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PICList Thread
'Supercap Backup'
1997\02\07@164504 by jim ruxton

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I'm building a system which requires memory backup for the PIC. ie. When
main power is removed the PIC will go to sleep and either wake up
priodically via watchdog interrupt and check to see if power is back by
polling a pin or I will use a hardware interrupt. My configuration now is as
follows:

                 Diode    .1 F cap
        +5 0------|>|---0--| |-------0 GND
                        |
                        |
                        0 To PIC VCC & MCLR

My problem is on power up the PIC doesn't start properly, I assume because
of the slow rise time. I'm considering using  a three terminal reset device.
This device would have to trip at a low voltage ie. 2.5 volts to allow the
capacitor voltage to droop while no external power is connected. I believe
the MCLR triggers at around 1.8 volts so as long the capacitor doesn't droop
this low I should be ok. Am I correct in my assumptions about MCLR? Is there
a way to avoid using an external reset device? Any help would sure be
appreciated. Thanks!
                                                                     Jim

1997\02\07@173310 by brentw1

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jim ruxton wrote:
{Quote hidden}

add two additional diodes one from +5 to the vcc of the pic and annother
from the cap diode juction you have now to vcc on the pic. you might
need to add a small resistor in series with the cap and diode to reduce
the inrush current if your powersource won't chanrge the capacitor
without saging

1997\02\08@043557 by Kalle Pihlajasaari

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Hi Jim,

> My problem is on power up the PIC doesn't start properly, I assume because
> of the slow rise time. I'm considering using  a three terminal reset device.
> This device would have to trip at a low voltage ie. 2.5 volts to allow the
> capacitor voltage to droop while no external power is connected. I believe
> the MCLR triggers at around 1.8 volts so as long the capacitor doesn't droop
> this low I should be ok. Am I correct in my assumptions about MCLR? Is there
> a way to avoid using an external reset device? Any help would sure be
> appreciated. Thanks!

The three terminal reset device will probably be cheaper than your
supercap.  It will be a good design choice for any battery powered
design.

Cheers
--
Kalle Pihlajasaari   spam_OUTkalleTakeThisOuTspamip.co.za   http://www.ip.co.za/ip
Interface Products   P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750   Fax: 402-7751    http://www.ip.co.za/people/kalle

DonTronics, Silicon Studio and Wirz Electronics uP Product Dealer

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