求助：MSP430G2452 Comparator A+输出波形的疑问

dear  all：

       我写了一个ca的小程序，可是输出波形有问题。为何比较器的输出（蓝色）周期只有原正弦波（黄色）的一半呢。而且高低电平也不对。求解答

#include <msp430g2452.h>

void clk_init(void)
{
//wdt
WDTCTL = WDTPW +WDTHOLD;//SHUTDOWN WDT

//basic clock system
DCOCTL = CALDCO_8MHZ;// calibration DCO ,8MHZ,(10FC)
BCSCTL1 = CALBC1_8MHZ;// calibration bcs, 8MHZ,(10FD)


//DISABLE INT
_DINT();

}

void port_init(void)
{
//p1.0->CA0 no need to setup
P1DIR |= BIT3;                            //P1.3 ->CAOUT
P1SEL |= BIT3;                           //DATASHEET 2X52
P1SEL2 |= BIT3;

}

void ca_init(void)
{


CACTL1 = CAEX +CAREF_3+ CAON ;                        //CAEX, CARSEL ->Vcaref(-terminal) ; CAREF_3->~0.55V


CACTL2 = P2CA1 + CAF;                                              //ca1 ->(+terminal) when CAEX =1;
}

int main(void)
{
clk_init();
port_init();

ca_init();

while(1)
{
;
}

}