Paul, some comments on your comments :-)

Paul Greenwood wrote:
> I have a regulator on my board and a big capacitor (10uF) to keep out
> noise, spikes, etc.  Well, I plug in my device, the cap. charges to 5V
> quickly.

I'm curious _why_ you have such a large capacitor on your board. A "filter"
capacitor is named such because it is configured as a low pass filter on the
input voltage. This is done in an effort to keep high frequency components
that are modulated on top of the input power from getting into the voltage
regulator. However, in some sense the regulator itself is the ultimate
low pass filter, passing only DC. The other purpose of the capacitor is
to act as a reservoir of energy when the current demands on the regulator
change suddenly and the regulator is connected to the primary source via
a network that has a limited response time. By providing the extra current
during the short period when the regulator starts demanding it and the
mains can produce it, the capacitor keeps the regulator operating in its
"normal" voltage range. Now, if all you have on the down wind side (the 5v
side) of this regulator is a PIC and a few CMOS chips, even the _worst_
case scenario could be handled by a much smaller capacitor. Easily 1uF and
probably .47uF.

> I checked this [DS1231] out but I really need something like a 5V regulator
> that will pull its output to 0V when the input falls below a threshold.

There are switching regulators with an on/off output. The other thing you
can do is to hook the Dallas part to a CMOS switch (like a 4066 or something
similar (and up to date :-)). Connect the PIC circuit in series with the
switch and have the Dallas part control the switch enable line. When the
reset delay has passed the Dallas part goes "on" and this turns on the CMOS
switch and that turns on the PIC circuit. You should probably be sure and
limit the current through the switch to under 100mA but there are other ways
to do this (with a MOSFET for example) that will work as well. In building
robots I find that good power management requires that I be able to turn
power _off_ to portions of the robot that are not being used (like the speech
synthesizer) to conserve batteries. I've started using Maxim Switching
regulators which are more efficient than 78xx linears and they have a logic
level "on/off" input. If you call Maxim ask about their evaluation board.

> I guess that, I MAY be able to ignore the line in the databook and use the
> reset pin and the DS1231.

All in all, probably the best way to go.

--Chuck