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'[PIC]: .1uf between Vcc and gnd ???'
2002\07\05@235531 by Gabriel Caffese

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Hello everyone,

       Does it make any sense to have a 0.1uf cap between Vcc and Gnd
if my circuit is gonna be powered by a battery ?

       Thanks in advance.-

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2002\07\05@235925 by Spehro Pefhany

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At 01:04 AM 7/6/02 -0300, you wrote:
>Hello everyone,
>
>         Does it make any sense to have a 0.1uf cap between Vcc and Gnd
>if my circuit is gonna be powered by a battery ?

Yes. Also parallel it with 100uF electrolytic, if possible.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\07\06@004638 by lexandre_Guimar=E3es?=

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Hi,

>         Does it make any sense to have a 0.1uf cap between Vcc and Gnd
> if my circuit is gonna be powered by a battery ?

   Yes, you still need it because your PCB traces still have resistance and
inductance regardless of how you put power in.. Always use a decoupling
capacitor for each cmos IC you have on your circuit. Those things switch
very fast and need the power close to them, put the cap as close as possible
to the power pins of the ic.

Best regards,
Alexandre Guimaraes

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2002\07\06@004828 by David Covick

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Gabriel,

I have a product on the market with NO caps across Vcc and Gnd.
It is a PIC12C672 run with a 3.6V lithium battery.
Thousands sold and around the world and not seen any issues from leaving
caps off.
I decided to do this to save parts count/cost.  Normally I use a 0.1uF, but
in this case I left it off due to the battery being very close to the PIC
(short track).
I'm sure someone on this list will tell me that what I did was wrong, but
since it works fine I'd be happy to hear from someone at to why it is a bad
idea.

David


{Original Message removed}

2002\07\06@004836 by Jinx

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> Does it make any sense to have a 0.1uf cap between Vcc and
> Gnd if my circuit is gonna be powered by a battery ?

As the battery runs down its impedance goes up, so any
components that draw high instantaneous currents such
as IR LEDs, relays, perhaps some switches, even other
ICs that the PIC controls the quiescent current of, may
cause a negative-going spike or even a slight temporary
droop of Vcc. 0.1uF may help some problems, but other
cases may require a larger cap, if you can stand the leakage
of an electrolytic. There could be instances of some unwanted
frequency being imposed or inducted onto Vcc and a small
filter cap may help there if there's a problem with that noise
affecting the circuit

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2002\07\06@010343 by Jinx

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> I'm sure someone on this list will tell me that what I did was wrong,
> but since it works fine I'd be happy to hear from someone at to
> why it is a bad idea

If yours works then it wasn't that much of a bad idea ;-) Better
some cash in your pocket than a non-essential part on the PCB

Curious to know what other components are on your board. Adding
a cap is "rule-of-thumb", you seem to get on fine without it so why
worry

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2002\07\06@024714 by David Covick

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> Curious to know what other components are on your board.

Piezo and Inductor that are pulsed.  Current is around 25mA during pulse.
I am using the watchdog though :)
I've never "seen" it timeout.


{Original Message removed}

2002\07\06@081345 by Spehro Pefhany

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At 09:24 PM 7/5/02 -0700, you wrote:
>Gabriel,
>
>I have a product on the market with NO caps across Vcc and Gnd.
>It is a PIC12C672 run with a 3.6V lithium battery.
>Thousands sold and around the world and not seen any issues from leaving
>caps off.
>I decided to do this to save parts count/cost.  Normally I use a 0.1uF, but
>in this case I left it off due to the battery being very close to the PIC
>(short track).
>I'm sure someone on this list will tell me that what I did was wrong, but
>since it works fine I'd be happy to hear from someone at to why it is a bad
>idea.

If you don't have any output lines, or there is some resistance in series
with them, then there is less need of bypass capacitors. PICs are pretty
tolerant of bypass caps being left off, but I don't do it except on the
bench. As someone else has already mentioned, the internal resistance of
the cell will go up as the battery gets older, so any problems would tend
to manifest themselves nearer the end of battery life. Some batches of
chips will be less tolerant than others to the resulting noise on the Vdd
line. Future mask revisions may be less or more tolerant. It doesn't seem
worth gnawing away at margins for the sake of a penny part that uses no
port pins and little space.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam.....interlog.com             Info for manufacturers: http://www.trexon.com
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2002\07\06@085803 by Joe Farr

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I might be totally wrong here but if capacitors have leakage, won't that affect the battery life ?


{Original Message removed}

2002\07\06@094225 by Spehro Pefhany

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At 01:57 PM 7/6/02 +0100, you wrote:
>I might be totally wrong here but if capacitors have leakage, won't that
>affect the battery life ?

