CCS/MSP432P401R: ADC On/Off Controlled by External Button Press and Timer A

Part Number: MSP432P401R

Tool/software: Code Composer Studio

I'm posting this as a related question because I accidentally clicked "This resolved my issue" on a comment on the other post when it did not actually resolve my issue.

While running the code below I get the following error:

#138 expression must be a modifiable value main.c /ADC_TEST line 77 C/C++ Problem

In reference to this line, where I am trying to turn the ADC off:

ADC14_CTL0_ENC = 0;   //turn off ADC

Furthermore, when I replace that instruction with a blinking LED (for debugging purposes) and then press the button I see one ADC value reported via UART and then the LED blinks continuously, regardless of how many additional times the button is pressed. Ideally, I want the ADC to turn on again when the button is pressed a second time. ANy help would be greatly appreciated.

/* Test ADC ON by external button and OFF by Timer A */

#include "msp.h"
#include "mytiming.h"

volatile unsigned int UART_flag = 0;
volatile unsigned int timer_flag = 0;
volatile unsigned int  button_flag = 0;
volatile unsigned int var = 0;
int millivolt = 0;

void UART0_init(void);
void delayMs(int n, int freq);
void setFreq(int freq);

int main(void) {

    setFreq(FREQ_3_MHZ);

    int mv_char_0 = 0;
    int mv_char_1 = 0;
    int mv_char_2 = 0;
    int mv_char_3 = 0;
    char uart_0;
    char uart_1;
    char uart_2;
    char uart_3;

    WDT_A->CTL = WDT_A_CTL_PW |             // Stop WDT
                 WDT_A_CTL_HOLD;

    // GPIO Setup
    P5->SEL1 |= BIT4;                       // Configure P5.4 for ADC
    P5->SEL0 |= BIT4;

    // Initialize Port 3 (button connections)
    P3->SEL0 = 0;  //clear register selection
    P3->SEL1 = 0;  //clear register selection
    P3->DIR = 0;
    P3->OUT = BIT0;
    P3->REN = BIT0;   //enable pull up resistor

    P3->IES = BIT0;  //interrupt on high-low transition
    P3->IFG = 0;     //clear any pending flags
    P3->IE = BIT0;   //enable port 3 interrupts

    UART0_init();

    // Enable global interrupt
    __enable_irq();

    // Enable ADC and Timer A0 interrupts in NVIC module
    NVIC_EnableIRQ(TA0_0_IRQn);
    NVIC_EnableIRQ(ADC14_IRQn);
    NVIC_EnableIRQ(PORT3_IRQn);

    // Sampling time, S&H=16, ADC14 on
    ADC14->CTL0 = ADC14_CTL0_SHT0_2 | ADC14_CTL0_SHP | ADC14_CTL0_ON;
    ADC14->CTL1 = ADC14_CTL1_RES_2;
    ADC14->MCTL[0] |= ADC14_MCTLN_INCH_1;   /* A1 ADC input select;
                                               Vref=AVCC */
    ADC14->IER0 |= ADC14_IER0_IE0;          /* Enable ADC conv
                                               complete interrupt */

    SCB->SCR &= ~SCB_SCR_SLEEPONEXIT_Msk;  /* Wake up on exit from
                                              ISR */

    while (1){
        if (button_flag){                      //button pushed, turn ADC on
            ADC14->CTL0 |= ADC14_CTL0_ENC | ADC14_CTL0_SC; //start sampling/conversion
            button_flag = 0;       //Reset flag
            P3->IFG = 0;           //clear any pending flags
            P3->IE |= BIT0;        //re-enable P3.0 interrupt
        }
        if (timer_flag){                      //time up, turn ADC off
            timer_flag = 0;
            ADC14_CTL0_ENC = 0;   //turn off ADC
        }
        if (UART_flag) {
            UART_flag = 0;
            millivolt = (int)((var + 1.46) / 1.2361);
            mv_char_3 = millivolt / 1000;
            mv_char_2 = millivolt / 100 - (mv_char_3 * 10);
            mv_char_1 = millivolt / 10 - (mv_char_3 * 100 + mv_char_2 * 10);
            mv_char_0 = millivolt - (mv_char_1 * 10 + mv_char_3 * 1000 + mv_char_2 * 100);
            uart_0 = (char) mv_char_0 + '0';
            uart_1 = (char) mv_char_1 + '0';
            uart_2 = (char) mv_char_2 + '0';
            uart_3 = (char) mv_char_3 + '0';
            while(!(EUSCI_A0->IFG & 0x02)) { }
            EUSCI_A0->TXBUF = uart_3;
            while(!(EUSCI_A0->IFG & 0x02)) { }
            EUSCI_A0->TXBUF = uart_2;
            while(!(EUSCI_A0->IFG & 0x02)) { }
            EUSCI_A0->TXBUF = uart_1;
            while(!(EUSCI_A0->IFG & 0x02)) { }
            EUSCI_A0->TXBUF = uart_0;
            while(!(EUSCI_A0->IFG & 0x02)) { }
            EUSCI_A0->TXBUF = 0x0D;
        }
    }
}

// ADC14 interrupt service routine
void ADC14_IRQHandler(void) {
    UART_flag = 1;
    var = ADC14->MEM[0];
}

//Timer A0 interrupt service routine
void TA0_0_IRQHandler(void){
    timer_flag = 1;         //flag when timer reaches value
    TA0CCTL0 &= ~CCIFG;     //clear pending interrupt flag
}

void PORT3_IRQHandler(void)       //Interrupt handler for P3.0
{
    button_flag = 1;              //Set flag to signal button press detected
    //configure Timer A0
    TA0CCR0 = 900000;             //Timer length = 300ms
    TA0CCTL0 |= CCIE;
    TA0CTL |= TASSEL_2 | MC_1;
    P3->IFG = 0;                  //Clear P3.0 pending interrupt flag
    P3->IE  &= ~BIT0;             //Disable interrupt for P3.0 for debouncing
}

void UART0_init(void)
{
    EUSCI_A0->CTLW0 |= 1;       // put in reset mode for config
    EUSCI_A0->MCTLW = 0;        // disable oversampling
    EUSCI_A0->CTLW0 = 0x0081;   /* 1 stop bit, no parity,SMCLK,byte
                                   data */
    EUSCI_A0->BRW = 26;         // 3,000,000 / 115200 = 26
    P1->SEL0 |= 0x0C;           // P1.3, P1.2 for UART
    P1->SEL1 &= ~0x0C;
    EUSCI_A0->CTLW0 &= ~1;      // take UART out of reset mode
}