BASIC TIMER中断不知道什么原因进不去

//******************************************************************************
//  MSP430x41x2 Demo - Basic Timer, Toggle P5.1 Inside ISR, 32kHz ACLK
//
//  Description: Toggles P5.1 by xor'ing P5.1 inside of a basic timer ISR.
//  ACLK provides the basic timer clock source. LED toggles every 125ms.
//  ACLK = LFXT1 = 32768Hz, MCLK = SMCLK = default DCO = 32 x ACLK = 1048576Hz
//  //* An external watch crystal between XIN & XOUT is required for ACLK *//
//
//               MSP430x41x2
//             -----------------
//         /|\|              XIN|-
//          | |                 | 32kHz
//          --|RST          XOUT|-
//            |                 |
//            |             P5.1|-->LED
//
//  P. Thanigai
//  Texas Instruments Inc.
//  January 2009
//  Built with CCE Version: 3.1 and IAR Embedded Workbench Version: 4.11
//*****************************************************************************
#include <msp430.h>

int main(void)
{
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
  FLL_CTL0 |= XCAP11PF;                     // Set load cap for 32k xtal
  P5DIR |= 0x02;                            // Set P5.1 as output
  BTCTL = BTDIV + BT_fCLK2_DIV16;           // ACLK/(256*16)
  IE2 |= BTIE;                              // Enable BT interrupt

  __bis_SR_register(LPM3_bits + GIE);       // Enter LPM3, enable interrupts
}

// Basic Timer Interrupt Service Routine
#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector=BASICTIMER_VECTOR
__interrupt void basic_timer_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(BASICTIMER_VECTOR))) basic_timer_ISR (void)
#else
#error Compiler not supported!
#endif
{
  P5OUT ^= 0x02;                            // Toggle P5.1
}

BT_fCLK2_DIV16是做什么用的呢？BTDIV不是已经是256分频了吗？

我把这个加上后，能用了。但是我在把它注释掉也能用了！！！请问你这是什么原因？谢谢

BT_fCLK2_DIV16的宏定义：