msp430 ADC12 怎样把内部参考电压设置成为2.5v,详细设置寄存器 谢谢

一个MSP430F54x使用内部2.5V参考电压的例程：

//******************************************************************************
// MSP430F54x Demo - ADC12, Using the Internal Reference
//
// Description: This example shows how to use the internal reference of ADC12.
// It 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
//
// MSP430F5438
// -----------------
// /|\| |
// | | |
// --|RST |
// | |
// Vin -->|P6.0/A0 |
// | |

//
// M Smertneck / W. Goh
// Texas Instruments Inc.
// September 2008
// Built with CCE Version: 3.2.2 and IAR Embedded Workbench Version: 4.11B
//******************************************************************************

#include "msp430x54x.h"

void main(void)
{
volatile unsigned int i;
WDTCTL = WDTPW+WDTHOLD; // Stop watchdog timer
P6SEL |= 0x01; // Enable A/D channel A0
ADC12CTL0 = ADC12ON+ADC12SHT0_0+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
}
}

ADC12CTL0 |= REFON + REF2_5V; //打开内部电压发生器 参考电压选择为2.5v