[参考译文] MSP430F5171：ACLK 神秘

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//******************************************************************************
//  MSP430F51x2 Demo - XT1 sources ACLK
//
//  Description: This program demonstrates using XT1 to source ACLK
//  ACLK = LFXT1 = 32,768Hz	
//  //* An external watch crystal between XIN & XOUT is required for ACLK *//	
//
//               MSP430F51x2
//             -----------------
//        /|\ |              XIN|-
//         |  |                 | 32kHz
//         ---|RST          XOUT|-
//            |                 |
//            |             P3.0|-->ACLK = ~10kHz
//            |                 |
//
//   Bhargavi Nisarga
//   Texas Instruments Inc.
//   Dec 2009
//   Built with CCS v4 and IAR Embedded Workbench Version: 4.21
//******************************************************************************
#include <msp430.h>

void Port_Mapping(void);

int main(void)
{
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
  Port_Mapping();

  P3DIR |= BIT0;                            // P3.0=ACLK set out to pins
  P3SEL |= BIT0; 
  PJSEL |= BIT4+BIT5;                       // Port select XT1

  UCSCTL6 &= ~(XT1OFF);                     // XT1 On
  UCSCTL6 |= XCAP_3;                        // Internal load cap
  UCSCTL3 = 0;                              // FLL Reference Clock = XT1

  // Loop until XT2 & DCO stabilizes - In this case loop until XT1 and DCo settle
  do
  {
    UCSCTL7 &= ~(XT1LFOFFG + XT1HFOFFG + DCOFFG);
                                            // Clear XT1,DCO fault flags
    SFRIFG1 &= ~OFIFG;                      // Clear fault flags
  }while (SFRIFG1&OFIFG);                   // Test oscillator fault flag
  
  UCSCTL6 &= ~(XT1DRIVE_3);                 // Xtal is now stable, reduce drive strength

  UCSCTL4 |= SELA_0;                        // ACLK = LFTX1 (by default)
  
  __bis_SR_register(LPM3_bits);             // Enter LPM3
  __no_operation();                         // For debugger
}

void Port_Mapping(void)
{
  __disable_interrupt();                    // Disable Interrupts before altering Port Mapping registers
  PMAPPWD = 0x02D52;                        // Enable Write-access to modify port mapping registers
  
  #ifdef PORT_MAP_RECFG                     
  PMAPCTL = PMAPRECFG;                      // Allow reconfiguration during runtime
  #endif  
  
  P3MAP0 = PM_TD1OUTH;
  
  PMAPPWD = 0;                              // Disable Write-Access to modify port mapping registers
  #ifdef PORT_MAP_EINT
  __enable_interrupt();                     // Re-enable all interrupts
  #endif  
}