Friday, August 5, 2011

Reading the supply voltage using the MSP430's internal Analog to Digital Converter

Many times you need to measure the supply voltage (Vdd) of the MSP430. The MSP430 has an ADC input channel which is connected to Vdd/2. The divide by two is required because the MSP430's internal reference is 2.5V, so Vdd must be divided by two for full resolution.

Two simple routines for reading the supply voltage of the MSP430 family are shown below. Some MSP430 devices use the 10-bit ADC, e.g. the MSP430F2274, and some use the 12-bit ADC, e.g. the MSP430F248. Refer to the datasheet for your part to determine which is used. 

Using the ADC10:
/** Reads the MSP430 supply voltage using the Analog to Digital Converter (ADC).
On ez430 boards, this is approx. 3600mV
@return Vcc supply voltage, in millivolts
unsigned int getVcc3()
ADC10CTL0 = SREF_1 + REFON + REF2_5V + ADC10ON + ADC10SHT_3;  // use internal ref, turn on 2.5V ref, set samp time = 64 cycles
ADC10CTL1 = INCH_11;                        
delayMs(1);                                     // Allow internal reference to stabilize
ADC10CTL0 |= ENC + ADC10SC;                     // Enable conversions
while (!(ADC10CTL0 & ADC10IFG));                // Conversion done?
unsigned long temp = (ADC10MEM * 5000l);        // Convert raw ADC value to millivolts
return ((unsigned int) (temp / 1024l));

Using the ADC12:
/** Private helper method to setup ADC for one-shot conversion and read out value according to registers.
Inserts a delay before beginning conversion if REFON
@return the raw ADC value with the specified commands.
@todo move the VREF warmup to startup and leave on to avoid 17mSec blocking delay each time?
unsigned int getAnalogInput(unsigned int adc12ctl0, unsigned int adc12ctl1, unsigned char adc12mctl0)
#define ADC_VREF_DELAY_MS 17   
ADC12CTL0 = adc12ctl0;
ADC12CTL1 = adc12ctl1;
ADC12MCTL0 = adc12mctl0;
if (adc12ctl0 & REFON)                    // if internal reference is used...
delayMs(ADC_VREF_DELAY_MS);           // 17mSec delay required to Vref capacitors
ADC12CTL0 |= ENC;                         // Enable conversions
ADC12CTL0 |= ADC12SC;                     // Start conversions
while (!(ADC12IFG & 0x01));               // Conversion done?
return ADC12MEM0;    // Read out 1st ADC value

/** Measures Vcc to the MSP430, nominally 3.3V
- ADC measures VCC/2 compared to 2.5V reference
- If Vcc = 3.3V, ADC output should be (1.65/2.5)*4095 = 2703
- Therefore (halfVcc/2.5)*4095 = ADC reading and (Vcc/2.5)*4095 = 2*ADC
- So Vcc*4096 = 5*ADC and VCC=5*ADC/4095

@return Vcc in millivolts
unsigned int getVcc3()
unsigned int ctl0 = REFON + REF2_5V + ADC12ON + SHT0_15;  // turn on 2.5V ref, set samp time=1024 cycles
unsigned int ctl1 = SHP;                                  // Use sampling timer, internal ADC12OSC
unsigned char mctl0 = SREF_1 + INCH_11;                   // Channel A10, Vcc/2
unsigned long vcc = (unsigned long) getAnalogInput(ctl0, ctl1, mctl0);
unsigned long mult = vcc * 5000l;
return ((unsigned int)(mult / 4096l));

1 comment:

  1. Hi, I had one doubt in the above mentioned code snippet for ADC10..If i use this code in Launchpad, it was resulting 2.1V..If i dont use the 2.5V voltage reference, then it results in Proper 3.6V. But if i theoretically calculate the same output with a formula given in the application report, then digital output value from the ADC10MEM register is totally different.
    The formula which i used for calculating the digital output is as follows.
    Wherein the launchpad receives a voltage of 3.5V and i kept the Vref as 1.5V...
    So the output goes like this : N=1023*(3.5/1.5)=2387, but if i read the value from registers as per the above program with 1.5Vref it shows 744.
    Can you help me in understanding the process.