MSP430之频率测量，误差1Hz

 1 #include "timerHz.h"
 2 #include "msp430g2553.h"
 3 
 4 #define _DEBUG_TIMERHZ_
 5 
 6 unsigned int capArray[16] = {0};
 7 unsigned char index = 2, flag = 0;
 8 float hz = 0;
 9 
10 /*
11  * 绝对值
12  */
13 int abs(int x)
14 {
15     return x>0?x:-x;
16 }
17 
18 /*
19  * 系统初始化
20  */
21 void HZ_Init()
22 {
23     P1SEL |= BIT1;        //P1.1复用
24     P1DIR &= ~BIT1;        //P1.1输入
25 //    P1SEL |= BIT2;        //P1.2复用
26 //    P1DIR &= ~BIT2;        //P1.2输入
27 
28     CCTL0 = CM_1 + SCS + CCIS_0 + CAP + CCIE;             //上升沿捕获,同步,信号源设置,捕获功能,允许捕获中断
29     TACTL = TASSEL_2 + MC_2;                        //定时器的时钟源选择SMCLK(16M)，连续计数
30 }
31 
32 /******************中断服务程序*****************************/
33 #pragma vector=TIMER0_A0_VECTOR
34 __interrupt void TIME0_A0_ISR(void)
35 {
36     capArray[index--] = TA0CCR0;
37     if(index == 0)
38     {
39         index = 2;
40         TA0CCR0 = 0;        // 清零计数器
41         flag = 1;
42     }
43 }
44 
45 #ifdef _DEBUG_TIMERHZ_
46 #include "12864.h"
47 #include "sys.h"
48 
49 void main()
50 {
51     double temp = 213.456;
52 //    unsigned char i;
53 
54     System_Init();
55     LCD_Init();
56     HZ_Init();
57 
58     LCD_Show_String("Measure freq:");
59     LCD_Set_Pos(1,0);
60     LCD_Show_Deci(temp);
61     LCD_Set_Pos(1,5);
62     LCD_Show_String("Hz");
63     LCD_Set_Pos(1,0);
64 
65     _EINT();
66     while(1)
67     {
68         if (flag == 1)
69         {
70 //            for (i = 0; i<=2; i += 2)
71 //                temp += abs(capArray[i+1] - capArray[i]);
72             temp = abs(capArray[1] - capArray[2]);
73             hz = ((double )(1000000))/temp;            //1MHZ,1/1000000s = 1us
74             LCD_Set_Pos(1,0);
75             LCD_Show_Deci(hz);        //显示
76             flag = 0;
77         }
78     }
79 }
80 #endif