Charles:

The 555 is not a gooc choice for a watch dog generator. What is does at power-up along will prevent its reliable use......and then any reasonable system still needs have an accurate Vcc brownout detect. And besides that the small 8 pin type reset generator / watch dog chips are cost effective against the 555 chip and all the other compoments around it. Remember that in production a quarter cent resistor still has a 8 -> 10 cent place cost.

Michael Karas