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...)


--
Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics