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PICList Thread
'PIC Power-up.'
1998\11\01@175602 by enpassant

picon face
As a newbee to PICs  I can't figure this one out.
It seems that the PIC 16F84 is very fickle about its power-up voltage.
I use an IBM PC CLONE power supply to power the chip.  It gives a steady
regulated +5V Supply.  If I set up the circuit and turn the power supply
on, nine times out of ten the chip acts unexpectedly.  If I then remove
the +5V power lead from the circuit and reconnect it, the code runs as
intended.

I surmise that the ramp-up voltage is causing this.  Has anyone else
seen this happen? Is there a simple circuit that will prevent any
voltage getting to the chip until it reaches +5V???  P.S. I tried a few
different power supplies and PICs- they do the same thing!

1998\11\01@182413 by Tony Nixon

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picon face
enpassant wrote:
>
> As a newbee to PICs  I can't figure this one out.


Probably the best course here is to use an external reset circuit.

You can use the a simple RC circuit or a reset chip such as MC34064 or
similar.

You may even get away with enabling the power up timer.


--
Best regards

Tony

Multimedia 16F84 Beginners PIC Tools.

http://www.picnpoke.com
Email spam_OUTpicnpokeTakeThisOuTspamcdi.com.au

1998\11\01@191117 by Mike Keitz

picon face
On Sun, 1 Nov 1998 17:55:04 -0500 enpassant <.....enpassantKILLspamspam@spam@AMERITECH.NET>
writes:
>As a newbee to PICs  I can't figure this one out.
>It seems that the PIC 16F84 is very fickle about its power-up voltage.
>I use an IBM PC CLONE power supply to power the chip.  It gives a
>steady
>regulated +5V Supply.  If I set up the circuit and turn the power
>supply
>on, nine times out of ten the chip acts unexpectedly.  If I then
>remove
>the +5V power lead from the circuit and reconnect it, the code runs as
>intended.

This is a dirty PIC secret.  If the power doesn't rise in the usual way
(i.e. go rapidly and monotonically from 0 to 5V), some internal nodes in
the PIC can  latch up.  The only cure is to turn the power off and back
on, even a MCLR reset won't clear it.  Microchip should wire MCLR to all
these nodes in future PIC chips so they can be reset.

Most power supplies are alright for the PIC, however some supplies don't
come up as sharply, and will latch the PIC up almost every time.  It does
help to hold the MCLR line low while the power is coming up, using an
external voltage detector chip.

If you use a PC clone supply, there are a couple of things to try.
First, most of these supplies work better with a minimum load on the +5V
output.  Try installing a power resistor of 5 ohms, 5 watts or so in
parallel with your PIC circuit's 5V.  Also, the PC supplies provide a
signal called Power Good that is ideal for holding the PIC in reset until
the power comes up.  It is used to reset the PC CPU as well.  The Power
Good line is on the end of the plug that goes to the mother board that
includes the + and - 12V lines (not the plug with 3 +5V and 3 ground
lines, the other one).  Connect Power Good directly to the PIC's MCLR
pin.

>
>I surmise that the ramp-up voltage is causing this.  Has anyone else
>seen this happen? Is there a simple circuit that will prevent any
>voltage getting to the chip until it reaches +5V???

Yes there are circuits to do that.  For example, you could use a
transistor circuit controlled by Power Good or an RC timer to not turn on
the PIC until after teh 5V has been present for a while.  Maybe even a
555 timer would work (it has 200 mA of output current, though the output
voltage won't rise quite to 5V).  A somewhat simpler possibility is to
use the +12V output of the supply and regulate it yourself with a 7805.


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1998\11\01@191923 by Les

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picon face
If you are not using the resistor in the xtal path  that can cause
it.Sometimes the xtal just doesn't oscillate without it or oscillates
erratic, It depends on the spec of the xtal or the pic. And always have
decoupling capacitors on the circuit you are building, don't just rely on
the PSU.Place the capacitor as close to the pic as possible a .1uf will
suffice and an electrolytic on the main voltage coming in, a tantalum if
possible but they are expensive and don't age very well.

1998\11\02@095755 by Dave Johnson

flavicon
face
>This is a dirty PIC secret.  If the power doesn't rise in the usual way
>(i.e. go rapidly and monotonically from 0 to 5V), some internal nodes in
>the PIC can  latch up.  The only cure is to turn the power off and back
>on, even a MCLR reset won't clear it.  Microchip should wire MCLR to all
>these nodes in future PIC chips so they can be reset.
What's the usual strategy in a real product?

Like, for instance, let's say I have a "friend" who's designing a battery
powered PIC-based product with a plain vanilla DC-DC boost converter...is
it a good idea to include some kind of auto-reset when the power comes up?

Purely theoretical, of course :-)

Dave Johnson

1998\11\03@125419 by Peter L. Peres

picon face
On Mon, 2 Nov 1998, Dave Johnson wrote:

> >This is a dirty PIC secret.  If the power doesn't rise in the usual way
> >(i.e. go rapidly and monotonically from 0 to 5V), some internal nodes in
> >the PIC can  latch up.  The only cure is to turn the power off and back
> >on, even a MCLR reset won't clear it.  Microchip should wire MCLR to all
> >these nodes in future PIC chips so they can be reset.
> What's the usual strategy in a real product?
>
> Like, for instance, let's say I have a "friend" who's designing a battery
> powered PIC-based product with a plain vanilla DC-DC boost converter...is
> it a good idea to include some kind of auto-reset when the power comes up?
>
> Purely theoretical, of course :-)

Well, what I sometimes do is, I use a 324 or comparator alongside the PIC
and I use one section of the comparator as voltage regulator for the PIC
only (often the reference is a LED ;) and I put some hysterezis into its
(positive) feedback loop so it starts and stops abruptly, and also drains
the charge in the PIC decoupling caps when turning off. This cures many
problems but it is not quite perfect. It's certainly better than a 78L05 +
prayers. The only problem is, one has to use smaller decoupling caps on
the PIC.

Peter

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