Editing Watchdog timer (section)

====Analog watchdogs====

[[File:Analog WDT.svg|thumb|Simple analog watchdog timer]]
[[File:Analog WDT timing.svg|thumb|Example timing diagram for analog WDT shown above. Four timely kick pulses keep ''V<sub>C</sub>'' below ''V<sub>TH</sub>''. When kicks cease, ''V<sub>C</sub>'' rises above ''V<sub>TH</sub>'' and causes a timeout.]]
Analog WDTs have a ''kick'' input and ''timeout'' output, but lack the clock input signal found in digital electronic watchdogs.

Circuitry and components vary widely among analog watchdogs, but in general, analog WDTs typically base their timing functions on [[capacitor]] charging rates. For example, in the analog watchdog circuit shown to the right, electric current ''i'' gradually charges capacitor ''C'', causing voltage ''V<sub>C</sub>'' to ramp up (rise at a constant rate). In normal operation, periodic "kick" pulses are applied to the kick input. Each kick causes capacitor ''C'' to discharge, thus restarting the voltage ramp-up. However, if the kicks cease or become spaced too far apart in time, ''V<sub>C</sub>'' will rise above threshold voltage ''V<sub>TH</sub>'' and, as a result, the [[voltage comparator]] will assert the ''timeout'' signal.