According to the graphs in the datasheet a PICF877-20 is rated at up to
20MHz as long as the voltage is greater than 4.5. It is derated to a maximum
of 16MHz between 4.0 and 4.5 volts. It is not rated to work AT ALL at
voltages below 4.0 volts.

Bob Ammerman
RAm Systems


----- Original Message -----
From: "Leo Dep" <leo.depalo.ro@LIBERO.IT>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, May 29, 2002 1:05 PM
Subject: Re: [PIC]:Powering the 16F877 with a 3.5 V, no quartz oscillate.


> >> In my understanding, BODEN will hold the chip in reset if Vcc is too
low.
> >> Its thresholds are set unconditionally to be compatible with 5v, so if
> you
> >> attempt to run at 3.3, BODEN will always hold the chip in reset.
> >>
> >> It would have been more useful to have a BODEN_3.3 and BODEN_5.0, or
> maybe a
> >> BODEN with an adjustable threshold, but Microchip did not do it that
way.
> >>
> >> Can someone else back me up on clearing BODEN for 3.3v designs? I may
be
> out
> >> on a limb here.
> >><<
> >
> > You are right.  You can't use the brownout detect feature with "low
> voltage"
> > designs because it will detect a brown out even though the power supply
is
> > correct.
> >
> > However, this guy's problem runs much deeper.  He is using the PIClist
as
> a
> > substitute for reading the manual.  Not knowing about brown out detect
is
> a
> > legitimate problem a newbie might stumble over, but attempting to run a
5V
> > chip at 3.5V without ever checking the electrical specs is just plain
> stupid
> > or lazy.  This is true of ANY chip, even a simple inverter.
>
> I do not agree with your way to evaluate people that ask help.
> I have asked help because I'm in trouble. You have partially reason saying
> from your poin of wiev this is a stupid problem, but I have been evaluate
> the possibility to use the 16F877 at lover voltage. On the manual on page
> 151, table 15.0 DS30292B and in the following page I have understand that
> the PIC should be used at lower voltage, and in particular I use the PIC
at
> 3,9 Volt.
>
> Now the problem of the oscillator seem solved, because with a couple of
> spare gate of a 74HC14 I have put the oscillator  to work in good shape,
but
> I would like understand why with a 4 Mhz clock the PIC refuse to start,
also
> making action on PCON.0  register as suggested by Brent.
>
> If at the end of this discussion the answer is "PIC CANNOT BE USEDWITH
POWER
> SOURCE LESS THAN 5 VOLT", for me is important, because I can decide to
power
> in other way.
>
> Regards
>
> Leo
>
> --
> http://www.piclist.com hint: The PICList is archived three different
> ways.  See http://www.piclist.com/#archives for details.
>
>
>

--
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