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'16F84 power consumption during RESET // Brownout d'
1999\02\26@073213 by Marc

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Hi.

I have a question about the 16F84 power consumption. What I need in my project
is to remember SRAM contents over short power failures. Storing the information
to EEPROM is not possible, because it is already full with other data.

To accomplish that I did decouple the 5V input with a series diode, and added
a capacitor on the PIC side. The (failure prone) 5V input is sensed with RB0/INT
.
A falling edge triggers the int, and the PIC 16F84 will go to sleep. This is
to make the capacitor charge last as long as possible.

The capacitor that I chose for size and cost reasons allows data retention of ab
out
30 seconds, which is plenty enough for the problem. However, the voltage falls
below the minimum allowed for the PIC to function properly (brown-out). There is
a window, about 8 seconds after power failure to about 15 seconds, in which the
16F84 will not restart properly. Before and after it works just fine.

I wonder what's the best way to avoid that. "Best" in my case is "minimum compon
ents".
Either I can connect MCLR to 5V input, which puts the 16F84 into RESET during
power failure. I can't find any power consumption figure in the data sheet for
that situation. But I guess, since the oscillator is supposed to start during
RESET, it will consume as much as during normal operation. Is that correct? Do
you have any information about this?

The other solution is to build a 5V rise detector that issues a reset pulse to
the 16F84. It must be very short in duration, though. The device is expected
to respond quickly. Can anyone of you suggest a VCC rise reset generator that
is available in SOT23 and does _not_ enlengthen the /RESET pulse into the ms
range (like the DS1233 for example does)? Possibly only as short as VCC takes
to rise above 4.5V? Or is a discrete circuit more appropriate to the problem,
as no reset is required on 5V =fall=, only on =rise=?

1999\02\26@085608 by Michael Rigby-Jones

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Could you not use the standard bown out reset controller as described in one
of the Micrchip app notes? I forget which but should be relatively easy to
find.  It consisted only of a transistor and a few resistors for the
simplest configuration.  I don't know how much power the PIC will consume
when held in reset but I guess not much at all.

Regards

Mike Rigby-Jones
spam_OUTmrjonesTakeThisOuTspamnortelnetworks.com


{Quote hidden}

1999\02\26@113957 by Matt Bonner

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Michael Rigby-Jones wrote:
>
> Could you not use the standard bown out reset controller as described in one
> of the Micrchip app notes? I forget which but should be relatively easy to
> find.

AN522

--Matt

1999\02\27@051707 by Marc

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> Could you not use the standard bown out reset controller as described in one
> of the Micrchip app notes? I forget which but should be relatively easy to
> find.  It consisted only of a transistor and a few resistors for the
> simplest configuration.  I don't know how much power the PIC will consume
> when held in reset but I guess not much at all.

I see no reason why it should consume power in the uA range. The oscillator is
running, which is very hungry. The brownout detector that you mention holds
the PIC in /RESET during the whole powerfailure time. I'd really prefer to
let the PIC sleep and reset it only at the _end_ of that time (ie differentiate
the 5V line, invert that and feed it into /MCLR, for example done with a
cap + resistor + NPN or as said preferably with a ready-to-buy SOT23 IC).
"Power-On reset" is a more appropriate word than "Brownout detector" probably :-
)

When I return to that project tomorrow, I'll measure the current during /RESET.
Maybe I have to rethink the whole design, as I'm very tightly limited on number
of components as well as the capacitor size.

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