I got a trouble when I powered ON a circuit with 555 which is used in monostable mode. The schematic is as follows. Fig. 1 Watchdog timer circuit with 555 Expected behaviour Trig in is connected to a digital output pin of a microprocessor. The microprocessor sets Trig in to Low every several hundreds of milliseconds for few milliseconds. Watchdog out is supposed to be Low only when the microprocessor fails to set the Trig in to Low within 1.
555 has been widely used for more than 40 years as a timer IC. There are bipolar type which is original, and CMOS type which is lower power consumption than original bipolar type. 555 can be used to make 10us to 100s timer which is configured by externally added R and C. Simplified block diagram is shown in Fig. 1. Fig. 1 Simplified block diagram and example usage Operation sequence As an initial state, Trigger input is more than 1⁄3 Vcc, and RS-FF is reset.