非常感谢大家的回复   我按照你们的思路在搞一下

参考单片机电子琴发声原理。高音叨是中音叨两倍频。

由图看出高频是低频的一倍
PCA不懂，如果频率不高可以用定时器软件模拟

1. void timer1_int (void) interrupt 3
   
2. {
   
3.         static uchar i = 0;
   
4.         TL1 = 0x18;                //设置定时初始值1ms@12.000MHz 12T
   
5.         TH1 = 0xFC;                //设置定时初始值1ms@12.000MHz 12T
   
6.         if(HLF_Sel)//0 = Low Feq, 1 = High Feq
   
7.         {
   
8.                 S_Out = i%2;//0=0,1=1,2=0,3=1
   
9.         }
   
10.         else
    
11.         {
    
12.                 S_Out = i>1;//0=0,1=0,2=1,3=1
    
13.         }
    
14.         if(++i==4)
    
15.         {
    
16.                 i=0;
    
17.         }
    
18. }

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思路是:定时器确定切换波频率的周期,改写PWM定时器，PWM只管输出就完了。
其实频占比都是50%的话，压根不用PWM，直接用T0CLK_OUT就可以了。

看图疑似倍频有2种方法1改变定时器初值2不变定时器初值修改计数值

最简单的方法是用定时器，按需要的脉冲宽度和变化周期做一个数组，定时器中断函数按序从数组中取值作重载值即可。示例如下：是一个按正弦规律变化的PWM波形。

1. #include <reg51.h>
   
2. sbit PWM=P1^0;
   
3. unsigned char code K_Sine[256]={
   
4. 0x00,0x01,0x03,0x04,0x06,0x07,0x09,0x0a,0x0c,0x0d,0x0f,0x10,
   
5. 0x12,0x13,0x15,0x16,0x18,0x19,0x1b,0x1c,0x1e,0x1f,0x21,0x22,
   
6. 0x24,0x25,0x27,0x28,0x2a,0x2b,0x2d,0x2e,0x2f,0x31,0x32,0x34,
   
7. 0x35,0x36,0x38,0x39,0x3b,0x3c,0x3d,0x3f,0x40,0x41,0x43,0x44,
   
8. 0x45,0x47,0x48,0x49,0x4a,0x4c,0x4d,0x4e,0x4f,0x50,0x52,0x53,
   
9. 0x54,0x55,0x56,0x57,0x58,0x5a,0x5b,0x5c,0x5d,0x5e,0x5f,0x60,
   
10. 0x61,0x62,0x63,0x64,0x65,0x66,0x67,0x67,0x68,0x69,0x6a,0x6b,
    
11. 0x6c,0x6d,0x6d,0x6e,0x6f,0x70,0x70,0x71,0x72,0x72,0x73,0x74,
    
12. 0x74,0x75,0x76,0x76,0x77,0x77,0x78,0x78,0x79,0x79,0x7a,0x7a,
    
13. 0x7b,0x7b,0x7b,0x7c,0x7c,0x7d,0x7d,0x7d,0x7d,0x7e,0x7e,0x7e,
    
14. 0x7e,0x7f,0x7f,0x7f,0x7f,0x7f,0x7f,0x7f,0x7f,0x7d,0x7b,0x7a,
    
15. 0x78,0x77,0x75,0x74,0x72,0x71,0x6f,0x6e,0x6c,0x6b,0x69,0x68,
    
16. 0x66,0x65,0x63,0x62,0x60,0x5f,0x5d,0x5c,0x5a,0x59,0x57,0x56,
    
17. 0x54,0x53,0x51,0x50,0x4f,0x4d,0x4c,0x4a,0x49,0x48,0x46,0x45,
    
18. 0x43,0x42,0x41,0x3f,0x3e,0x3d,0x3b,0x3a,0x39,0x37,0x36,0x35,
    
19. 0x34,0x32,0x31,0x30,0x2f,0x2e,0x2c,0x2b,0x2a,0x29,0x28,0x27,
    
20. 0x26,0x24,0x23,0x22,0x21,0x20,0x1f,0x1e,0x1d,0x1c,0x1b,0x1a,
    
21. 0x19,0x18,0x17,0x17,0x16,0x15,0x14,0x13,0x12,0x11,0x11,0x10,
    
22. 0x0f,0x0e,0x0d,0x0c,0x0c,0x0b,0x0a,0x0a,0x09,0x09,0x08,0x08,
    
23. 0x07,0x07,0x05,0x05,0x04,0x04,0x04,0x03,0x03,0x03,0x02,0x02,
    
24. 0x02,0x02,0x02,0x01,0x01,0x01,0x01,0x01,0x00,0x00,0x00,0x00,
    
25. 0x00,0x00,0x00,0x00};    //kindred sine wave
    
26. 
    
27. void InitTimer0(void)
    
28. {
    
29.     TMOD = 0x01;
    
30.     TH0 = 0xff;
    
31.     TL0 = 0x00;
    
32.     EA = 1;
    
33.     ET0 = 1;
    
34.     TR0 = 1;
    
35. }
    
36. 
    
37. void main(void)
    
38. {
    
39.         InitTimer0();
    
40.         while(1);
    
41. }
    
42. 
    
43. void Timer0Interrupt(void) interrupt 1
    
44. {
    
45.         static unsigned char i;
    
46.         PWM=~PWM;
    
47.         TH0 = 0xff;
    
48.         TL0 = K_Sine[i];
    
49.         i++;
    
50. }

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是不是只有两种频率？我看占空比都是50%，没必要用PWM了，搞个定时器就行

用不同定时中断输出高低电平实现。