Ceramic caps have negligible leakage compared to the circuit or even
the PCB unless it is very clean (>10,000M ohms). Electrolytics have
more, but still typically << 1uA, depending on the type. For
something like alkaline AA cells you can put the cheapest aluminum
e-cap in there and not worry. For 10+ year life from a Li battery
you might want to look more carefully.

So, the effect on battery life is not much. If the circuit
stops working due to the cell internal resistance increasing, that,
effectively, results in much more reduction in battery life.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
EraseMEspeffspam_OUTspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
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2002\07\06@094630 by Dave Dilatush

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David Covick wrote...

>I'm sure someone on this list will tell me that what I did was wrong, but
>since it works fine I'd be happy to hear from someone at to why it is a bad
>idea.

Leaving out bypass capacitors is not a "bad idea" if you're
willing to risk the problems that can result from leaving them
out.  In the case you cited, you accepted that risk and got
lucky.

But you don't always get lucky.  And when inadequate bypassing
does cause problems, those problems can be an absolute screaming,
drooling nightmare to troubleshoot: wierd, baffling, erratic
symptoms that defy all logic and come and go seemingly at random.

To paraphrase the unfortunate Dan Quayle: the mind is a terrible
thing to lose- especially over something as basic as good power
supply hygiene.

Skimping on bypass capacitors may be a good way to drive yourself
crazy; but it's not a cost-saving technique that I would ever,
EVER choose- no matter how much pressure I was under to save
money.  Been there.  Done that.  It ain't worth the pain.

DD

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2002\07\06@151103 by Larry G. Nelson Sr.

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Yes. It will help with the high frequency current spikes needed and lower
the noise in the rest of your circuit.

At 01:04 AM 7/6/02 -0300, you wrote:
{Quote hidden}

Larry G. Nelson Sr.
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http://www.ultranet.com/~nr

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2002\07\07@085620 by Olin Lathrop

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>         Does it make any sense to have a 0.1uf cap between Vcc and Gnd
> if my circuit is gonna be powered by a battery ?

It can, depending on what kind of circuit the battery is powering.  The
100nF cap will reduce the apparent impedence of the battery at high
frequencies.


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(978) 742-9014, http://www.embedinc.com

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2002\07\07@090712 by Olin Lathrop

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> I might be totally wrong here but if capacitors have leakage,
> won't that affect the battery life ?

The original question asked about a 100nF, presumably ceramic, capacitor.
These have essentially zero leakage in this context.  However, electrolytics
have more leakage and it can be an issue depending on the size of the
battery, etc.


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2002\07\07@182253 by Richard Mellina

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If there is anything in the circuit that could create variations in the
supply voltage. I once made a circuit for controlling the speed and
direction of a motor but had no filtering caps and I spent hours debugging
the thing only to find my stupid mistake. It's cheap insurance.

{Original Message removed}

2002\07\08@160446 by Micro Eng

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From my experiance....the caps across the parts are not really for surge
suppression, but more to keep the voltage from sagging when the parts are
switching.  The caps store the energy and when needed....gives it.


{Quote hidden}

>{Original Message removed}

2002\07\08@172931 by Gabriel Caffese

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Thanks for the answers, but now.....

If I´m feeding and i2c EEPROM from an I/O line, (RB1)  would it be better
too,
to put a .1uf ceramic capacitor between RB1 and Gnd ?

       Thanks again.....

-----Mensaje original-----
De: pic microcontroller discussion list
[TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU]En nombre de Micro Eng
Enviado el: Lunes, 08 de Julio de 2002 17:04
Para: RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU
Asunto: Re: [PIC]: .1uf between Vcc and gnd ???


>From my experiance....the caps across the parts are not really for surge
suppression, but more to keep the voltage from sagging when the parts are
switching.  The caps store the energy and when needed....gives it.


{Quote hidden}

>{Original Message removed}

2002\07\08@174443 by Spehro Pefhany

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At 06:21 PM 7/8/02 -0300, you wrote:
>Thanks for the answers, but now.....
>
>If I´m feeding and i2c EEPROM from an I/O line, (RB1)  would it be better
>too,
>to put a .1uf ceramic capacitor between RB1 and Gnd ?

I take this to mean that you are *powering* the EEPROM from a port pin..
this is a bit of a problem situation. When the port pin switches, you
will briefly draw the short-circuit current of the port pin from your
power supply. I think I'd tend to a 0.01uF capacitor across the EEPROM
(close to the EEPROM, not the PIC) with a 0.1uF or 0.22uF ceramic cap
very close to the PIC power pins. If you have analog stuff on the same
power supply you might have to do more in the way of filtering than
usual.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
RemoveMEspeffspam_OUTspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
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2002\07\08@174953 by Jim

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David Covick wrote:

 "I left it off due to the battery being very close
  to the PIC"

I wonder what the internal capacitance of the battery is
at the highed frequencies?

This just might be working in your favor!

Jim


{Original Message removed}

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