shù)不就完了嗎？呵呵，程序如下：

module div(clk, clk_div);

input clk;

output clk_div;

reg [15:0] counter;

always @(posedge clk)

if(counter==56817) counter <= 0;

else counter <= counter+1;

assign clk_div = counter[15];

endmodule

下面我們來算一下它的占空比：我們清楚地知道，這個(gè)輸出波形在counter為0到32767的時(shí)候?yàn)榈停?2767到56817的時(shí)候?yàn)楦?，占空比?0%多一些，如果我們需要占空比為50%，那么怎么辦呢？不用急，慢慢來。

我們?cè)僭O(shè)定一個(gè)參數(shù)，使它為56817的一半，使達(dá)到它的時(shí)候波形翻轉(zhuǎn)，那不就完了嗎？呵呵，再看看：

module div(clk, clk_div);
input clk;
output clk_div;
reg [14:0] counter;
always @(posedge clk)
if(counter==28408) counter <= 0;
else counter <= counter+1;
reg clk_div;
always @(posedge clk)
  if(counter==28408) clk_div <= ~clk_div;
endmodule

占空比是不是神奇地變成50%了呢？呵呵。

繼續(xù)讓我們來看如何實(shí)現(xiàn)任意占空比，比如還是由50 M分頻產(chǎn)生880 Hz，而分頻得到的信號(hào)的占空比為30%。

56818×30%=17045

module div(clk,reset,clk_div,counter);
input clk,reset;
output clk_div;
output [15:0] counter;
reg [15:0] counter;
reg clk_div;
always @(posedge clk)
if(!reset) counter <= 0;
else if(counter==56817) counter <= 0;
else counter <= counter+1;
always @(posedge clk)
if(!reset) clk_div <= 0;
else if(counter<17045) clk_div <= 1;
else clk_div <= 0;
endmodule

三分頻的Verilog實(shí)現(xiàn)

//很實(shí)用也是筆試面試時(shí)?？嫉?，已經(jīng)經(jīng)過仿真

占空比要求50%和不要求占空比差別會(huì)很大，先看一個(gè)占空比50%的描述

module div3(CLKIN,CLKOUT,RESETn);
input CLKIN,RESETn;
output CLKOUT;
 
//internal counter signals
reg[1:0] count_a;
reg[1:0] count_b;
reg      CLKOUT;
 
always @(negedge RESETn or posedge CLKIN)
begin
    if (RESETn==1'b0)
        count_a<=2'b00;
    else
        if (count_a==2'b10)
            count_a<=2'b00;
        else
            count_a<=count_a+1;
end
 
always @(negedge RESETn or negedge CLKIN)
begin
    if (RESETn==1'b0)
        count_b<=2'b0;
    else
        if (count_b==2'b10)
            count_b<=2'b00;
        else
            count_b<=count_b+1;
end
 
always @(count_a or count_b or RESETn)
begin
 if (RESETn==1'b0)
   CLKOUT=1'b0;
 else if((count_a+count_b==4)||(count_a+ count_b==1))
   CLKOUT=~CLKOUT;
end
endmodule

0 1 2 0 1 2
/ / / /
0 1 2 0 1 2

下面是一個(gè)非50%的描述，只用了上升沿

module div3(CLKIN,CLKOUT,RESETn);
input CLKIN,RESETn;
output CLKOUT;
 
 
wire d;
reg     q1,q2;
wire         CLKOUT;
 
always @(negedge RESETn or posedge CLKIN)
begin
    if (RESETn==1'b0)
        q1<=1'b0;
    else
        q1<=d;
end
 
always @(negedge RESETn or posedge CLKIN)
begin
    if (RESETn==1'b0)
        q2<=1'b0;
    else
        q2<=q1;
end
assign d=~q1 & ~q2;
assign CLKOUT=q2;
endmodule

占空比不是50%，只用了單沿觸發(fā)器，寄存器輸出。

至于其他奇數(shù)要求50%的或者不要求的占空比的，都可以參照上面兩個(gè)例子做出。
占空比為50%的一個(gè)更好的實(shí)現(xiàn)。

module div3(CLKIN,CLKOUT,RESETn);
input CLKIN,RESETn;
output CLKOUT;
//internal counter signals
reg[1:0] count_a;
reg            b,c;
//reg        CLKOUT;
wire CLKOUT;
always @(negedge RESETn or posedge CLKIN)
begin
    if (RESETn==1'b0)
        count_a<=2'b00;
    else
        if (count_a==2'b10)
            count_a<=2'b00;
        else
            count_a<=count_a+1;
end
always @(negedge RESETn or negedge CLKIN)
begin
    if (RESETn==1'b0)
        b<=1'b0;
    else
        if (count_a==2'b01)
            b<=2'b0;
        else
            b<=1'b1;
end
always @(negedge RESETn or posedge CLKIN)
begin
    if (RESETn==1'b0)
        c<=1'b0;
    else
        if (count_a==2'b10)
            c<=1'b1;
        else if (count_a==2'b01)
            c<=1'b0;
end
assign          CLKOUT=b & c;
endmodule

時(shí)鐘選擇器的Verilog寫法!

