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'[PIC]: Decoupling capacitor'
2001\01\13@200947 by Jose S. Samonte Jr.

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I'm a beginner in PIC programming, and I just want to know what's the real use
of a decoupling capacitor. How does it affect the operation of the PIC? What
capacitance value should I use? What should I use, an electrolytic cap or a
ceramic (non-polarized) cap?

Thank you very much. God Bless.

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2001\01\13@220740 by David VanHorn

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At 03:32 PM 1/13/01 -0700, Jose S. Samonte Jr. wrote:
>I'm a beginner in PIC programming, and I just want to know what's the real use
>of a decoupling capacitor. How does it affect the operation of the PIC? What
>capacitance value should I use? What should I use, an electrolytic cap or a
>ceramic (non-polarized) cap?


The decoupling caps provide short bursts of current when the micro needs it.
Wiring has inductance, which means that the longer the wire, the more
difficult it is to quickly change the amount of current flowing. The result
is that the voltage, at the processor, swings over quite a range, because
of this series impedance.

Generally, electrolytics are a bad choice for micro bypasses. Open one up,
and you'll see long rolled up foils. The length makes inductance.

Ceramic disc is a pretty good choice. Mount the cap at the VSS lead, and
take VCC to the cap, and then to the processor VCC pin.  Don't connect
anything else to the processor VCC pin.

Now, the high speed current only needs to travel between the cap and the
processor, over a short wire.  So, the rest of the power supply wiring is
"decoupled" from the processor.

Don't forget that ground side though. It's just as important as VCC.
Current flows in a loop, and it MUST get back to the source.  (It's easier
to talk about this with pictures...)


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2001\01\14@111452 by Olin Lathrop

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>>
I'm a beginner in PIC programming, and I just want to know what's the real
use
of a decoupling capacitor. How does it affect the operation of the PIC? What
capacitance value should I use? What should I use, an electrolytic cap or a
ceramic (non-polarized) cap?
<<

Sounds like you should go find a good basic electronics book.  You are never
far from the hardware in an embedded system.

Digital ICs can often change the amount of current they draw by a large
factor very rapidly.  The regulator in the power supply can take care of
longer term variations in the current demand, but there is a frequency
beyond which it can not respond to fluctuations anymore.  Also, wires or PC
board traces are not perfect conductors.  They have some finite resistance
and inductance.  All these effects together make it so that a digital chip
can cause significant glitches on the power supply voltage near the chip.  A
capacitor accross the power supply leads right at the chip will attenuate
these transients, thereby "decoupling" the chip from other chips being fed
from the same power supply.

Decoupling capacitors are commonly 10 to 100 nF ceramic.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, spam_OUTolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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