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'[PIC] A few questions about crystals'
2012\01\17@131103 by Nathan House

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I'm making a board for a PIC18 microcontroller and have a few
questions about crystal selection. Firstly, on Digi-Key, under the
heading "Load Capacitance," there is the option "Series" and then a
bunch of different capacitor options below that. Will a series crystal
work, or should I get one that uses capacitors? One other question I
have is regarding the crystal frequency. Most PIC18 microcontrollers
(all of the ones I've used, at least) have a PLL built in, so a
variety of different crystals can be used with the same resulting
system clock speed. If the PIC I'm using has a PLL that would allow me
to use, say, a 4MHz, 8MHz, 12MHz, 20MHz, or 24MHz crystal, is there
any reason to use a higher speed crystal as opposed to a lower speed
one?

Appreciate your advice!

Nathan

-- Student Hobbyist
http://www.roboticsguy.co

2012\01\17@132848 by Isaac Marino Bavaresco

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Em 17/1/2012 16:11, Nathan House escreveu:
> I'm making a board for a PIC18 microcontroller and have a few
> questions about crystal selection. Firstly, on Digi-Key, under the
> heading "Load Capacitance," there is the option "Series" and then a
> bunch of different capacitor options below that. Will a series crystal
> work, or should I get one that uses capacitors?


Series resonant crystals need a different type of circuit. MCUs always
use parallel resonant crystals. Use a crystal with a load capacitance
bigger than your MCU's pin capacitance and add external capacitors that
make the total capacitance match the crystal's capacitance.


{Quote hidden}

Lower external frequencies mean less EMI radiation, but the PLL will add
jitter to your clock.


Best regards,

Isaac

2012\01\17@142605 by jim

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Nathan,

Regarding your question about capacitance.  There are series cut
crystals and parallel cut crystals.  You can use either one in a given
circuit and it will work.  However, it won't be ideal,  and if you use a
parallel crystal in a series circuit, the frequency of the crystal won't
be what it is stated to be.  and vice versa for a series cut crystalin a
parallel circuit.  What this boils down to is use the correct crystal
for the circuit you have.

Regarding your question about using the PLL is a PLL generates some
amount of noise.  The bulk of this noise results from jitter of the
incoming frequency being processed (up or down converted by the PLL). If
the circuit you are working with can accept this noise, then by all
means use it.  The original intent of using a PLL in a microcontroller
has to do with RF radiation.  With a crystal of let's say 20Mhz
connected directly to the microcontroller, if there is any nonlinearity
in the oscillator circuit, there will be some spurious frequencies
generated, and radiated.  These spurs will generally be harmonics of the
fundamental.  ie 40Mhz, 60Mhz...etc.  By using a PLL, internal to the
microcontroller, you can use a lower crystal frequency connected to the
microcontroller, but it can run faster internally, and the generated
and/or radiated RFI will be less.  So, you can get the best of both
worlds, assuming you application and circuit can work with the extra
noise.  Chances are it can.  Only in cases where low noise is a critical
factor would you probably not get by using the internal PLL, and in
those cases you'd almost have to use a crystal cut for the frequency you
want to operate at.

Hope this helps you out.

Regards,

Jim
Regards,

Jim

> ---{Original Message removed}

2012\01\17@170102 by Sean Breheny

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Just to add to what others have said: there is actually no such thing,
from what I've read, as a series crystal vs a parallel one. All
crystals have both resonant modes. However, they are at two slightly
different frequencies and the manufacturer has to choose one or the
other to tune to the desired frequency. For example, a particular
crystal might start out with a series resonant frequency of 9.900MHz
and a parallel resonant frequency of 9.905MHz with no external
capacitance. Let's say the manufacturer is trying to make a 10MHz
crystal - they can shave off tiny bits of material or make some other
kind of mechanical adjustment to shift the frequencies up or down but
the two modes (series and parallel) will always be different so the
manufacturer has to choose whether to tune the series mode to 10MHz or
the parallel mode to 10MHz, and they also have to decide on a specific
external capacitance to add (if parallel mode is intended) because
that is how circuits which are intended to use the parallel mode
typically work - that is, they add a few 10s of picofarads of external
capacitance.

Sean


On Tue, Jan 17, 2012 at 1:11 PM, Nathan House <spam_OUTnathanpiclistTakeThisOuTspamgmail.com> wrote:
{Quote hidden}

>

2012\01\17@173048 by Dave
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You want parallel crystals, and you take the spec they give for loading capacitance, double that, subtract about 5pf, and use that value or as close as reasonable for the loading caps.

Do not use 22pf caps on a 22pf crystal.  The bad news is that it will probably run that way.  Why that is bad news is left as an exercise for the student.


