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'Stupid question'
1998\10\12@110751 by Dushara Jayasinghe

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Hi all,

I'm a beginner in electronics and this may very well be a stupid question. I'm u
sing the 16F84 chip and In most circuits that I've seen, there are two capacitor
s connected to OSC1 and OSC2 (33pF). But I have tried connecting the xtal withou
t these capacitors and the controller still seems to work. My question is, what
is the reason for these two capacitors?

Thank you
Dushara

1998\10\12@115224 by Dave VanHorn

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I'm a beginner in electronics and this may very well be a stupid
question. I'm using the 16F84 chip and In most circuits that I've
seen, there are two capacitors connected to OSC1 and OSC2 (33pF). But
I have tried connecting the xtal without these capacitors and the
controller still seems to work. My question is, what is the reason for
these two capacitors?

They provide some phase shift in the circuit, to assure that the
crystal always starts, and they provide the loading capacitance to the
crystal so that it is operating on the specified frequency.

1998\10\12@120430 by Jan Derogee

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Hi Dushara,
the ceramic capacitors reduce any HF-interferrence and give a more "stable"
resonance-frequency. The bigger the capacitors the more stable is your freq.
But beware you cannot increase your capacitor to any wanted value, when you
exceed a certian limmit your oscillator will not oscillate. You can leave-out
the capacitors but don't be supprised when you have some unwanted side-effects.
The best is to use a 22pF ceramic capacitor at least thats mine experience,
based on other examples of oscillators. By the way why should you not use the
capacitors the aren't expensive or difficult to apply.

Happy PIC'n, Jan Derogee



> Hi all,
>
> I'm a beginner in electronics and this may very well be a stupid question. I'm
using the 16F84 chip and In most circuits that I've seen, there are two capacit
ors connected to OSC1 and OSC2 (33pF). But I have tried connecting the xtal with
out these capacitors and the controller still seems to work. My question is, wha
t is the reason for these two capacitors?
>
> Thank you
> Dushara
>

1998\10\12@133615 by Dave VanHorn

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>the ceramic capacitors reduce any HF-interferrence and give a more
"stable"
>resonance-frequency. The bigger the capacitors the more stable is
your freq.
>But beware you cannot increase your capacitor to any wanted value,
when you
>exceed a certian limmit your oscillator will not oscillate. You can
leave-out
>the capacitors but don't be supprised when you have some unwanted
side-effects.
>The best is to use a 22pF ceramic capacitor at least thats mine
experience,
>based on other examples of oscillators. By the way why should you not
use the
>capacitors the aren't expensive or difficult to apply.


This is not a valid explanation of what the xtal caps are for.

They have nothing to do with "HF-interference", except that of you
route your PCB badly, they can cause it. These caps should return to
the uP's ground pin with their own separate track.  Ideally, the chip
mfgr will put the osc pins very near the ground pin.

The statement that "The bigger the capacitors the more stable is your
freq." is completely wrong.
Xtals are cut for a specific loading capacitance. They will perform
within their spec, if the actual value of capacitance in the circuit
is correct. For most microcontroller projects, we aren't worried about
absolute stability, so a general purpose parallel xtal, usually rated
for 18-22pF load, with 15-18pF caps, will suffice.
In timekeeping applications, you really need to load the xtal
properly. This is usually done with a trimmer, since the actual value
of stray capacitance is hard to predict, and will vary from unit to
unit somewhat.
Adding additional capacitance only makes problems, it's as bad as not
having enough.

<ALL> crystals have a parallel and a series resonant frequency. These
two frequencies are never the same (can't be).  A series xtal in a
parallel circuit can't be put on it's specified frequency, and a
parallel xtal in a series circuit likewise can't be made to sing the
other tune.

Crystals are also sensitive to drive level. They are usually specified
for some specific power in microwatts. The small "watch crystals"
(fork type) are very sensitive to drive, and can actually be broken by
too much drive level.  Too much or too little drive will also affect
the operating frequency, but in this case less is better than more.

The biggest effect you're likely to notice from leaving the caps out
is that the osc will not come to the proper frequency, and it may not
start at all, or it may not start reliably.

