Electronic – reading voltage using TI’s MSP430 FF22x4

adcmsp430voltage measurement

I am trying to measure a power device's voltage using TI's MSP430. The voltage source is connected to A1 (i.e. P4) and ground (i.e. P1) on the target board.

Here is the relevant code:

ADC10CTL1 = INCH_1 + CONSEQ_0; //A1, single measurement
ADC10CTL0 = SREF_1 + ADC10SHT_3 + REFON + ADC10ON + ADC10IE + ADC10SR; //same as sample temperature sensor code
ADC10CTL1 &= ~ADC10DF; //setting binary format for ADC10MEM
ADC10CTL0 |= ENC + ADC10SC; //Sampling and conversion start
 __bis_SR_register(CPUOFF + GIE);        // LPM0 with interrupts enabled
//read result in a uint8_t[2]
incomingVoltage[0] = ((uint8_t *)&ADC10MEM)[0]; //this is Least-significant-byte
incomingVoltage[1] = ((uint8_t *)&ADC10MEM)[1]; //this is most-significant-byte

I am having following problems:

I am printing the ADC10MEM contents on the AP, and I see that the leading 6 bits of ADC10MEM are all 1, instead of 0. I am unable to get the reason for the same.
If I consider only the last 10 bits of ADC10MEM, I can see that the value increases and decreases with rise and fall in voltage, but if I obtain Vin using the formula:

N = 1023 * ((Vin – VR- ) / (VR+ – VR-)), I do not get the correct value. (VR+ = 1.5V, VR- = 0V, as batteries power the target board)
N: the value in ADC10MEM, in decimal

I am unable to find where I am going wrong. Do I have to enable the pin for analog input (ADC10AE0 |= 0x02), and set direction (P4DIR &= 0x00) as well?

Thanks!

EDIT 1: Vin Values:
These are the ADC10MEM values I am getting, considering last 10 bits.
Following calculations use Vr+ = 1.5, Vr- = 0.
For input voltage 0.38V, N=1100000010 i.e. 770, and Vin=1.12.
For input voltage 0.5V, N=1101010110 i.e. 854, and Vin=1.25.
For input voltage 1V, N=1110101001 i.e. 937, and Vin = 1.37.
For input voltage 1.45V, N=1111011000 i.e. 984, and Vin = 1.44.
As I said, the value is increasing/decreasing with increase/decrease in input, but it isn't correct.

EDIT 2: Interrupts:
I am modifying the sample temperature sensor code. I see

__bis_SR_register(LPM3_bits+GIE);  // LPM3 with interrupts enabled

at the start of while(1) loop in linkTo()/main_ED.c, and there's

 __bis_SR_register(CPUOFF + GIE);        // LPM0 with interrupts enabled

after 'sampling and conversion start' and before 'read results'

EDIT 3: Code based on msp430x22x4_adc_10_02.c from TI's code samples

ADC10CTL1 = INCH_1 + CONSEQ_0;
ADC10CTL0 = SREF_1 + ADC10SHT_2 + REFON + ADC10ON + ADC10IE;
ADC10AE0 |= 0x02;
P4DIR |= 0x00;
ADC10CTL0 |= ENC + ADC10SC; //Sampling and conversion start

I am getting similar values even with this code.

Best Answer

This is the code that worked:

ADC10CTL1 = INCH_1 + CONSEQ_0;
ADC10CTL0 = SREF_1 + ADC10SHT_2 + REFON + ADC10ON + ADC10IE;
for (countDown = 240; countDown > 0; countDown--); //delay to allow references to settle
ADC10AE0 |= 0x02;
ADC10CTL0 |= ENC + ADC10SC; //Sampling and conversion start
__bis_SR_register(CPUOFF + GIE);        // LPM0 with interrupts enabled
result = ADC10MEM; //result is a int
incomingVoltage[0] = result&0xFF; //incoming voltage is an array of uint8_t, send it to AP
incomingVoltage[1] = (result>>8)&0xFF; //this and previous line is similar to temperature sensing code sample

Thanks for the help!

Related Solutions

Electronic – Reading from multiple channels using ADC10 and DTC

I now have this working. ADC10 part is now similar to TI's samples.

This is the code that can read successfully from 4 pins simultaneously:

unsigned int res[5]; //for holding the conversion results
void main (void)
{ 

  //code for connecting to AP

  //reading voltage
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
  ADC10CTL1 = INCH_4 + CONSEQ_1;            // A4/A3/A2/A1/A0, once multi channel
  ADC10CTL0 = REF2_5V + ADC10SHT_2 + MSC+ REFON + ADC10ON + ADC10IE; //2.5 reference voltage
  ADC10AE0 = 0x1F;                          // P2.0,1,2,3,4 ADC option select
  ADC10DTC1 = 0x5;                         // 5 conversions
  P1DIR |= 0x01;                            // Set P1.0 output

  for (;;)
  {
    P1OUT |= 0x01;                          // Set P1.0 LED on
    ADC10CTL0 &= ~ENC;
    while (ADC10CTL1 & BUSY);               // Wait if ADC10 core is active
    ADC10SA = (int)res;                        // Data buffer start
    ADC10CTL0 |= ENC + ADC10SC;             // Sampling and conversion ready
    __bis_SR_register(CPUOFF + GIE);        // LPM0, ADC10_ISR will force exit
    P1OUT &= ~0x01;                         // Clear P1.0 LED off
  }

}

#pragma vector=ADC10_VECTOR
__interrupt void ADC10_ISR(void)
{
    //code from reading from res and sending it to AP
  __bic_SR_register_on_exit(CPUOFF);        // Clear CPUOFF bit from 0(SR)
}

Thanks for the help!

Electronic – Resistive sensor reading ADC

If you have a reference voltage of (say) 3V, the range your input will see is: -

3V * 10k/13k = 2.307V (sensor at 3kohms)
3V * 10k/70k = 0.428V (sensor at 60kohms)

If you used a 50uA current excitation and a grounded sensor, the range your input will see is: -

50e-6 * 3k = 0.15V (sensor at 3kohms)
50e-6 * 60k = 3.0V (sensor at 60kohms)

With a current excitation the percentage of your 3V reference range used is 95%. With voltage and resistor excitation you only get 63% of the range.

If your reference voltage is lower or higher, the above "range" statements are still true.

enter image description here

Here is an example. An ADC input is directly connected to the sensor. The sensor is fed with 50uA via the PNP transistor. The 50uA is measured across "R" and compared with "V" by the op-amp. The op-amp keeps the current through R at a value that generates a voltage "V" across it. Values could be R=10k and V=0.5V or R=20k and V=1V. Op-amp should be chosen that has close-to-either-rail IO performance like an AD8605 (used many times by me in this same configuration for strain gauge excitation).

Here's a quick DC simulation for 60k and 3k loads: -

enter image description here

Transistor used is BC547C or BC847C for those with good eye-sight.

Note that due to small base currents in the transistor, the current isn't 50uA but 49.846uA with 60kohm load and 49.849uA when loaded at 3kohms. Note also the voltages across the sensor - 2.991V on 60k load and 149.5mV on 3k load.

Best Answer

Related Solutions

Electronic – Reading from multiple channels using ADC10 and DTC

Electronic – Resistive sensor reading ADC

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