Nathan House <.....nathanpiclistKILLspamspam@spam@gmail.com> wrote:

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>

2012\01\18@061211 by alan.b.pearce

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> Regarding your question about capacitance.  There are series cut crystals and
> parallel cut crystals.  You can use either one in a given circuit and it will work.
> However, it won't be ideal,  and if you use a parallel crystal in a series circuit,
> the frequency of the crystal won't be what it is stated to be.  and vice versa for a
> series cut crystalin a parallel circuit.  What this boils down to is use the correct
> crystal for the circuit you have.

My understanding is that the crystal manufacturers not only tweak the resonant frequency so that the stated frequency is on the required parallel or series resonant point, but that they can make other tweaks that make the crystal more reliable in the designed mode of operation, and if used in the other mode may not oscillate reliably, or may jump to other frequency spurs that may not affect the designed mode.
-- Scanned by iCritical.

2012\01\18@132628 by Electron

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At 20.25 2012.01.17, you wrote:
{Quote hidden}

What do You mean with noise exactly? What is the concern, additional EMI
susceptibility?

Thanks.


{Quote hidden}

>> ---{Original Message removed}

2012\01\18@135050 by Bob Ammerman

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regarding noise introduced by PLL:


What really happens here is you get some jitter in the clock. That means that although the frequency of the clock over a short period of time (a few cycles or so) is accurate the individual cycles of the clock may vary slightly in length.

In other words, given a program that toggles an output quickly you might notice a bit of 'fuzz' on the rising and falling edges of the signal on an oscilloscope. This is caused by the clock jitter which is, more or less, unavoidable with a PLL.

In a funny way clock jitter can help you pass EMI testing. This is because the spectrum of the clock frequency and its multiples is 'spread' due to the instantaneous variations in the clock.

-- Bob Ammerman
RAm Systems

2012\01\18@140337 by David VanHorn

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Fundamental mode crystals aren't available easily in high frequencies,
so PLLs are used to take a more easily available crystal and derive a
higher (sometimes MUCH higher) frequency.

In my experience, the PLL can be EMI sensitive, in that it's loop
filter can be disturbed by external noise if you don't pay careful
attention to layout, avoiding any AC currents flowing through its
ground, or nearby.  With on-chip PLL filters, you get what you get,
and it's usually fine.

I have not seen any detectable noise created, in terms of radiated
noise from the PLL, in routine EMI scans.
Using a sensitive narrowband receiver like the IC-R8500 I can detect
the PLL emissions from directly on top of the chip, but it's nothing
to worry about.

As bob said, the clock will have some jitter, but that's not usually a
concern for clocking a uC

2012\01\24@151845 by Nathan House

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When using a microcontroller that operates at a high clock speed, such
as an 80MHz PIC32, is there an alternative to using a PLL? I don't see
any 80MHz crystals on Digikey..

Thanks,

Nathan

-- Student Hobbyist
http://www.roboticsguy.co

2012\01\24@153755 by Luis Moreira

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Hi Nathan,
The use of the PLL, is to improve problems wit noise and cross talking,
having such a large speed clock running around your board without causing
problems is not easy. The PLL allows the device to run at high speed, but
that high speed clock is only used internally by the the device in a way
that it as been optimized by the manufacturer.
So in short, you want to be using that PLL. :-)
Best Regards
                      Luis
On Jan 24, 2012 8:23 PM, "Nathan House" <nathanpiclistspamKILLspamgmail.com> wrote:

{Quote hidden}

>

2012\01\24@162533 by Herbert Graf

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On Tue, 2012-01-24 at 15:18 -0500, Nathan House wrote:
> When using a microcontroller that operates at a high clock speed, such
> as an 80MHz PIC32, is there an alternative to using a PLL? I don't see
> any 80MHz crystals on Digikey..

Actually there are higher value crystals out there. The question is
would the PICs crystal driver work at those speeds?

That said, the are alot of benefits to running with a lower crystal and
then using a PLL to get up to the speed you really want. You localize
the high speed clock, and PLLs can do lots of fancy stuff to ensure your
clock is clean.

If you REALLY don't want to use a PLL, use an oscillator. Tons of
options out there. I regularly use 100 and 125MHz oscillators.

TTYL

2012\01\29@134005 by Nathan House

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> If you REALLY don't want to use a PLL, use an oscillator. Tons of
> options out there. I regularly use 100 and 125MHz oscillators.

I don't have anything against using the PLL :-) Are there advantages
to using oscillator, aside from a higher speed?