1998\10\12@161113 by daniel dourneau

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At 00:56 13/10/98 +1000, you wrote:

>I'm a beginner in electronics and this may very well be a stupid question.
I'm using the 16F84 chip and In most circuits that I've seen, there are two
capacitors connected to OSC1 and OSC2 (33pF). But I have tried connecting
the xtal without these capacitors and the controller still seems to work.
My question is, what is the reason for these two capacitors?

Oscillators are a little like magic or black art. You will find out that,
in most cases, the oscillator will work without these capacitors. But if
you are starting mass producing the things by the million, you will find
that sometimes and for no reason some of them will not work.
The reason is in the design of the oscillator circuit and some parameters
linked to the crystal (or ceramic resonator). The little caps are there to
introduce some phase shift and help the thing to start. Them will also
filter harmonics and help make sure that it will not start oscillating in
some crazy modes like third harmonic.
The diagram is fairly simple but do not try to complete theoritical
calculations, they will lead you nowhere. Just pick out some ideas on
schematics and copy them.
There was a pretty comprehensive application note in one of the early
embedded handbooks from Intel (back in the days when I8051 meant $!). I do
not know if I still have it in one of my cupboards. If I find it I shall
let you know the reference.
Daniel Dourneau

1998\10\12@185012 by tsk3000

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daniel dourneau wrote:
>
> The little caps are there to
> introduce some phase shift and help the thing to start. Them will also
> filter harmonics and help make sure that it will not start oscillating in
> some crazy modes like third harmonic.
Okay, I know almost _nothing_ about xtals.  All I know is that
they're handy for producing a high frequency oscillation for parts
of circuits, like CPUs and MCUs, but aside from that, I know
nothing.  Could someone explain to me what phase shift means and
what these harmonics are?  I'm a tad confused about these things,
but still interested...

Just a little question.  :)

Cheers,
--
~Keith
spam_OUTtsk3000TakeThisOuTspamProdigy.Net
ICQ UIN 15590177
http://pages.prodigy.net/tsk3000/

1998\10\12@203954 by Dave VanHorn

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>> The little caps are there
>Okay, I know almost _nothing_ about xtals.  All I know is that
>they're handy for producing a high frequency oscillation for parts
>of circuits, like CPUs and MCUs, but aside from that, I know
>nothing.  Could someone explain to me what phase shift means and
>what these harmonics are?  I'm a tad confused about these things,
>but still interested...


Xtals, two main flavors, series resonant, and parallel resonant.
Almost all uP designs use parallel resonant.  ALL xtals have a series
and a parallel resonant point, and they are not the same (cannot be!),
but their behaviour is only guaranteed for one point.

When they are made, they are designed to be on-frequency, with a
specific loading capacitance, values of 18-22pF are common.  The
circuit itself, and the uP chip provide some stray capacitance, so a
good guess might be 18pF for a 22pF rock.  If you need finer accuracy,
say for timekeeping, then you'll need a trimmer cap.   Sticking any
old cap on, or "bigger is better" will cause the xtal to try to run at
some other frequency, and may result in it not running at all.

The crystal itself provides phase shift for an amplifier in the uP.
Depending on the exact crystal, it may not provide enough phase shift
by itself, so the caps help out here as well. (that part I'm a little
fuzzy on myself)
The general theory is that if you have gain (from the amp) feedback
(through the xtal) and phase shift (from the xtal and the caps) then
you'll get oscillation.

Drive level is also an issue, especially with the "watch xtals". Their
tuning fork design is more delicate, and you can shift the operating
frequency, or cause them to physically break by overdriving them.


'Stupid Question'
2002\09\24@165933 by Matt Johnson
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I am new to this. I was wondering for this beginner circuit:

http://www.piclist.com/techref/piclist/cheapic/bincnt.htm

does the C = 33pf designate a Crystal? if not, if it is a capacitor then I
am confused by the drawing because it says 32.768 kHz but does not show an
icon for the crystal.

Which leads me to a generic question. When designing a circuit how do you
choose which crystal to use? do all projects use the same generally? what
brand and part number? Also, what other components need to be ordered
(like 5v regulator, etc). I am going to make an order to digikey and I
dont want to get the stuff and realize I have to place a whole other order
becasue I forgot some stupid support component...

