This is something that would be typically used to measure Battery voltage over time.

Here’s a complete stand-alone module that can be used to investigate this feature.

// ===========================================
// o Measure VDD using BAND GAP Voltage, 1.2V
// on PIC24FJ256GB004 PIM w/ Explorer 16 Board
// o ICD3, MPLAB v8.85, C30 v3.31
// o June 2012
// o Note:  Please see Microchip Application Note AN1072
//    for an more in-depth example of this using a PIC16F690
// ===========================================

#include <p24fxxxx.h>

#ifdef __PIC24FJ64GB004__ //Defined by MPLAB when using 24FJ64GB004 device
_CONFIG1( JTAGEN_OFF & GCP_OFF & GWRP_OFF & FWDTEN_OFF & ICS_PGx1)
_CONFIG2( FCKSM_CSDCMD & OSCIOFNC_OFF & POSCMOD_XT & FNOSC_PRI & I2C1SEL_SEC & IOL1WAY_ON)
_CONFIG3(SOSCSEL_SOSC & WUTSEL_LEG & WPDIS_WPDIS & WPCFG_WPCFGDIS)
_CONFIG4(DSWDTEN_OFF & RTCOSC_SOSC & DSWDTPS_DSWDTPS6 & DSWDTOSC_SOSC & DSBOREN_OFF)

#else
#error ERROR: INCORRECT PIC selected for this project: Use PIC24FJ64GB004.
#endif
unsigned long VIN_Result,VDD_Result;
unsigned long Count;

int main(void)
{

AD1CON2 = 0; // AVdd, AVss, int every conversion, MUXA only
AD1CON3 = 0x1F05; // 31 Tad auto-sample, Tad = 5*Tcy
AD1CON1 = 0x80E4; // Turn on, auto sample start, auto-convert
AD1CHS = 15; // VBG 1.2V
AD1PCFGbits.PCFG15 = 0; // Disable digital input on VBG Channel
AD1CHS = 15; // CHAN for VBG 1.2V
AD1CSSL = 0; // No scanned inputs

while (1)
{
VIN_Result = VDD_Result = 0;

// ———————————————————————
// Wait for conversion to complete
// ———————————————————————
while(!AD1CON1bits.DONE)
;

// ———————————————————————
// Read A2D result
// ———————————————————————
Count = (unsigned long) ADC1BUF0;

// ———————————————————————
// FYI Only – Let’s see what VIN would be, assuming VDD is 3.3V
// ———————————————————————
VIN_Result = (Count * 3300)/1024;

// ———————————————————————
// Scale 1.2V to 1200 for calculation.
// ———————————————————————
VDD_Result = (unsigned long)((1200 * 1023.0))/(Count);

// ———————————————————————
// Set Breakpoint here…
// ———————————————————————
Nop();
Nop();
Nop();
Nop();

} // End of while(1)…

} // End of main()…

One of the easiest ways to config the PIC32 is to use this function call found in <plib.h>:

SYSTEMConfigPerformance(SYS_FREQ);

My setup:
Explorer 16 Demo Board
PIM PIC32MX795F512L
MLPAB v8.85
C32 v2.20
ICD3

<CODE>
#include <p32xxxx.h>
#include <plib.h>

// Configuration Bit settings
// SYSCLK = 80 MHz (8MHz Crystal/ FPLLIDIV * FPLLMUL / FPLLODIV)
// PBCLK = 40 MHz
// Primary Osc w/PLL (XT+,HS+,EC+PLL)
// WDT OFF
// Other options are don’t care

#pragma config FPLLMUL = MUL_20, FPLLIDIV = DIV_2, FPLLODIV = DIV_1, FWDTEN = OFF
#pragma config POSCMOD = HS, FNOSC = PRIPLL, FPBDIV = DIV_1

#define SYS_FREQ (80000000L)

int main(void)
{

SYSTEMConfigPerformance(SYS_FREQ);

mJTAGPortEnable(0); // Disable JTAG which is ON by default.

TRISAbits.TRISA0 = 0; // PORT PIN A0 = OUTPUT

while (1)
{
// ———————————————————————
// Too S…L…O…W…
// LATAbits.LATA0 ^= 1;
// ———————————————————————

// ———————————————————————
// F…A…S…T
// ———————————————————————
LATAINV = 1; // AUTOMICALLY invert/toggle A0

}
}
</CODE>