Smartphones can be rebooted with a power-cycle off and on (usually when you need to make an urgent call), but how do you power down the MCU that runs the cruise control in your car? Turning off the car would definitely reboot the cruise control loop, but this option is obviously not safe at a high rate of speed. The first WDT, if not reset, may trigger consequences akin to a soft-boot rather than a hard boot. A bit unusual for embedded WDTs, this nested WDT gives the system up to 3 chances to reset the WDT before resetting the computer. Figure 1: This computer WDT has 3 stages of successive WDTs. A watchdog can be an external component in a separate package from the integrated circuit (IC) that houses the MCU (best), or a watchdog can be found inside the IC but on a different circuit from the MCU, however a WDT that’s dependent on the same resources as the MCU might not be a good idea for obvious reasons. It’s best to keep a watchdog external and unreachable by MCU code.

But as long as the MCU is running, it will continue to ping the watchdog to reset the timer. Like a bomb, the watchdog timer is set to count down and if it times out, it resets the MCU, dumping programs and rebooting the MCU and probably other areas in the system that work in tandem with the MCU. The MCU checks in with the watchdog timer at a set interval to show that it’s still on the job. Watchdog timers (WDTs), or watchdogs, are circuits external to the processor that can detect and trigger a processor reset (and/or another event) if necessary. (The BSoD happens much less often these days.) Embedded systems are different from desktop computers, however, in that there is rarely going to be a human around who will know how to reboot the failing device. In “the old days,” the Windows operating system would occasionally crash (experience a fatal error) and put up what was called the Blue Screen of Death (BSoD) where after it would reboot to prevent damage to the computer such as writing over vital boot code or similarly dangerous events. If a program goes haywire or into an infinite loop, it needs a way to check and see if things are still running.

In the world of microcontrollers (MCUs), sometimes things go wrong.

What is watch dog