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PICList Thread
'Battery Backup'
1997\01\21@184849 by James Ruxton

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I was wondering if anyone has any simple solutions for battery backup. I'm
using a 16C74 that is displaying info. on an alphanumeric led display.
There are times when I'll have to change the main battery , or turn off the
main battery for one or two days. My problem is I don't want to lose the
data in the PIC .  What would people reccomend in this case. Should I add
eeprom? Should I turn off the display  put the pic to sleep and run off a
cap or small battery? What is the simplest way to detect the main battery
being removed from the system and still have time to save important data?
If anyone has any thoughts on this issue I'd love to hear them. Thanks
--
Jim Ruxton              (416)927-7679
Cinematronics
Toronto Canada
spam_OUTcinetronTakeThisOuTspampassport.ca

1997\01\21@190934 by TONY NIXON 54964

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The simplest way is to have a diode in series with a backup battery
connected to the supply rail to the PIC.

If external components are controlled by the PIC which draw unwanted
current then the PIC can monitor the main supply rail with an IO pin.
When power is turned off, the backup battery takes over, and because
of the low input, the PIC can disable all external devices. The IO
pin may need a resistor to ground.

Memory backup caps (SuperCaps) can also be diode isolated and feed
the PIC power pins when the main supply is disconnected. The PIC
should then be put to sleep to minimise current draw. The PIC may not
function properly at voltages lower than 5V, but the RAM holds its
data OK. (1.5V RAM retention specified).

If the project is mains powered, then this power can charge a NiCad
or similay battery via a resistor in parallel with the supply
regulator.

More sophisticated ways can be employed using supply rail monitors
(from Maxim for example). These will automatically connect a backup
battery in the event of a power failure.

Regards
Tony


Just when I thought I knew it all,
I learned that I didn't.

1997\01\22@083947 by Mark A. Corio

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In a message dated 97-01-21 18:50:59 EST, you write:

>I was wondering if anyone has any simple solutions for battery backup. I'm
>using a 16C74 that is displaying info. on an alphanumeric led display.
>There are times when I'll have to change the main battery , or turn off the
>main battery for one or two days. My problem is I don't want to lose the
>data in the PIC .  What would people reccomend in this case. Should I add
>eeprom? Should I turn off the display  put the pic to sleep and run off a
>cap or small battery? What is the simplest way to detect the main battery
>being removed from the system and still have time to save important data?
>If anyone has any thoughts on this issue I'd love to hear them. Thanks
>--
>Jim Ruxton

Jim,
If your data that you want to maintain is not changing too often, eeprom
works well.  We are currently building a product where we want to power up
with the configurations that were set when the unit was powered down.  We
save the configuration to eeprom each time the user changes any settings.
This way they are always available at power up.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel:  (716) 328-5850 --- Fax:  (716) 328-1144
http://www.frontiernet.net/~rmi/

***** Designing Electronics For Research & Industry *****

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