Code Recipe: Reading MSP430 power supply voltage level

While playing with solar cells and MSP430 Launchpad I needed to check if MCU’s power supply level is high enough to switch to higher clock speed and do some power-hungry stuff. As MSP430G2553 comes with built-in 10-bit analog-to-digital converter and precise 1.5V and 2.5V internal voltage references, the task was quite easy.

My specific power source (solar cell + 0.47F supercap + 3.3V LDO) provides rather wide range of voltages – from 0V to ~3.25V. The MSP microcontroller starts from about 1.8V. To measure the whole range, I had to use both voltage references: 1.5V for voltages lower than 3V and 2.5V for voltages higher (or equal) than 3V. Sort of a dual range voltage meter

This C function reads and returns Vcc level in millivolts.
UPDATE: To properly power down ADC after measurement, ENC flag must be cleared before clearing ADC10ON. Code updated

uint16_t Msp430_GetSupplyVoltage(void)
{
	uint16_t raw_value;
	// first attempt - measure Vcc/2 with 1.5V reference (Vcc < 3V )
	ADC10CTL0 = SREF_1 | REFON | ADC10SHT_2 | ADC10SR | ADC10ON;
	ADC10CTL1 = INCH_11 | SHS_0 | ADC10DIV_0 | ADC10SSEL_0;
	// start conversion and wait for it
	ADC10CTL0 |= ENC | ADC10SC;
	while (ADC10CTL1 & ADC10BUSY) ;
	// stop conversion and turn off ADC
	ADC10CTL0 &= ~ENC;
	ADC10CTL0 &= ~(ADC10IFG | ADC10ON | REFON);
	raw_value = ADC10MEM;
	// check for overflow
	if (raw_value == 0x3ff) {
		// switch range - use 2.5V reference (Vcc >= 3V)
		ADC10CTL0 = SREF_1 | REF2_5V | REFON | ADC10SHT_2 | ADC10SR | ADC10ON;
		// start conversion and wait for it
		ADC10CTL0 |= ENC | ADC10SC;
		while (ADC10CTL1 & ADC10BUSY) ;
		raw_value = ADC10MEM;
		// end conversion and turn off ADC
		ADC10CTL0 &= ~ENC;
		ADC10CTL0 &= ~(ADC10IFG | ADC10ON | REFON);
		// convert value to mV
		return ((uint32_t)raw_value * 5000) / 1024;
	} else
		return ((uint32_t)raw_value * 3000) / 1024;
}