Role of decopling capacitance

Thomas Guest #2167640
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Hi all,
I always see decoupling capacitors connected at the voltage supply line 
of uControllers. I basically know that these capacitors eliminate the 
noise so that the voltage at the input of the uC is smooth.
My question now is, how does these capacitors work? Sometimes an 
additional resistor is connected in parallel. What is the latter for? 
What can be the reason that the voltage at the input of the uC gets 
noisy and unsmooth?

And why do we never connect the uC directly to the supply but we always 
use a voltage regulator between them?

Thanks
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Martina Guest #2167665
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>My question now is, how does these capacitors work?

They act as an extra reservoir for fast current spikes and they short 
circuit the large inductance of the supply voltage wiring. By this, 
large loops are avoided.

>Sometimes an additional resistor is connected in parallel.

Not in parallel, but in series. This forms an RC-filter, which is way 
more effective than a simple cap, correct dimensioning provided.

>What can be the reason that the voltage at the input of the uC gets
>noisy and unsmooth?

It's the micro itself that creates lots of noise on the supply line, by 
the internal switchings!

>And why do we never connect the uC directly to the supply but we always
>use a voltage regulator between them?

In battery powered applications this often done. If, on the other hand, 
a regulator is used then usually to prevent the micro from being 
destroyed by a too high supply voltage.
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