By CD4060 and 32.768Hz composed of quartz oscillator pulse generating circuit generates 4Hz pulse source. 74LS293 counter 4Hz pulse source is counted, its output Q is connected to the microcontroller reset terminal RESET, certain I / O terminal is connected to the microcontroller 74LS293's clear terminal R, the program within 1.5 seconds of the first output from low to high and because of narrow pulses from high to low, the output Q has been low, otherwise the output is high .TTL type watchdog circuit :
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