28335测试超声波遇到的问题，请指导

#include "DSP28x_Project.h"     // Device Headerfile and Examples Include File

#include "systimer.h"

#define  LED1GpioDataRegs.GPBDAT.bit.GPIO60

#define  LED2GpioDataRegs.GPBDAT.bit.GPIO61

#define   Trig  GpioDataRegs.GPADAT.bit.GPIO12

#define   Echo  GpioDataRegs.GPADAT.bit.GPIO13

interrupt void xint2_isr(void);

void scic_echoback_init(void);

void scic_fifo_init(void);

void scic_xmit(int a);

void scic_msg(char *msg);

void InitLED(void);

void Init_ultrasonic(void);

Uint32 begin_time=0;

Uint32 over_time=0;

Uint32 period=0;

float displacement=0.0;

Uint32 H_displacement=0;

Uint32 L_displacement=0;

Uint16 flag=0;

void main(void)

{

   char *msg;

   

// 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

   InitScicGpio();

// Step 3. Clear all interrupts and initialize PIE vector table:

// Disable CPU interrupts

   DINT;

// Initialize the 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;

   InitPieVectTable();

   

   EALLOW;// This is needed to write to EALLOW protected registers

   PieVectTable.XINT2 = &xint2_isr;

   EDIS;   // This is needed to disable write to EALLOW protected registers

// Interrupts that are used in this example are re-mapped to

// ISR functions found within this file.

// Step 4. Initialize the Device Peripheral. This function can be

//         found in DSP2833x_CpuTimers.c

   InitCpuTimers();   // For this example, only initialize the Cpu Timers

   InitSystimer(&CpuTimer1);   //初始化CPUTimer1

// To ensure precise timing, use write-only instructions to write to the entire register. Therefore, if any

// of the configuration bits are changed in ConfigCpuTimer and InitCpuTimers (in DSP2833x_CpuTimers.h), the

// below settings must also be updated.

   StartCpuTimer1();

// Step 5. User specific code, enable interrupts:

   PieCtrlRegs.PIECTRL.bit.ENPIE = 1;          // Enable the PIE block

   PieCtrlRegs.PIEIER1.bit.INTx5 = 1;          // Enable PIE Gropu 1 INT2

   IER |= M_INT1;       

 

// Enable global Interrupts and higher priority real-time debug events:

   EINT;   // Enable Global interrupt INTM

//  ERTM;   // Enable Global realtime interrupt DBGM

   scic_fifo_init();   // Initialize the SCI FIFO

   scic_echoback_init();  // Initalize SCI for echoback

//   msg = "\r\n\n\nHello\0";

//   scic_msg(msg);

   InitLED();

   Init_ultrasonic();

   LED1=0;

   LED2=1;

   Trig=0;

   

// Step 6. IDLE loop. Just sit and loop forever (optional):

   for(;;)

   { 

     Trig=1;

     DELAY_US(10);

     Trig=0;

/*    while(1)

     {

     if(Echo==1)

     {

     //begin_time=CpuTimer1Regs.PRD.all;

     //StartCpuTimer1();   

     begin_time=CpuTimer1Regs.TIM.all;

     flag=1;

     break;

     }

     if(flag==1&&Echo==0)

     {

     //StopCpuTimer1();

     over_time=CpuTimer1Regs.TIM.all;

     period=begin_time-over_time;

     displacement=340*period*100/1000/2;

     H_displacement=(displacement>>16);

      L_displacement=displacement;

      scic_xmit(H_displacement>>8);

      scic_xmit(H_displacement); 

      scic_xmit(L_displacement>>8);

      scic_xmit(L_displacement);  

       flag=0;

     }   

      break;   

   }*/

   //period=begin_time-over_time;

   displacement=170*period*0.00667*1000;

   H_displacement=((int)displacement>>16);

   L_displacement=(int)displacement;

   scic_xmit(H_displacement>>8);

   scic_xmit(H_displacement); 

   scic_xmit(L_displacement>>8);

   scic_xmit(L_displacement);  

  }

}

void scic_echoback_init()

{

    // Note: Clocks were turned on to the SCIA peripheral

    // in the InitSysCtrl() function

 ScicRegs.SCICCR.all =0x0007;   // 1 stop bit,  No loopback

                                   // No parity,8 char bits,

                                   // async mode, idle-line protocol

ScicRegs.SCICTL1.all =0x0003;  // enable TX, RX, internal SCICLK,

                                   // Disable RX ERR, SLEEP, TXWAKE

ScicRegs.SCICTL2.all =0x0003;

ScicRegs.SCICTL2.bit.TXINTENA = 1;

ScicRegs.SCICTL2.bit.RXBKINTENA =1;

#if (CPU_FRQ_150MHZ)

     ScicRegs.SCIHBAUD    =0x0001;  // 9600 baud @LSPCLK = 37.5MHz.

     ScicRegs.SCILBAUD    =0x00E7;

#endif

#if (CPU_FRQ_100MHZ)

      ScicRegs.SCIHBAUD    =0x0001;  // 9600 baud @LSPCLK = 20MHz.

      ScicRegs.SCILBAUD    =0x0044;

#endif

ScicRegs.SCICTL1.all =0x0023;  // Relinquish SCI from Reset

}

// Transmit a character from the SCI

void scic_xmit(int a)

{

    while (ScicRegs.SCIFFTX.bit.TXFFST != 0) {}

    ScicRegs.SCITXBUF=a;

}

void scic_msg(char * msg)

{

    int i;

    i = 0;

    while(msg != '\0')