可调频正弦波发生器

这是利用DAC0832 和DAC0832 做的正弦波发生器，P2口是用于DAC0832的数据端，DAC0832用的是直通方式。利用改变2个取点之间的时间从而改变它的波形频率！
然后用Proteus 仿真，但为什么当我按S7或S8时，波形都没有改变的呢，求指导！是我的方法错了，还是我的代码有问题！求各位指导
下面是代码：
#include <reg52.h>
#include<absacc.h>
#define uchar unsigned char
#define uint  unsigned int
sbit s7=P1^6;        //按键7
sbit s8=P1^7;        //按键8
uint p;
uint n;
uint ft,ff;
uchar code sintab[256]=
{
0x80,0x83,0x86,0x89,0x8d,0x90,0x93,0x96,
0x99,0x9c,0x9f,0xa2,0xa5,0xa8,0xab,0xae,
0xb1,0xb4,0xb7,0xba,0xbc,0xbf,0xc2,0xc5,
0xc7,0xca,0xcc,0xcf,0xd1,0xd4,0xd6,0xd8,
0xda,0xdd,0xdf,0xe1,0xe3,0xe5,0xe7,0xe9,
0xea,0xec,0xee,0xef,0xf1,0xf2,0xf4,0xf5,
0xf6,0xf7,0xf8,0xf9,0xfa,0xfb,0xfc,0xfd,
0xfd,0xfe,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xfe,0xfd,
0xfd,0xfc,0xfb,0xfa,0xf9,0xf8,0xf7,0xf6,
0xf5,0xf4,0xf2,0xf1,0xef,0xee,0xec,0xea,
0xe9,0xe7,0xe5,0xe3,0xe1,0xde,0xdd,0xda,
0xd8,0xd6,0xd4,0xd1,0xcf,0xcc,0xca,0xc7,
0xc5,0xc2,0xbf,0xbc,0xba,0xb7,0xb4,0xb1,
0xae,0xab,0xa8,0xa5,0xa2,0x9f,0x9c,0x99,
0x96,0x93,0x90,0x8d,0x89,0x86,0x83,0x80,
0x80,0x7c,0x79,0x76,0x72,0x6f,0x6c,0x69,
0x66,0x63,0x60,0x5d,0x5a,0x57,0x55,0x51,
0x4e,0x4c,0x48,0x45,0x43,0x40,0x3d,0x3a,
0x38,0x35,0x33,0x30,0x2e,0x2b,0x29,0x27,
0x25,0x22,0x20,0x1e,0x1c,0x1a,0x18,0x16,
0x15,0x13,0x11,0x10,0x0e,0x0d,0x0b,0x0a,
0x09,0x08,0x07,0x06,0x05,0x04,0x03,0x02,
0x02,0x01,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x01,0x02,
0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,
0x0a,0x0b,0x0d,0x0e,0x10,0x11,0x13,0x15,
0x16,0x18,0x1a,0x1c,0x1e,0x20,0x22,0x25,
0x27,0x29,0x2b,0x2e,0x30,0x33,0x35,0x38,
0x3a,0x3d,0x40,0x43,0x45,0x48,0x4c,0x4e,
0x51,0x55,0x57,0x5a,0x5d,0x60,0x63,0x66,
0x69,0x6c,0x6f,0x72,0x76,0x79,0x7c,0x80       
};
void delay (uchar k);
void boxingkey();        //按键
void zhengxian();   //正弦波
void main()
{
    n=0;
    p=1;
    P1=0xff;
    ff=20;
    ft=1000000/256/ff;     //2点之间的时间
    zhengxian();
   while(1)  
    {   
        boxingkey();
   }   
}

void boxingkey()
{
  if(s7==0)
  {   
        delay(10);
        if(s7==0)
        {
            TR0=0;
            ff=ff+10;                //频率+
            ft=1000000/256/ff;
            TH0=(65536-ft)/256;
            TL0=(65536-ft)%256;
            TR0=1;
        }
                while(！s7);
                delay(10);
                while(！s7);       
    }
    else if(s8==0)
    {  
        delay(10);
        if(s8==0)
        {
        TR0=0;
        ff=ff-10;                //频率-
        ft=1000000/256/ff;
        TH0=(65536-ft)/256;
        TL0=(65536-ft)%256;
        TR0=1;
            }
                while(！s8);
                delay(10);
                while(！s8);       
    }
}

void zhengxian()   //正弦波
{
        TMOD=0x01;
        TH0=(65536-ft)/256;
        TL0=(65536-ft)%256;
        EA=1;
        ET0=1;
        TR0=1;                  
}                                       
void delay (uchar k)        //延时
{
uchar j;
while((k--)！=0)
  {
    for(j=0;j<125;j++);
   }
}
void external() interrupt 1        //中断
{
        TH0=(65536-ft)/256;
        TL0=(65536-ft)%256;
        n++;
        P2=sintab[n];
        if(n>=256)n=0;
}                                 

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