ADS8638写寄存器时同时有数据输出

ADS8638根据手册上看写寄存器的时候，DOUT应该是0x0000的，但是为什么我写寄存器的时候，同时有数据输出？


以下是我的代码
#include "DSP2833x_Device.h"     // DSP2833x Headerfile Include File#include "DSP2833x_Examples.h"   // DSP2833x Examples Include File#include "SciSetup.h"#define          LedReg        (*((volatile  Uint16 *)0x41FF))Uint16    *ExRamStart = (Uint16 *)0x100000;void senddata(Uint16 a, int num);#define ALPD GpioDataRegs.GPADAT.bit.GPIO26// Prototype statements for functions found within this file.// interrupt void ISRTimer2(void);void delay_loop(void);void spi_xmit(Uint16 a);void spi_fifo_init(void);void spi_init(void);void error(void);unsigned int Spi_TxReady(void);unsigned int Spi_RxReady(void);void main(void)[   Uint16 sdata;  // send data   Uint16 rdata;  // received data   // Step 1. Initialize System Control:// PLL, WatchDog, enable Peripheral Clocks// This example function is found in the DSP2833x_SysCtrl.c file.   InitSysCtrl();// Step 2. Initalize GPIO: // This example function is found in the DSP2833x_Gpio.c file and// illustrates how to set the GPIO to it's default state.// InitGpio();  // Skipped for this example  // Setup only the GP I/O only for SPI-A functionality// This function is found in DSP2833x_Spi.c   InitXintf16Gpio();        //zq   InitSpiaGpio();   EALLOW;      GpioCtrlRegs.GPAMUX2.bit.GPIO26 = 0;      GpioCtrlRegs.GPAPUD.bit.GPIO26=0;      GpioCtrlRegs.GPADIR.bit.GPIO26 = 1;      EDIS;// Step 3. Clear all interrupts and initialize PIE vector table:// Disable CPU interrupts    DINT;// Initialize PIE control registers to their default state.// The default state is all PIE interrupts disabled and flags// are cleared.  // This function is found in the DSP2833x_PieCtrl.c file.   InitPieCtrl();// Disable CPU interrupts and clear all CPU interrupt flags:   IER = 0x0000;   IFR = 0x0000;   // Initialize the PIE vector table with pointers to the shell Interrupt // Service Routines (ISR).  // This will populate the entire table, even if the interrupt// is not used in this example.  This is useful for debug purposes.// The shell ISR routines are found in DSP2833x_DefaultIsr.c.// This function is found in DSP2833x_PieVect.c.   InitPieVectTable();        // Step 4. Initialize all the Device Peripherals:// This function is found in DSP2833x_InitPeripherals.c// InitPeripherals(); // Not required for this example   spi_fifo_init();          // Initialize the Spi FIFO   spi_init();                  // init SPI  // Step 5. User specific code:// Interrupts are not used in this example.         LedReg = 0xFF;                        ALPD=1;                  //PowerUP//                 spi_xmit(0x080E);//PowerDown//                spi_xmit(0x0800);//PowerUp     for(;;)      [        // Transmit data             spi_xmit(0x0201);//自动采集              ]]         // Step 7. Insert all local Interrupt Service Routines (ISRs) and functions here:        void delay_loop()[    long      i;    for (i = 0; i < 1000000; i++) []]void spi_init()[            SpiaRegs.SPICCR.all =0x000F;                     // Reset on, falling edge, 16-bit char bits        SpiaRegs.SPICTL.all =0x0006;                         // Enable master mode, normal phase,                                                 // enable talk, and SPI int disabled.        SpiaRegs.SPIBRR =0x00FF;        SpiaRegs.SPICCR.all =0x008F;                 // relinquish SPI from reset    SpiaRegs.SPIPRI.bit.FREE = 1;                // Set so breakpoints don't disturb xmission]void spi_xmit(Uint16 a)[        Uint16 rdata1;  // received data        if(Spi_TxReady() == 1)        [                SpiaRegs.SPITXBUF=a;        ]        while(Spi_TxReady() != 1)        [        ]        if(Spi_RxReady() == 1)        [           rdata1 = SpiaRegs.SPIRXBUF;         ]        delay_loop();]    void spi_fifo_init()                                                                                [// Initialize SPI FIFO registers    SpiaRegs.SPIFFTX.all=0xA040;    SpiaRegs.SPIFFRX.all=0x204f;    SpiaRegs.SPIFFCT.all=0x0;]  unsigned int Spi_TxReady(void)        //发送就绪[        unsigned int i = 0;        if(SpiaRegs.SPISTS.bit.BUFFULL_FLAG == 1)[        i = 0;]else[        i = 1;]        return (i);]unsigned int Spi_RxReady(void)        //接收就绪[        unsigned int i = 0;        if(SpiaRegs.SPISTS.bit.INT_FLAG == 1)[        i = 0;]else[        i = 1;]        return (i);]//===========================================================================// No more.//===========================================================================