A dc-triggered reset of a watchdog circuit is prone to failure. If the watchdog program hangs up, then the reset signal becomes activated continuously, and the microprocessor has no way to escape the situation. We found that a simple solution uses an ac trigger to reset the watchdog circuit (Figure 1). We used an RC oscillator consisting of a 74C14 gate to generate active-low reset signals to the microprocessor at approximately 10-msec intervals. High-level pulses at the base of the transistor switch Q₁ reset the charging capacitor C₁ to a low level. If the watchdog trigger remains in a high state for a longer period than you want, the oscillator generates an active-low reset pulse. You may believe that a reset signal from a watchdog circuit to a microprocessor is equivalent to a power-on reset, but it is not. The warm-boot and cold-boot programs in embedded microprocessors significantly differ. Warm-boot watchdog signals are prone to hang-up. The circuit in Figure 1 can activate the microprocessor even if the watchdog signal hangs up.

Is this the best Design Idea in this issue? Vote at www.ednmag.com.