MSP430G2553寄存器配置方式，ACLK作为定时器Timer0的时钟源

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//******************************************************************************
//  MSP430G2xx3 Demo - Timer_A, Toggle P1.0, Overflow ISR, 32kHz ACLK
//
//  Description: Toggle P1.0 using software and the Timer_A overflow ISR.
//  In this example an ISR triggers when TA overflows. Inside the ISR P1.0
//  is toggled. Toggle rate is exactly 0.5Hz. Proper use of the TA0IV interrupt
//  vector generator is demonstrated.
//  ACLK = TACLK = 32768Hz, MCLK = SMCLK = default DCO
//  //* An external watch crystal on XIN XOUT is required for ACLK *//	
//
//           MSP430G2xx3
//         ---------------
//     /|\|            XIN|-
//      | |               | 32kHz
//      --|RST        XOUT|-
//        |               |
//        |           P1.0|-->LED
//
//  D. Dang
//  Texas Instruments Inc.
//  December 2010
//   Built with CCS Version 4.2.0 and IAR Embedded Workbench Version: 5.10
//******************************************************************************

#include <msp430.h>

int main(void)
{
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
  P1DIR |= 0x01;                            // P1.0 output
  TACTL = TASSEL_1 + MC_2 + TAIE;           // ACLK, contmode, interrupt

  __bis_SR_register(LPM3_bits + GIE);       // Enter LPM3 w/ interrupt
}

// Timer_A3 Interrupt Vector (TA0IV) handler
#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector=TIMER0_A1_VECTOR
__interrupt void Timer_A(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(TIMER0_A1_VECTOR))) Timer_A (void)
#else
#error Compiler not supported!
#endif
{
  switch( TA0IV )
  {
    case  2:  break;                        // CCR1 not used
    case  4:  break;                        // CCR2 not used
    case 10:  P1OUT ^= 0x01;                // overflow
              break;
  }
}