A Simple Latching Watchdog Timer

Most watchdog-timer ICs produce a single limited-duration output pulse when the watchdog timeout expires. That works fine for triggering resets or interrupts in a microprocessor, but some applications require the output (failure indicator) to latch. The simple circuit shown in the figure provides a latched failure indication in response to a loss of the input pulse stream.

Based on a microprocessor-supervisor/watchdog IC, this circuit is suitable for monitoring a fan (based on the fan's tachometer output), an oscillator or the software execution of a microprocessor.

During power-up, the RESET pin remains low until V_CC stabilizes and the reset timeout delay expires. Capacitor C charges through R until the FET's gate voltage reaches its threshold (V_TH), which turns on the FET and enables the latching capability. To prevent false triggers, you should set the delay of RC much longer than the reset timeout.

The WDI input (pin 6) must toggle at a minimum rate set by the capacitor C_SWT. If that fails to happen, RESET goes low, turns on the LED, and pulls the RESET IN connection low, thereby latching RESET. The circuit remains in this condition until you cycle V_CC or push the switch. Either action turns off the FET and allows RESET IN to go high.

To monitor the open-drain tachometer signal of a fan, connect a 10-kΩ pull-up resistor from WDI to V_CC (pin 8), and connect WDI to the fan's tachometer output. Because the fan requires some time to spin up, the watchdog circuit needs to be deactivated for a short-delay interval.

You can create this delay by placing a capacitor (C2) from RESET IN to ground. Notice that this delay must be shorter than the RC delay mentioned previously, or RESET will latch prematurely.

For a fan monitor, the value of C_SWT sets the maximum tachometer pulse period according to the formula t_WD=5.06 × 10⁶ C_SWT, where t_WD is in seconds and C_SWT is in farads. If the fan speed drops below this threshold, the RESET output asserts low and latches.