On Fri, 20 Feb 1998 10:03:58 -0000 Mark Birks <m.birks@OMNIBUS.CO.UK>
writes:
>HI,
>
>I intend to have a 3V Lithium as a backup battery which is intended to
>run a PIC for just long enough to allow it to save some data to an
>external EEPROM and then go into SLEEP mode.

You don't need a battery for that, a moderately large capacitor will do.
On the other hand, if you have a battery, you don't need an EEPROM since
the data in the PIC RAM will not be lost when the power is off.

The size of the capacitor required can be calculated:

C = I*t / V

For example, if the writing process takes 100 ms and 10 mA, and the
capacitor voltage can drop by 1V (from 4.5 to 3.5V say) during the
process, C = 100E-3 * 10E-3 / 1 = 1E-3 F = 1000 uF.
>
>Is 3V enough as I will have a series diode as well :
>
>
>5V ---------diode---------------------------------Vcc
>                          |
>                       diode
>                          |
>                          |
>             3V Lithium button battery
>                          |
>0V ------------------------------------------------- 0V
>
>I also intend to use a DALLAS DS1233M to monitor the 5V rail and give
>a
>'power fail' signal to initiate the message writing. Is this a good
>way
>to do this ?

Dallas also has chips which contain both a low voltage a circuit to
switch over to battery.  They use a FET switch so the voltage drops in
the diode circuit are not present.  If cost isn't too important you may
want to use one of them.
>

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