下面是TI公司给的源码示例，判断OSC 起震是否正常，OFIFG中断标志位是否有中断产生。问题是：为啥整段代码中没有开启IE1的OFIE，而且如果开启了OFIE，就走不下去？

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 *******************************************************************************
 * 
 *                       MSP430 CODE EXAMPLE DISCLAIMER
 *
 * MSP430 code examples are self-contained low-level programs that typically
 * demonstrate a single peripheral function or device feature in a highly
 * concise manner. For this the code may rely on the device's power-on default
 * register values and settings such as the clock configuration and care must
 * be taken when combining code from several examples to avoid potential side
 * effects. Also see www.ti.com/grace for a GUI- and www.ti.com/msp430ware
 * for an API functional library-approach to peripheral configuration.
 *
 * --/COPYRIGHT--*/
//******************************************************************************
//  MSP-FET430P140 Demo - Basic Clock, MCLK Sourced from HF XTAL XT2
//
//  Description: Proper selection of an external HF XTAL for MCLK is shown
//  by first polling the OSC fault until XTAL is stable - only then is MCLK
//  sourced by XT2.  MCLK is on P5.4.
//  ACLK= n/a, MCLK= XT2, SMCLK= DCO,
//  //* HF XTAL NOT INSTALLED ON FET *//
//  //* Min Vcc required varies with MCLK frequency - refer to datasheet *//	
//
//               MSP430F149
//            -----------------
//        /|\|            XT2IN|-
//         | |                 | XTAL (455k - 8Mhz)
//         --|RST        XT2OUT|-
//           |                 |
//           |             P5.4|-->MCLK = XTAL
//
//
//  M. Buccini
//  Texas Instruments Inc.
//  Feb 2005
//  Built with CCE Version: 3.2.0 and IAR Embedded Workbench Version: 3.21A
//******************************************************************************

#include <msp430.h>

int main(void)
{
  volatile unsigned int i;
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
  P5DIR |= 0x10;                            // P5.4= output direction
  P5SEL |= 0x10;                            // P5.4= MCLK option select
  BCSCTL1 &= ~XT2OFF;                       // XT2= HF XTAL

  do
  {
  IFG1 &= ~OFIFG;                           // Clear OSCFault flag
  for (i = 0xFF; i > 0; i--);               // Time for flag to set
  }
  while ((IFG1 & OFIFG));                   // OSCFault flag still set?

  BCSCTL2 |= SELM_2;                        // MCLK= XT2 (safe)

  for (;;);                                 // Do nothing
}