- Matt

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2002\09\24@171604 by Joan Ilari

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> http://www.piclist.com/techref/piclist/cheapic/bincnt.htm
>
> does the C = 33pf designate a Crystal?

No : it designates the 2 capacitors at each side of the quartz crystal

> if not, if it is a capacitor then I
> am confused by the drawing because it says 32.768 kHz but does not show an
> icon for the crystal.

yes, there is an icon for the crystal (have a close look at the drawing)
32.768 kHz makes reference to the crystal which is in the middle of the 2
capacitors

> Which leads me to a generic question. When designing a circuit how do you
> choose which crystal to use?

Microchip gives you all the info you need in their technical notes
(crystal value, capacitor values needed to oscillate ...)

Best regards

Joan Ilari

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2002\09\24@172350 by Pic Mailingliste

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Hello Matt,

it's a bad drawing imho :-)

    Crystal
OSC1 --|||-- OSC2
    |      |
    _      -
  C _      - C
    |      |
VDD  -----------

So there are two capacitors called C both with 33pF and there's one
crystal, with a frequency of 32.768 kHz which requires those two
capacitors to start oscillating (take a look at the PIC 16F84
datasheet PDF page 39).

BUT what I'm wondering about is why the schematic on the URL shows the
capacitors connected to VDD (+4.5V) instead to VSS (GND) as shown in
the Microchip datasheet ..... I think I would connect 'em to VSS (GND)
as shwon in the datasheet !!!

Now to the general questions:

Instead of the crystal with 2 capacitors you could use an RC-network
(take a look at the datasheet page 39/40)

Which crystal to use ... well this depends on what you would like to
do with the circuit ... the 32.768 kHz crystal is a good choice for
building a clock or timing circuit, because counting with a
timer-interrupt up to 32768 means counting 1 second. As you can see
the 32.678 kHz gives you a very easy 1 second interval.

But for other projects it could be interesting to use a much faster
crystal if you've got a critical timing ...

As stated before the speed of the crystal depends on the project you
want to build !

If you want to build a standalone circuit you would need some
5V-regulator (7805) some capacitors (100nF) an transformer 230 or 110V
to 7-9V some rectifiers ... This should be ok to build a first
PIC-circuit as far as I can remember :-)

Greets,
      Michael

MJ> I am new to this. I was wondering for this beginner circuit:

MJ> http://www.piclist.com/techref/piclist/cheapic/bincnt.htm

MJ> does the C = 33pf designate a Crystal? if not, if it is a capacitor then I
MJ> am confused by the drawing because it says 32.768 kHz but does not show an
MJ> icon for the crystal.

MJ> Which leads me to a generic question. When designing a circuit how do you
MJ> choose which crystal to use? do all projects use the same generally? what
MJ> brand and part number? Also, what other components need to be ordered
MJ> (like 5v regulator, etc). I am going to make an order to digikey and I
MJ> dont want to get the stuff and realize I have to place a whole other order
MJ> becasue I forgot some stupid support component...

MJ> - Matt

MJ> --
MJ> http://www.piclist.com hint: PICList Posts must start with ONE topic:
MJ> [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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2002\09\24@180700 by Herbert Graf

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> I am new to this. I was wondering for this beginner circuit:
>
> http://www.piclist.com/techref/piclist/cheapic/bincnt.htm
>
> does the C = 33pf designate a Crystal? if not, if it is a capacitor then I
> am confused by the drawing because it says 32.768 kHz but does not show an
> icon for the crystal.

       No, the C's are capacitors, one goes between Vdd and OSC1, one between VDD
and OSC2. The "other" symbol is the ~32k crystal.

> Which leads me to a generic question. When designing a circuit how do you
> choose which crystal to use? do all projects use the same generally? what
> brand and part number?

       With a freq as low as 32k about any crystal out there should work.

> Also, what other components need to be ordered
> (like 5v regulator, etc). I am going to make an order to digikey and I
> dont want to get the stuff and realize I have to place a whole other order
> becasue I forgot some stupid support component...

       From what I see you should need nothing other then what is shown (except
perhaps an IC socket for the PIC and off course the battery thing). TTYL

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