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'[PIC] - Why are there 2 Vdd and Vss on PIC18F452'
2003\02\18@015038 by Lim KK

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Does anyone know why micochip has 2 Vdd pins and 2 Vss pins for the 18F452
microcontroller? Is it sufficient to tie just one set to the power supply or
is there a need to tie the other set as well?

Thanx!
Mark





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2003\02\18@021700 by Josh Koffman

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I'm not 100% sure why there are two of each, but they must both be tied
to the correct signal or you can expect flakey, if any operation of your
chip.

Josh
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Lim KK wrote:
>
> Does anyone know why micochip has 2 Vdd pins and 2 Vss pins for the 18F452
> microcontroller? Is it sufficient to tie just one set to the power supply or
> is there a need to tie the other set as well?

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2003\02\18@070454 by Ian McLean

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Just to confirm, yes it is very important that the Vdd's and the Vss's are
tied together.

The simple reason Microchip have done this is to increase the amount of
current that the PIC can draw for operating peripheral ports and built in
functions.

{Original Message removed}

2003\02\18@090825 by Francisco Ares

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I guess that there shoud have been some internal routing problems, so
the easier way to get round was to route it "externaly", i.e, in the
custumer board.

Francisco


Lim KK wrote:

{Quote hidden}

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2003\02\18@150432 by Herbert Graf

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> Does anyone know why micochip has 2 Vdd pins and 2 Vss pins for the 18F452
> microcontroller? Is it sufficient to tie just one set to the
> power supply or
> is there a need to tie the other set as well?

       The reason there are two remains pretty much unclear. It is known though
that on some PICs if you don't connect one set certain parts of the chips do
not get power properly. Connect both of them. TTYL

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2003\02\18@150845 by Jai Dhar

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Quoting Herbert Graf <spam_OUTmailinglistTakeThisOuTspamFARCITE.NET>:

> > Does anyone know why micochip has 2 Vdd pins and 2 Vss pins for the 18F452
> > microcontroller? Is it sufficient to tie just one set to the
> > power supply or
> > is there a need to tie the other set as well?
>
>         The reason there are two remains pretty much unclear. It is known
> though
> that on some PICs if you don't connect one set certain parts of the chips do
> not get power properly. Connect both of them. TTYL
>

I would be really interested to know why also - I use an f877 and haven't
noticed any erratic operation thus far. Then again, I can't claim to have used
all the available components.

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2003\02\18@151849 by Herbert Graf

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> >         The reason there are two remains pretty much unclear.
> It is known
> > though
> > that on some PICs if you don't connect one set certain parts of
> the chips do
> > not get power properly. Connect both of them. TTYL
> >
>
> I would be really interested to know why also - I use an f877 and haven't
> noticed any erratic operation thus far. Then again, I can't claim
> to have used
> all the available components.

       The last "story" I heard was that if you leave one pair unconnected certain
parts of the chip will not receive power "properly", the thing is some WILL
still receive power (due to the odd paths power can flow in an IC, ie. body
diodes) but their power supply will be horribly noisy resulting in erratic
operation. Personally I don't understand why there is even a question as to
why to connect them: Microchip says connect them, so I connect them. It's
not like they're a pain to route? TTYL

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2003\02\18@152931 by Wouter van Ooijen

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I recall that in at least one datasheet of a recent flash PIC one pair
was marked as digital, the other pair as analog.

Wouter van Ooijen

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2003\02\22@025447 by jim barchuk

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Hi Mark!

> Does anyone know why micochip has 2 Vdd pins and 2 Vss pins for the 18F452
> microcontroller? Is it sufficient to tie just one set to the power supply or
> is there a need to tie the other set as well?

Related to scale, as compared to an itty bitty 14 pin dip with one power
and one ground pin, I found a page with pinouts for a Pentium II. 121
pins, 19 Vcc_Core, 30 Vss, and a few other odd Vcc_ pins. That's fully
*1/4* of the pins used just to power the silly thing! :)

Might be related to an analog/digital thing, but I powered up a 16f877
with one pin powered and the other not, then the other powered and the one
not, and saw Vcc at the unpowered pins. So they're connected directly or
indirectly internally.

It's most likely to allow for power surges when the chip is just idling
along and then all of a sudden is asked to do a lot of stuff. Even a
component lead can suffer voltage drop. Not much, but the analog portions
especially don't like that at all.

For peace of mind, always always always connect and properly decouple all
power and ground pins. It'll remove one potential point of contention when
other weird circuit stuff happens and you're unsure where to start
troubleshooting. Wire and caps are cheaper than time. :)

Have a :) day!

jb

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2003\02\22@061735 by michael brown

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----- Original Message -----
From: "jim barchuk" <jbspamKILLspamJBARCHUK.COM>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Saturday, February 22, 2003 1:47 AM
Subject: Re: [PIC] - Why are there 2 Vdd and Vss on PIC18F452


> Hi Mark!
>
> > Does anyone know why micochip has 2 Vdd pins and 2 Vss pins for the
18F452
> > microcontroller? Is it sufficient to tie just one set to the power
supply or
> > is there a need to tie the other set as well?
>
> Related to scale, as compared to an itty bitty 14 pin dip with one
power
> and one ground pin, I found a page with pinouts for a Pentium II. 121
> pins, 19 Vcc_Core, 30 Vss, and a few other odd Vcc_ pins. That's fully
> *1/4* of the pins used just to power the silly thing! :)

The socket 478 P4 has 85 Vcc pins and 181 Vss pins making for over half
of the pins being used for power.  But then again it needs 50A of power.
:-O

> Might be related to an analog/digital thing, but I powered up a 16f877
> with one pin powered and the other not, then the other powered and the
one
> not, and saw Vcc at the unpowered pins. So they're connected directly
or
> indirectly internally.
>
> It's most likely to allow for power surges when the chip is just
idling
> along and then all of a sudden is asked to do a lot of stuff. Even a
> component lead can suffer voltage drop. Not much, but the analog
portions
> especially don't like that at all.
>
> For peace of mind, always always always connect and properly decouple
all
> power and ground pins. It'll remove one potential point of contention
when
{Quote hidden}

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