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PICList Thread
'PIC detecting power down...'
1998\02\20@051015 by Mark Birks

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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.

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 ?

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1998\02\20@123736 by Mike Keitz

picon face
On Fri, 20 Feb 1998 10:03:58 -0000 Mark Birks <.....m.birksKILLspamspam@spam@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.
{Quote hidden}

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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1998\02\20@174246 by wwl

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On Fri, 20 Feb 1998 10:03:58 -0000, you wrote:

>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.
>
>Is 3V enough as I will have a series diode as well :
>
>
>5V ---------diode---------------------------------Vcc
>                          |
>                       diode
>                          |
>                          |
>             3V Lithium button battery
>                          |
>0V ------------------------------------------------- 0V
Why not use a capacitor (not even a supercap type)- I assume once the
data is written, no further action is needed. Unless you are writing a
lot of data, I doubt a battery would be needed. A hundred uF or two
should be plenty to write a good few bytes to the eeprom. Be careful
to ensure the voltage holds up long enough after the last byte write
to ensure the eeprom completes its internal write cycle.
>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 ?
Probably a bit expensive - you could use the comparator in a 16C620 to
do the same thing more cheaply. If the power always goes fairly
quickly (i.e. not a battery rundown), you could just compare the
voltage on the PIC reservoir cap (or battery) with the incoming supply
- when the incoming supply drops below the PIC supply, trigger the
shutdown event.  

An alternative approach to an eeprom -  if there isn't much data, keep
the PIC running permanently (asleep) on a battery, and store the data
in the PIC's RAM instead of an EEProm.


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