Significance of watchdog timer in Embedded Systems

1)Watchdog timer is basically a timing device that is set for predefined time interval and some event should occur during that time interval else the device generates a time out signal.

2)One application where it is most widely used is when the mobile phone hangs and no activity takes place, in those cases watchdog timer performs a restart of the system and comes to the rescue of the users.
 
3)It is used to reset to the original state whenever some inappropriate events take place such as too many commands being given at the same time or other activities that result in malfunctioning of the GUI. It is usually operated by counter devices.

Watchdog_Timer_Example

#include "embedded.h"
// LEDs used to indicate code activity and reset source
DigitalOut myled1(LED1); //in main loop part 1
DigitalOut myled2(LED2); //in main loop part 2 (where fault occurs)
DigitalOut myled3(LED3); //The pushbutton or power on caused a reset
DigitalOut myled4(LED4); //The watchdog timer caused a reset
 
// Simon's Watchdog code from
// http://mbed.org/forum/mbed/topic/508/
class Watchdog {
public:
// Load timeout value in watchdog timer and enable
    void kick(float s) {
        LPC_WDT->WDCLKSEL = 0x1;                // Set CLK src to PCLK
        uint32_t clk = SystemCoreClock / 16;    // WD has a fixed /4 prescaler, PCLK default is /4
        LPC_WDT->WDTC = s * (float)clk;
        LPC_WDT->WDMOD = 0x3;                   // Enabled and Reset
        kick();
    }
// "kick" or "feed" the dog - reset the watchdog timer
// by writing this required bit pattern
    void kick() {
        LPC_WDT->WDFEED = 0xAA;
        LPC_WDT->WDFEED = 0x55;
    }
};
 
// Setup the watchdog timer
Watchdog wdt;
 
int main() {
    int count = 0;
// On reset, indicate a watchdog reset or a pushbutton reset on LED 4 or 3
    if ((LPC_WDT->WDMOD >> 2) & 1)
        myled4 = 1; else myled3 = 1;
        
// setup a 10 second timeout on watchdog timer hardware
// needs to be longer than worst case main loop exection time
    wdt.kick(10.0);  
 
// Main program loop - resets watchdog once each loop iteration
// Would typically have a lot of code in loop with many calls
    while (1) {
        myled1 = 1; //Flash LEDs 1 & 2 to indicate normal loop activity
        wait(.05);
        myled1 = 0;
        myled2 = 1;
        wait(.05);
// Simulate a fault lock up with an infinite while loop, but only after 25 loop iterations
        if (count == 25) while (1) {};
// LED 2 will stay on during the fault
        myled2 = 0;
        count ++;
// End of main loop so "kick" to reset watchdog timer and avoid a reset
        wdt.kick();
    }
}