MSP-EXP430F5529LP: Using ADC12 and ACS712 Current Sensor

Hello Forums,

I am trying to get the MSP430F5529 on the LaunchPad mentioned (MSP-EXP430F5529LP) to read the voltage that is output from a 5A ACS712 current sensor. The sensor is power using the 5V and GND pins on the LaunchPad (in the 40-pin Booster Pack section). The sensor output can reach close to 5V when 5A of current flows through it, so I used a potential divider (with 3k resistors) at the sensor output to half the output voltage. The output of the divider is fed to P6.0 on the LaunchPad.

The relevant code I wrote for this is:

//  Function name: ADC_Init
//  Purpose: Initializes and configures MSP430 ADC for conversion of ACS712 voltage reading to current
//  Inputs: none
//  Returns: none

void ADC_Init(void){

    REFCTL0 &= ~REFMSTR;                    //Set voltage reference control to ADC_12 registers

    //ADC Control Registers Setup
    ADC12CTL0 = ADC12SHT0_9|ADC12REFON|ADC12REF2_5V|ADC12ON;        // Sample and hold time, adc reference generator on and set to 2.5V, adc on
    ADC12CTL1 = ADC12SHP;                                           // SAMPCON signal source
    ADC12CTL2 = ADC12RES_2;                                         // 12-bit resolution

    ADC12MCTL0 = ADC12SREF_1 + ADC12INCH_0;                         // REF: Vref+ to MCU GND; Port 6 pin 0
    P6DIR = 0x00;               // P6.0 as input
    P6SEL |= 0x02;              // P6.0 to be used by ADC
    ADC12CTL0 &= ~ADC12SC;      // Clear 'Start Conversion' bit, to ensure conversion not started
    ADC12CTL0 |= ADC12ENC;

}

//  Function name: Get_ACS712_Value
//  Purpose: Reads ACS 712 current sensor output voltage via ADC, then convert voltage to current
//  Inputs: none
//  Returns: Current value from ACS 712 Sensor

void Get_ACS712_Value(void){

    ADC12CTL0 |= ADC12SC;                               //Start ADC conv.
    while (ADC12CTL1 & ADC12BUSY){                      // Poll busy bit waiting for conversion to complete
        __no_operation();
    }

    unsigned int acs_voltage = ADC12MEM0 & 0x0FFF;                      // keep only low 12 bits

    float acs_volt2 = 2 * ( (float) acs_voltage) * (2.5 / 4095.0);

    bat_states.current = (acs_volt2 - 2.525) / 0.185;    // x10 for 5V range on acs_712; x20 for 2.5V range on acs_712
}

Using this setup, and the code above, I am not getting the correct value. The value I am getting is -13.3A, which is way off, since it is out of the range of the sensor. Removing the sensor output from P6.0 gives a value of -13.6A. I am not sure where this offset is coming from, as it does not appear to be a calculation error.

 I can verify that current sensor/potential divider setup works, and outputs approximately 1.25V for 0A through it, as expected. Connecting the sensor output to P6.0 does seem to drop the sensor output voltage to a very low value, around 0.03V.

Does anyone know what may be happening here?

Am I attempting to draw too much current from the LaunchPad, or is it a calculation error?

I did not configure P6.0 to use a pull-down resistor, so I am not sure what is causing this.

Regards