I found this excellent page on crystals,
http://www.sxlist.com/techref/clocks.htm, and have a few more
questions! The page I just linked to suggested splitting the
calculated capacitor into two separate capacitors (on each side of the
crystal) and connecting one to GND and one to VCC. Is that good
advice?

Also, say I want to use the PLL to bump the system clock up to 48 MHz.
If I used an 8 MHz crystal instead of a 4 MHz crystal (assuming both
are valid options to use with the PLL), would that cut down the
jitter/noise by a factor of two?

Thanks,

Nate

-- Student Hobbyist
http://www.roboticsguy.co

2012\01\29@151246 by Matt Bennett

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On Sun, January 29, 2012 12:40 pm, Nathan House wrote:
>> If you REALLY don't want to use a PLL, use an oscillator. Tons of
>> options out there. I regularly use 100 and 125MHz oscillators.
>
> I don't have anything against using the PLL :-) Are there advantages
> to using oscillator, aside from a higher speed?

The 2-16 MHz range has historically been a sweet spot with respect to low
cost and accuracy.  The higher you go, the physically smaller and thinner
the crystal had to be, so it was more sensitive to microphonics and damage
from physical motion.  Go lower, and the crystal gets proportionately
bigger (I believe that your typical 32KHz watch crystal is a somewhat
different technology than what you see in the MHz range, so it isn't quite
apples-apples).

> I found this excellent page on crystals,
> http://www.sxlist.com/techref/clocks.htm, and have a few more
> questions! The page I just linked to suggested splitting the
> calculated capacitor into two separate capacitors (on each side of the
> crystal) and connecting one to GND and one to VCC. Is that good
> advice?

Honestly, I've seen many, many crystal circuits, and I've never seen the
GND/VDD split used. At the frequencies crystals run, GND and VDD are
equivalent (if they're not, you've got bigger problems than crystal
stability to face). It makes routing much harder, as well.  If you think
about what the crystals are doing, there should be a 180 degree phase
shift between ends of the crystal- whatever current is going in to one
crystal should be coming out of the other- I'd think that keeping both
capacitors tied to the same plane would be better as it keeps the overall
loop of current much smaller.

> Also, say I want to use the PLL to bump the system clock up to 48 MHz.
> If I used an 8 MHz crystal instead of a 4 MHz crystal (assuming both
> are valid options to use with the PLL), would that cut down the
> jitter/noise by a factor of two?

Ideally, yes, but crystals aren't particularly known for their cycle-cycle
stability- their stability is more apparent in a longer term sense.
Cycle-cycle, RC oscillators can be quite stable. If you want to learn lots
about oscillators, look up papers by John R. Vig (here's one for example:
<http://www.umbc.edu/photonics/Menyuk/Phase-Noise/Vig-tutorial_8.5.2.2.pdf>).
In a real circuit, without a $100,000 analyzer, you'll probably never
notice a difference.



Matt Bennett
Just outside of Austin, TX
30.51,-97.91

The views I express are my own, not that of my employer, a large
multinational corporation that you are familiar with

2012\01\29@164103 by William \Chops\ Westfield

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On Jan 29, 2012, at 12:12 PM, Matt Bennett wrote:

> At the frequencies crystals run, GND and VDD are equivalent

So has anyone run their crystals' caps to Vcc instead of Gnd?
IIRC, there was once where this would have made the pcb layout much easier, and I was very tempted, based on the above "theory" from school.  But I think I chickened out.

BillW

2012\01\30@140204 by Herbert Graf

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On Sun, 2012-01-29 at 13:40 -0500, Nathan House wrote:
> > If you REALLY don't want to use a PLL, use an oscillator. Tons of
> > options out there. I regularly use 100 and 125MHz oscillators.
>
> I don't have anything against using the PLL :-) Are there advantages
> to using oscillator, aside from a higher speed?

Not really, they tend to use MORE power, so if you want to use more
juice then you have to... :)

TTYL


'[PIC] A few questions about crystals'
2012\02\02@173309 by Electron
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By the way, someone suggested to use much bigger caps than you read
in the datasheet: why's that? What are the pro's and the con's?

2012\02\02@194750 by IVP

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> By the way, someone suggested to use much bigger caps than you
> read in the datasheet: why's that? What are the pro's and the con's?

I don't know about "much" bigger, but AIUI one pro is stability and
one con is slower start-up. My tendency is to find out what the mftr
recommends and go with that, trusting that they've done the sum

2012\02\02@210919 by Dave

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Not exactly.  I suggested interpreting the data sheet properly.
Electron <.....electron2k4KILLspamspam.....infinito.it> wrote:

>
>By the way, someone suggested to use much bigger caps than you read
>in the datasheet: why's that? What are the pro's and the con's?
>
>

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