CCS/MSP430G2553: Measuring a signal from Timer_A with an oscilloscope

Part Number: MSP430G2553

Tool/software: Code Composer Studio

Hi there,

I am recently working with the Timer_A interrupt.

Hardware settup: includes a 32 kHz external crystal connected to XIN and XOUT (I am working with the launch pad). Also the RXD and TXD Jumpers are configurated for USB UART communication. Workinstation is a Laptop.

Software: The software uses the extern source to generate the ACLK signal to suply the Timer_A. I use the timer in a compare mode to generate a 1 Hz frequency and would love to see that on my oszilloskop. In generall the code works and a LED which toggles in this frequency works. For messuring I toggle P1.5 with the 1 Hz frequency as well.

Messuring settup: The P1.5 and the upper J6 GND are connected to my oszilloskop.

Problem: Funny thing is, that the toggling LED stops if i connect GND. Also I am not able to see the frequency on the oszilloskopes display. Also funny is that i am working on a laptop connected with the launchpad. If i connect my recharger to my laptop the msp led stops blinking as well. I suppose its something with the ground of the device but i have no clue how to interprete it.

Can please anybody help me with that? It drives me crazy!

An image of my messure setup:

From time to time the messurement works. But as seen it shows a 1 Hz frequency even if i chose a compare value of 32000/2 so it should actually be 2 Hz. Still my setup is very sensitive about the ground pin. if i touch it or remove the ground connection to my osci, the behavior changes.

To make it complete, here my code:

#include <msp430.h>
#define LED_rot BIT0
#define LED_gruen BIT6
#define Button BIT3
#define Testpoint BIT5

bool schalter =true;

int main(void)
{
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
 
  P1DIR = LED_rot + LED_gruen + Testpoint; // P1.0 output
  P1OUT = 0;
 
  //Button setup
  P1REN = Button;                 // Enable internal pull-up/down resistors
  P1OUT = Button;                 //Select pull-up mode for P1.3
  P1IE = Button;                       // P1.3 interrupt enabled
  P1IES = Button;                     // P1.3 Hi/lo edge
  P1IFG &= ~Button;                  // P1.3 IFG cleared
 
  //ACLK with LFXT1CLK; 32768 Hz external crystal
  BCSCTL1 = 0;
  BCSCTL1 |= XT2OFF;
  BCSCTL2 = 0;
  BCSCTL3 =0;
  BCSCTL3 |= XCAP_0;
 
  //Compare value -> 1 Hz Toggle frequence
  TACCR0 = 32768 / 2;
 
  // C/C Register settings: no Capture; compare mode;  
  TACCTL0 = CM_0 + CCIE;
 
  //Timer A setiings: Source = ACLK; Upmode; Interrupt enabled
  TACTL = 0;
  TACTL = TASSEL_1 + MC_1;

  __bis_SR_register(GIE);       // Enter LPM3 w/ interrupt
 
  while(1)
  {
  }
}

// Timer A0 interrupt service routine
#pragma vector = TIMER0_A0_VECTOR
__interrupt void Timer0_A0_CCR0_ISR (void)
{
    P1OUT ^= LED_rot;
    P1OUT ^= Testpoint;
}

#pragma vector=PORT1_VECTOR
__interrupt void Port_1(void)
{
    if ( schalter )
    {
        TACTL ^= MC_1;
        schalter = false;
        P1OUT ^= LED_gruen;
    }
    else
    {
        TACTL ^= MC_1;
        schalter = true;
        P1OUT ^= LED_gruen;
    }
    P1IES ^= Button;                     // P1.3 Hi/lo edge
    P1IFG &=~Button;                        // P1.3 IFG cleared
}