Self-measure supply voltage from PIC16F153xx

VCC-PIC16F153xx.c

// DIA addresses
#define TSLR2 	0x8113
#define FVRA1X	0x8118
#define FVRA2X	0x8119

// Read from Device Information Area
uint16_t DIARead(uint16_t addr) {
	// Access DIA
	NVMCON1 = 0b01000000;
	// Set address
	NVMADRH = (addr >> 8) & 0xFF;
	NVMADRL = addr & 0xFF;
	// Start reading
	NVMCON1bits.RD = 1;
	
	// Wait for reading end
	while (NVMCON1bits.RD);
	
	// Return data
	return (NVMDATH << 8) | NVMDATL;
}

/*   MEASURE VCC   */
// FVR enabled, Temperature sensor disabled, low range, comparator FVR disabled, ADC FVR buffer 1024mV
FVRCON = 0b10000001;
// Wait for FVR to be ready
while (!FVRCONbits.FVRRDY);

// ADC source is FVR buffer 1, conversion stopped, ADC enabled
ADCON0 = 0b11111001;
// Result right aligned, clock is ADCRC, ref is VCC
ADCON1 = 0b10110000;
// No auto conversion
ADACT = 0;

// Required for ADC value settling
__delay_us(200);

// Start conversion
ADCON0bits.GO = 1;

// Wait for conversion end
while (ADCON0bits.GO);

// Calculate
uint16_t adc = (ADRESH << 8) | ADRESL;

// FVR real value
uint16_t fvrCalib1 = DIARead(FVRA1X);

// Vcc = Vfvr * ADC dynamic / ADC value
float voltage = (fvrCalib1 * 1024.0) / adc;
//uint16_t voltage = ((8192 / adc) * 1024) / 8;
printf("\nSupply voltage %fmV\r", voltage);