MSP430F5529单片机ADC外部电压怎么设置成为参考电压

ADC12REF2_5v是使能内部的2.5v参考电压，ADC12SREF是选择参考电压的通道

参考电压的范围：

ADC12REF2_5是内部基准源电压选择位。选择该位的时候，必须置位REFON

ADC12SREFX是参考源选择位

使用内部参考电压的话，您需要打开参考源，选择ADC12SREF_1 即为内部基准参考源，然后需要使能 REFON，最后选择参考电压ADC12REF2_5  1.5V or 2.5V）

建议您结合例程看一下

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//******************************************************************************
//  MSP430F552x Demo - ADC12, Using the Internal Reference
//
//  Description: This example shows how to use the internal reference of REF 
//  module using the ADC12 control registers.
//  The ADC12 uses the internal 2.5V reference and performs a single conversion
//  on channel A0. The conversion results are stored in ADC12MEM0. Test by
//  applying a voltage to channel A0, then setting and running to a break point
//  at the "__no_operation()" instruction. To view the conversion results,
//  open an ADC12 register window in debugger and view the contents of ADC12MEM0
//
//  NOTE:REFMSTR bit in REFCTL0 regsiter is reset to allow the ADC12_A reference
//    control regsiters handle the reference setting. Upon resetting the REFMSTR
//    bit, all the settings in REFCTL are 'dont care' and the legacy ADC12  
//    control bits (ADC12REFON, ADC12REF2_5, ADC12TCOFF and ADC12REFOUT) control 
//    the reference system. 
//
//                MSP430F552x
//             -----------------
//         /|\|                 |
//          | |                 |
//          --|RST              |
//            |                 |
//     Vin -->|P6.0/CB0/A0      |
//            |                 |
//
//   Bhargavi Nisarga
//   Texas Instruments Inc.
//   April 2009
//   Built with CCSv4 and IAR Embedded Workbench Version: 4.21
//******************************************************************************

#include <msp430.h>

int main(void)
{
  volatile unsigned int i;
  WDTCTL = WDTPW+WDTHOLD;                   // Stop watchdog timer
  P6SEL |= 0x01;                            // Enable A/D channel A0
  REFCTL0 &= ~REFMSTR;                      // Reset REFMSTR to hand over control to 
                                            // ADC12_A ref control registers
  ADC12CTL0 = ADC12ON+ADC12SHT02+ADC12REFON+ADC12REF2_5V;
                                            // Turn on ADC12, Sampling time
                                            // On Reference Generator and set to
                                            // 2.5V
  ADC12CTL1 = ADC12SHP;                     // Use sampling timer
  ADC12MCTL0 = ADC12SREF_1;                 // Vr+=Vref+ and Vr-=AVss

  for ( i=0; i<0x30; i++);                  // Delay for reference start-up

  ADC12CTL0 |= ADC12ENC;                    // Enable conversions

  while (1)
  {
    ADC12CTL0 |= ADC12SC;                   // Start conversion
    while (!(ADC12IFG & BIT0));
    __no_operation();                       // SET BREAKPOINT HERE

  }
}