CPRI有3種數(shù)據(jù)時(shí)鐘,61.44M 122.88M 245.76M,需要模塊能夠根據(jù)外部的速率指示信號(hào)(一個(gè)2位的輸入信號(hào),由模塊外部給定)選擇其中的一種時(shí)鐘作為模塊的工作時(shí)鐘

但由于所選用的FPGA工作頻率關(guān)系,不能超過400M,無法通過寄存器方式實(shí)現(xiàn)時(shí)鐘源的選擇.

使用雙邊觸發(fā)的方式將最高的頻率進(jìn)行分頻,代碼如下,已經(jīng)通過ModelSim的仿真，可以實(shí)現(xiàn)。

module clk_div(
                  reset,        //復(fù)位信號(hào)
                  data_rate,    //數(shù)據(jù)速率指示
                  clk2,         //245.76M的時(shí)鐘輸入
                  time1,        //分頻計(jì)數(shù)器
                  clk           //選擇后的時(shí)鐘輸出
                  );
input       reset;
input [1:0] data_rate;
input       clk2;
output [1:0] time1;
output      clk;
reg [1:0] time1;
reg       clk;                  
                  
always@(clk2 or reset)
if(reset)
   begin
     time1=2'b00;
     clk=1'b0;
   end
else
   begin
     time1=time1+1'b1;
     case(data_rate)
       2'b00:if(time1==2'b00)
                clk=~clk;
             else
                clk=clk;  
       2'b01:if(time1[0]==1'b1)
                clk=~clk;
             else
                clk=clk;
       2'b10:clk=clk;
       2'b11:clk=~clk;
     endcase
   end
endmodule

verilog 實(shí)現(xiàn)gray碼計(jì)數(shù)器

//16位gray碼計(jì)數(shù)器,gary碼狀態(tài)改變時(shí)候每次只改變一個(gè)bit
//，可以有效防止競爭和毛刺的產(chǎn)生。
module gray_counter(clk,clr,start,stop,q,cout);
 input clk;
 input clr;
 input start,stop;
 output reg [3:0] q;
 output reg cout;
 reg flag=1;
 reg [3:0] s,next_s;
 
 //parameter S0=0, S1=1, S2=2, S3=3, S4=4, S5=5, S6=6, S7=7;
 //parameter S8=8, S9=9, S10=10, S11=11, S12=12, S13=13, S14=14, S15=15;
 parameter s0=0000, s1=0001, s2=0011, s3=0010;
 parameter s4=0110, s5=0111, s6=0101, s7=0100;
 parameter s8=1100, s9=1101, s10=1111,s11=1110;
 parameter s12=1010, s13=1011, s14=1001, s15=1000;
 always @(posedge clk)
 begin
  if (clr) s <= s0;
  else   s <= next_s;
 end 
 /*always @(posedge start or posedge stop)
 begin
  if(start) flag=1;
  else if (stop) flag=0;
 end*/
 always @(s or flag)     /*該進(jìn)程實(shí)現(xiàn)狀態(tài)的轉(zhuǎn)換*/
 begin    
  case (s)
   s0: begin
   if (flag)  next_s <=s1;
   // else       next_s <=s0;
    end
   s1: begin
   if (flag)    next_s <= s2;
   end
   s2: begin
   if (flag)   next_s <=s3;
   ////else        next_s <=s0;
   end
   s3: begin
   if (flag)    next_s <=s4;
   //else        next_s <=s3;
   end
   s4: begin
   if (flag)     next_s <= s5;
   end
   s5: begin
   if (flag)   next_s <=s6;
   //else        next_s <=s0;
   end
   s6: begin
   if (flag)   next_s <=s7;
   //else        next_s <=s3;
   end
   s7: begin
   if (flag)  next_s <=s8;
   //else       next_s <=s0;
   end
   s8: begin
   if (flag)    next_s <= s9;
   end
   s9: begin
   if (flag)   next_s <=s10;
   ////else        next_s <=s0;
   end
   s10: begin
   if (flag)    next_s <=s11;
   //else        next_s <=s3;
   end
   s11: begin
   if (flag)     next_s <= s12;
   end
   s12: begin
   if (flag)   next_s <=s13;
   //else        next_s <=s0;
   end
   s13: begin
   if (flag)   next_s <=s14;
   //else        next_s <=s3;
   end
   s14: begin
   if (flag)   next_s <=s15;
   ////else        next_s <=s0;
   end
   s15: begin
   if (flag)    next_s <=s0;
   //else        next_s <=s3;
   end
   default:    next_s <=s0;   /*default語句*/
  endcase
 end
 always @(s)          /*該進(jìn)程定義組合邏輯（FSM的輸出）*/  
 begin
  case(s)
  s0: q=0;
  s1: q=4;
  s2: q=2;
  s3: q=3;
  s4: q=4;
  s5: q=5;
  s6: q=6;
  s7: q=7;
  s8: q=8;
  s9: q=9;
  s10: q=10;
  s11: q=11;
  s12: q=12;
  s13: q=13;
  s14: q=14;
  s15: q=15;    
  default:q=0;     /*default語句，避免鎖存器的產(chǎn)生*/
  endcase
 end
 always @(s)
    begin
   if (s==s15) cout=1;//assign cout=q;
   else cout=0;
 end
endmodule

審核編輯：郭婷

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