MSP430FR2311: ADC raw data to milli/micro volt code conversion

Hi Walter,

When converting from ADC data to an equivalent input voltage, you'll need to know the reference voltage (VREF) and the ADC resolution (n). For example, the minimum LSB size in Volts = VREF / (2^n) = 1.5 V / (2^10) = 1.465 mV. Keep in mind that VREF can be different (e.g. 1.5 V or DVCC) depending on the device and/or settings. Also, a lower VREF for the same ADC resolution (n) results in a lower LSB size, but the input range is lower and limited to VREF. Check out Section 2.1.2 in the Designing With MSP430FR58xx/FR59xx/68xx/69xx ADC app note.

The 'msp430fr231x_adc10_05.c' code example should be what you're looking for. You can find it inside CCS by going to TI Resource Explorer > Software > MSP430Ware > Devices > MSP430FR2XX_4XX > MSP430FR2311 > Peripheral Examples > Register Level. You may need to install MSP430Ware in order to import the project into your CCS workspace. I've copied it below for your reference.

msp430fr231x_adc10_05.c

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//******************************************************************************
//  MSP430FR231x Demo - ADC, Lo_Batt, Set P1.0 if AVcc < 2.50V
//
//  Description: A single sample is made on A13 (Vref) with reference set to
//  DVCC. Software sets ADC10SC to start sample and conversion
//  - ADC10SC automatically cleared at EOC. Once conversion is completed, the
//  following equation is used to calculate DVCC = (1023 * 1.5) / adcResult.
//  if DVCC is < 2.5V, P1.0 set indicating a lo_Batt condition, else reset.
//
//                MSP430FR2311
//             -----------------
//         /|\|              XIN|-
//          | |                 |
//          --|RST          XOUT|-
//            |                 |
//            |A13 (Vref)   P1.0|-->LED
//
//   Darren Lu
//   Texas Instruments Inc.
//   July 2015
//   Built with IAR Embedded Workbench v6.30 & Code Composer Studio v6.1 
//******************************************************************************
#include <msp430.h>

unsigned int adcResult;                     // Temporarily stores the ADC value
                                            // that was read from ADCMEM0
int main(void)
{
    volatile unsigned long dvccValue;       // Calculated DVCC value

    WDTCTL = WDTPW | WDTHOLD;               // Stop WDT

    P1DIR |= BIT0;                          // Set P1.0 to output direction

    // Disable the GPIO power-on default high-impedance mode to activate
    // previously configured port settings
    PM5CTL0 &= ~LOCKLPM5;

    // Configure ADC10
    ADCCTL0 &= ~ADCENC;                     // Disable ADC
    ADCCTL0 = ADCSHT_2 | ADCON;             // ADCON, S&H=16 ADC clks
    ADCCTL1 = ADCSHP;                       // ADCCLK = MODOSC; sampling timer
    ADCCTL2 = ADCRES;                       // 10-bit conversion results
    ADCIE = ADCIE0;                         // Enable ADC conv complete interrupt
    ADCMCTL0 = ADCINCH_13 | ADCSREF_0;      // A13 ADC input select = 1.5V Ref
                                            // Vref = DVCC
    // Configure reference module located in the PMM
    PMMCTL0_H = PMMPW_H;                    // Unlock the PMM registers
    PMMCTL2 |= INTREFEN;                    // Enable internal reference
    while(!(PMMCTL2 & REFGENRDY));          // Poll till internal reference settles

    while(1)
    {
        ADCCTL0 |= ADCENC | ADCSC;          // Sampling and conversion start
        __bis_SR_register(LPM0_bits | GIE); // Enter LPM0, ADC_ISR will force exit
        __no_operation();                   // For debug only

        // To calculate DVCC, the following equation is used
        // DVCC = (1023 * 1.5) / adcResult
        // The following equation is modified to use only integers instead
        // of using float. All results needs to be divided by 100 to obtain
        // the final value.
        // DVCC = (1023 * 150) / adcResult
        dvccValue = ((unsigned long)1023 * (unsigned long)150) / (unsigned long) (adcResult);

        if (dvccValue < 250)                // DVCC < 2.50V?
            P1OUT |= BIT0;                  // Set P1.0 LED on
        else
            P1OUT &= ~BIT0;                 // Clear P1.0 LED off
    }
}

// ADC interrupt service routine
#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector=ADC_VECTOR
__interrupt void ADC_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(ADC_VECTOR))) ADC_ISR (void)
#else
#error Compiler not supported!
#endif
{
    switch(__even_in_range(ADCIV, ADCIV_ADCIFG))
    {
        case ADCIV_NONE:
            break;
        case ADCIV_ADCOVIFG:
            break;
        case ADCIV_ADCTOVIFG:
            break;
        case ADCIV_ADCHIIFG:
            break;
        case ADCIV_ADCLOIFG:
            break;
        case ADCIV_ADCINIFG:
            break;
        case ADCIV_ADCIFG:
            adcResult = ADCMEM0;            // Read ADC memory
            __bic_SR_register_on_exit(LPM0_bits);// Exit from LPM
            break;
        default:
            break;
    }
}

Regards,

James

Hi james,

This is one is what I need precisely because I was trying to detect low battery level. 

If say, I will not use 1.5 reference and use only DVCC as reference. Will it still work if I am measuring DVCC but it is also the reference in this case?

Regards,

Walter 

Hi Marshall, 

I see it can work using 1.5V reference using AD13.

I want to know the delay in reference settling like the one below. I  cannot find in data sheet or user guide.

// Configure reference
PMMCTL0_H = PMMPW_H; // Unlock the PMM registers
PMMCTL2 |= INTREFEN; // Enable internal reference
__delay_cycles(400); // Delay for reference settling??

How did they get the optimum value.

Regards,

Walter

Hi Walter,

The reference settling time should be provided in the datasheet. Then, depending on your CPU clock speed, you can adjust the number of cycles to achieve that settling time delay.

How __delay_cycles() works

Regards,

James

Hi Team,

I am talking only about the optimum value for the 1.5V reference to settle in msp430fr2311 device.

I cannot find in specs.

Regards,

Walter

If you have any other questions, please post them in a new thread. I think we've resolved your original question. Thanks.

Regards,

James