MSP430之频率测量，误差1Hz

#include "timerHz.h"
#include "msp430g2553.h"

#define _DEBUG_TIMERHZ_

unsigned int capArray[16] = {0};
unsigned char index = 2, flag = 0;
float hz = 0;

/*
 * 绝对值
 */
int abs(int x)
{
    return x>0?x:-x;
}

/*
 * 系统初始化
 */
void HZ_Init()
{
    P1SEL |= BIT1;        //P1.1复用
    P1DIR &= ~BIT1;        //P1.1输入
//    P1SEL |= BIT2;        //P1.2复用
//    P1DIR &= ~BIT2;        //P1.2输入

    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
    TACTL = TASSEL_2 + MC_2;                        //定时器的时钟源选择SMCLK(16M)，连续计数
}

/******************中断服务程序*****************************/
#pragma vector=TIMER0_A0_VECTOR
__interrupt void TIME0_A0_ISR(void)
{
    capArray[index--] = TA0CCR0;
    if(index == 0)
    {
        index = 2;
        TA0CCR0 = 0;        // 清零计数器
        flag = 1;
    }
}

#ifdef _DEBUG_TIMERHZ_
#include "12864.h"
#include "sys.h"

void main()
{
    double temp = 213.456;
//    unsigned char i;

    System_Init();
    LCD_Init();
    HZ_Init();

    LCD_Show_String("Measure freq:");
    LCD_Set_Pos(1,0);
    LCD_Show_Deci(temp);
    LCD_Set_Pos(1,5);
    LCD_Show_String("Hz");
    LCD_Set_Pos(1,0);

    _EINT();
    while(1)
    {
        if (flag == 1)
        {
//            for (i = 0; i<=2; i += 2)
//                temp += abs(capArray[i+1] - capArray[i]);
            temp = abs(capArray[1] - capArray[2]);
            hz = ((double )(1000000))/temp;            //1MHZ,1/1000000s = 1us
            LCD_Set_Pos(1,0);
            LCD_Show_Deci(hz);        //显示
            flag = 0;
        }
    }
}
#endif