ADS1247寄存器数据读不出来

我这有个参考代码，你看看。

#include <msp430f5529.h>

long a[512];

unsigned int j;

  unsigned char nop=0xff;

  unsigned char rdatac=0x14;

int test;

int main(void)

{

  volatile unsigned int i;

  WDTCTL = WDTPW+WDTHOLD;                   // Stop watchdog timer

  P1IES |= BIT0;                           //P5.1口作为ADC的DRDY接口，下降沿触发中断

  P1IE |= BIT0;                            //P5.1中断使能

  P1IFG &= ~BIT0;

  P6DIR |= BIT7;                             //P1.1 as start signal

  P6OUT |= BIT7;

  test=0;

for(i=0;i<512;i++)

{

  a[i]=0;

}

  P4SEL |= BIT4+BIT5+BIT0;                       // P4.0,4,5 option select

  UCA1CTL1 |= UCSWRST;                      // **Put state machine in reset**

  UCA1CTL0 |= UCMST+UCSYNC+UCMSB;    // 3-pin, 8-bit SPI master

  UCA1CTL1 |= UCSSEL_2;                     // SMCLK

  UCA1BR0 = 0;                           //

  UCA1BR1 = 0;                              //

  UCA1MCTL = 0;                             // No modulation

  UCA1CTL1 &= ~UCSWRST;                     // **Initialize USCI state machine**

  j=0;

  for(i=50000;i>0;i--);                        // Wait for slave to initialize

  while (!(UCA1IFG&UCTXIFG));

  UCA1TXBUF = 0x42;

  while (!(UCA1IFG&UCTXIFG));

  UCA1TXBUF = 0x01;

  while (!(UCA1IFG&UCTXIFG));

  UCA1TXBUF = 0x60;            //MUX1

  while (!(UCA1IFG&UCTXIFG));           

  UCA1TXBUF = 0x02;            //SYS0

  while (!(UCA1IFG&UCTXIFG));

  UCA1TXBUF = 0x4A;            

  while (!(UCA1IFG&UCTXIFG));

  UCA1TXBUF = 0x01;           

  while (!(UCA1IFG&UCTXIFG));

  UCA1TXBUF = 0x04;            //IDAC0

  while (!(UCA1IFG&UCTXIFG));

  UCA1TXBUF = 0x23;            //IDAC1

    while (!(UCA1IFG&UCTXIFG));               // USCI_A0 TX buffer ready?

    UCA1TXBUF = rdatac;                     // Transmit first character

    for(i=0;i<100;i++);

  while(1)

  {

    if((P1IN & BIT0)==0)

    {

      for(i=0;i<3;i++)

      {

        while (!(UCA1IFG&UCTXIFG));               // USCI_A0 TX buffer ready?

        UCA1TXBUF = nop;                     // Transmit first character

        while (!(UCA1IFG&UCRXIFG));

        a[j] = a[j]*256+UCA1RXBUF;

      }

      j++;

      if(j>511)

      {

        j=0;

        for(i=0;i<100;i++);

        while(1);

        for(i=0;i<512;i++)

        {

         a[i]=0;

        }

      }

    }

  }

}