Say what? Decoupling - To take away, remove from, or provide a path to
separate. IMHO and experience decoupling capacitors remove noise which is
external of a device on it's input. Decoupling capacitors provide a low
impedance path for resonate frequency (Xc) noise, which is why we use
several sizes of caps. Pull-up & pull-down resistors and series inductors
provide extra/boost current for switching that takes place in a device.

Ray

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
> Behalf Of Vasile Surducan
> Sent: Sunday, May 25, 2014 1:41 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Decoupling capacitors
>=20
> "The primary purpose of the decoupling capacitor is to supply the
> transient current needed when chip outputs switch from low to high."
>=20
> The primary purpose of any decoupling capacitor is to maintain a low
> impedance in the supply point of the potential aggressor (switching for
> instance). Basically you should know (by measurement) the noise
> spectrum on your board and compute the value of the decoupling
> capacitor for those particular frequencies. This operation is
> difficult, so most people are throwing on the board 100nF placed near
> the supply pins of the FPGA, microcontrollers or whatever else. There
> are situations which this method does not solve the problem. So the
> questions "near the Vcc" or "near the GND" has both (in my opinion)
> wrong answers. You need low impedance which should be created by
> generous traces thickness or better GND and VCC planes.
>=20
>=20
> On Sun, May 25, 2014 at 7:07 PM, Dave Tweed <pic@dtweed.com> wrote:
>=20
> > David C Brown wrote:
> > > On a board without power planes and using chips with GND and Vcc on
> > > opposite corners is it best to place the decoupling capacitor
> > > nearest the GND pin or nearest the Vcc pn?
> >
> > Nearest the Vcc pin.
> >
> > The primary purpose of the decoupling capacitor is to supply the
> > transient current needed when chip outputs switch from low to high.
> > The path for this current is from the Vcc pin of the driving chip,
> > through the high-side transistor, through the PCB trace to the
> load(s)
> > and then to ground. So in order to be most effective, the inductance
> > (and trace length) between the capacitor and the Vcc pin needs to be
> as small as possible.
> >
> > If you don't have a ground plane, you might think you have a similar
> > situation ond the Gnd pin with respect to high-to-low transitions;
> the
> > pull-down current needs a low-impedance path, too. But the situation
> > is not symmetrical; the return path for this current is through
> > ground, too, so you're still at the mercy of the overall impedance of
> > the ground network between the chips in question.
> >
> > In other words, make your ground network as beefy as possible, then
> be
> > liberal with the decoupling caps on the Vcc pins.
> >
> > -- Dave Tweed
> > --
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