Hi Rob,

The simplest circuit I can think of is (Please excuse the ASCII Art):


9V DC Source  ------|>|------+---------  To PIC Regulator
                             |
                             |
                            ---
                             ^
                            / \
                            ---
                             |
                             |

                            9 V
                          Battery

The two diodes can be just about anything (INS914 is what I would use
because they're cheap).

When the input power drops in this circuit, the 9 Volt battery automatically
takes over.

Ideally, I would say you should use a > 9.7 Volt supply (10 or 12 Volts) to
make sure the circuit doesn't oscillate between taking power from the DC
Source and the Battery.

Sensing the input power and switching sources by PIC control for this
application is probably overkill.

myke

>Hi,
>
>I plan on using the DS1233 (or variant) as a brownout detection to reste
>MCLR on a PIC project i have using a 16C73 as Myke Predko explained).  I
>remember others on the list having problems with the PIC brwonout circuits,
>so this seems like a good method.  However, I wanted to add a battery
>backup to the PIC, as this chip will normally be powered by a 9V DC power
>supply that is connected to AC.  The 9V will go through a 5V fixed
>regulator before the PIC of course.  My question is, what kind of circuit
>can I build that would switch in or out a 9v battery as a backup?  I was
>thinking a switched MOSFET, except that depending on how variable the 9Vdc
>was, the MOSFET might go into the active region and get pretty hot if it
>dropped to lets say 8Vdc.
>
>ALso, there aren't any chips similiar to the Dallas brownout chip that have
>the battery backup?  I've looked, but the onew with battery backup seem
>rather complicated.
>
>Thanks for any thoughts or suggestions!
>
>Rob
>
>

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