Low power in LMP3 and Touch Sensing in MSP430G2553

Hi folks,

i´ve been working on capacitive RC touch sensing and looking for the lowest power for a few days with no good results. I´m using MSP430G2553 on the Launchpad and to sum up this is what i´m trying to do:

1 º Use VLO as a source for Timer_A0 to generate a 4Hz interrupt.

2º Use this 4Hz interrupt to start a scanning of the electrode using the RC method. In this case, the DCO at 16MHz is the source for the Timer_A1, so you can measure the difference in the number of cycles for the Pin-negative-edge-triggered interrupt.

3º When the pin-negative-edge-triggered interrupt is detected, the difference in cycles is measured and compared to a threshold so you can decide if there is a finger or no.

This is the code:


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#include <msp430g2553.h>


unsigned int timer_count;

void main(){

    WDTCTL = WDTPW + WDTHOLD;                         // Stop watchdog timer
    BCSCTL1 = CALBC1_16MHZ;                           // Set DCO to 1, 8, 12 or 16MHz
    DCOCTL = CALDCO_16MHZ;


    BCSCTL3 |= LFXT1S_2;


    // CONFIGURO EL TIMER 0 Y EL CCR0
    TA0CCTL0 &= ~CCIE;                                 // CCR0 interrupt disable
    TA0CTL = TASSEL_2 + MC_0;                        // SMCLK and Continous up

    // CONFIGURO EL TIMER 0 Y EL CCR0
    TA1CCTL0 |= CCIE;                                 // CCR0 interrupt enable para 32768Hz es un segundo
    TA1CCR0 = 5000;                                    // The number of timer counts in the period is TACCR0+1.
    TA1CTL = TASSEL_1 + MC_1;                        // ACLK and up to CCR0

    P2SEL &= ~BIT6 + ~BIT7;                            // GPIO function
    P1DIR = 0xFF;                                    // out
    P1OUT = 0;                                        //  0

    P2DIR = 0xFF;                                    // out
    P2OUT = 0;                                        //  0

    ADC10CTL0 &= ~ENC;                                // Disable ADC conversion
    ADC10CTL0 &= ~(REFON + ADC10ON);                  // Ref and ADC10 off

    _BIS_SR(GIE);                 // Enter LPM0 w/ interrupt


    while(1){
        /* Take the active key high to charge the pad */
        TA0R = 0;
        TA0CTL = TASSEL_2 + MC_2;                        // SMCLK and Continous up
        P1OUT |= BIT4;
        _NOP();
        _NOP();
        _NOP();
        _NOP();
        _NOP();
        _NOP();
        _NOP();
        _NOP();
        // Enable interrupts (edge set to low going trigger)
        // set the active key to input (was output high), and start the
        // timed discharge of the pad.
        P1IES |= BIT4;                                    //-ve edge trigger
        P1IE |= BIT4;
        P1DIR &= ~BIT4;
        /* Take a snaphot of the timer... */
        timer_count = TA0R;
        LPM0;
        P1IE &= ~BIT4;                                    // disable active key interrupt
        P1OUT &= ~BIT4;                                   // switch active key to low to discharge the key
        P1DIR |= BIT4;                                    // switch active key to output low to save power
        LPM3;
    }

}

#pragma vector=PORT1_VECTOR
__interrupt void port_1_interrupt(void)
{
    TA0CTL = TASSEL_2 + MC_0;                        // SMCLK and Continous up
    timer_count = TA0R - timer_count;
    if((timer_count > 2500) && (P1IFG == BIT4)){
        P1OUT ^= BIT0;
    }
    P1IFG = 0;                                        // clear flag
    LPM3_EXIT;                                        // Exit LPM3 on reti

}

// Timer_A2 Interrupt Vector (TA0IV) handler
#pragma vector=TIMER1_A0_VECTOR
__interrupt void Timer1_A0(void)
{
    _BIC_SR_IRQ(LPM3_bits);                   // Clear LPM3 bits from 0(SR)

}

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I achieved 67uA, but i think i´m doing something wrong, because as i see in some application notes, the current consumption in LPM3 is in the order of the hundreds of nA. This code stays in LPM3 most of the time so i think the current ought be much lower than 67uA.

Any advice?? Anything would be very appreciated! Thanks in advanced!

Alvaro G