Watchdog Timer

Overview

• Watchdog Timer: component of a microcontroller to monitor and supervise the functions of other system components and the program
• In case of an error an interrupt or a system reset can be executed.
• The watchdog timer needs to be resetted continuously, otherwise the interrupt or reset will be executed.
• If there is a error and the program holds, the watchdog is no longer resetted and restarts the program.
• The WDT of the MSP430 can be used as a continuous interrupt generator in the so-called interval mode.
• When ever you want to change the value of the WDTCTL register, you need to set the WDTPW bit to the correct value. Otherwise an MSP430 will be resetted.

Watchdog Mode

Watchdog Mode of the MSP430 Watchdog Timer

int main(void) {
    WDTCTL = WDTPW | WDTHOLD;    // stop watchdog timer

    P1DIR |= BIT0;
    P1OUT |= BIT0;
    __delay_cycles(100000L);
    P1OUT &= ~BIT0;

    P1DIR &= ~BIT3;
    P1OUT |= BIT3;
    P1REN |= BIT3;
    P1IES |= BIT3;
    P1IFG &= ~BIT3;
    P1IE |= BIT3;

    WDTCTL = WDTPW + WDTCNTCL + WDTSSEL;

    __enable_interrupt();

    while (1) {
        __low_power_mode_3();
    }
}

#pragma vector=PORT1_VECTOR
__interrupt void PORT1_ISR() {
    if (P1IFG & BIT3) {
        P1IFG &= ~BIT3;
        WDTCTL = WDTPW + WDTCNTCL + WDTSSEL;
    }
}

• LED on P1.0 will blink only on the beginning of the program.
• When the LED blinks the MSP430 was resetted by the Watchdog Timer
• WDTCTL = WDTPW + WDTCNTCL + WDTSSEL;
  • The Watchdog Timer is enabled.
  • As clock source the ACLK is used (WDTSSEL).
  • After 32768 clock cycles = 1 second the watchdog timer will trigger the reset.
• When the button on P1.3 is pressed an interrupt will be executed.
• This interrupt resets the Watchdog Timer.
• You need to press the button once every, otherwise the MSP430 will be resetted.

Interval Mode

Interval Mode of the MSP430 Watchdog Timer

int main(void) {
    WDTCTL = WDTPW | WDTHOLD; // stop watchdog timer

    P1DIR |= BIT0;
    P1OUT &= ~BIT0;

    WDTCTL = WDTPW + WDTTMSEL + WDTCNTCL + WDTSSEL;
    IE1 |= WDTIE;

    __enable_interrupt();

    while (1) {
        __low_power_mode_3();
    }
}

#pragma vector=WDT_VECTOR
__interrupt void WDT_ISR() {
  P1OUT ^= BIT0;
}

• After 32 768 clock cycles = 1 second og the Watchdog Timer the WDT_ISR() is executed.
• The LED is toggled once every second.

Info

Watchdog Timer Interval Select

WDTISx	Divider	Interval @ACLK=32.768 kHz	Interval @SMCLK=1 MHz
00	/32768	1000 ms	32.768 ms
01	/8192	250 ms	8.192 ms
10	/512	15.625 ms	512 µs
11	/ 64	≈ 1.95 ms	64 µs

Question

Modify the previous program. Change the LED toggling period to 250 ms.

Solution

WDTCTL = WDTPW + WDTTMSEL + WDTCNTCL + WDTSSEL + WDTIS0;