Each device has three inputs permitting the choice of either leading-edge or trailing edge triggering. An internal connection from CLR to the input gate makes it possible to trigger the circuit by a positive-going signal on CLR as shown in the Truth Table. Once triggered, the basic pulse width may be extended by retriggering the gated active-LOW transition or active-HIGH transition inputs or be reduced by use of the active-LOW transition clear input. An external resistor RX and external capacitor CX are required for proper operation. The value of CX may vary from 0 to any necessary value.
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If you want genuine rad hard parts you may need to start with a list of suitable ICs and go from there. Re using a small microcontroller as mentioned below - if the uC has its own watchdog, may it not be as suitable? Maybe not. Is it possible to adequately shield a very small core of key components? Old iron: The datasheet here seems to be pretty much a denizen of another age and not at all comfortable at the 10 second range. A number of ICs contain oscillators and digital counters and can provide monostable functionality out to many hours.
A better watchdog: Very suitable in many cases and possibly most cost effective would be a small microcontroller dedicated to the watchdog task. One which itself has a quality watchdog to protect it from falling over would be a good idea.
Achievable results: A microcontroller solution can achieve delays of any sensibly conceivable length and complexity of pattern. As an example of achievable watchdog complexity that would be hard to achive any other way - "A watchdog may usually allow say 10 seconds between triggering events but if it was takem to more than 9 second twice in a row or 6 seconds more than 4 times in a row or more than 3 seconds twenty or more times in the last hour it should force a reset.
Programming is required, but in any volume the net cost of implementation is liable to be lower than most alternatives. The table below from the Onsemi datasheet here provides the useful table below at the bottom of page The MCB datasheet here at 1 uS to 23 hours recommended "looks good" [tm]. Similarly the MC Note that the is shown as recommended to 1 second but usable up to 10 seconds.
The main limitation seems to be leakage. Datasheet here from OnSemi. They say pulse wdths from 1 uS to 10 seconds. Longer for he excessively enthused.
Electronic Engineering Dictionary
IC TTL 74123
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