Dear

I've 2 stupid questions related to crytstal

From my understanding, crystal is used to generate CLK for MCU. But what
is the relationship between crystal and PLL? And what can be measured in
the crystal pin when the MCU is in reset or run state?

Also I would like to know how to read crystal value marked on the crystal

Thanks

JC

On Feb 15, 3:31 pm, "jackycheung" <jackymaill...@gmail.com> wrote:

> I've 2 stupid questions related to crytstal

A mastery of understatement. You need to learn to read datasheets.

> From my understanding, crystal is used to generate CLK for MCU. But what

Maybe.

> is the relationship between crystal and PLL? And what can be measured in

Maybe nothing. Does your micro have a PLL? Does it have a DCO? Are you
using either, or are you using the crystal as a raw clock source?

> the crystal pin when the MCU is in reset or run state?

There is no way of answering that generically.

> Also I would like to know how to read crystal value marked on the crystal

Crystals are generally marked with characters from the Roman alphabet
<http://en.wikipedia.org/wiki/Roman_alphabet> and digits from the
Arabic numeral set <http://en.wikipedia.org/wiki/Arabic_numerals>. In
rarer cases you might find letters from the Cyrillic alphabet <http://
en.wikipedia.org/wiki/Cyrillic_alphabet>.

Those are the generalities; of course for specifics you would refer to
the crystal manufacturer's datasheet, would you not?

On Sun, 15 Feb 2009 14:31:05 -0600, jackycheung wrote:

> Dear
> 
> I've 2 stupid questions related to crytstal
> 
> From my understanding, crystal is used to generate CLK for MCU. 

The MCU uses the crystal as the frequency determining element for its 
clock generation circuitry.  The crystal itself is a passive device, and 
does not generate anything directly.

> But what is the relationship between crystal and PLL?

Whatever the MCU manufacturer makes it.  To find out, read the MCU data 
sheet -- it varies wildly by manufacturer and part, so there is little 
generic information to be had.  At best, I can promise you that usually 
if the MCU has a PLL it's output frequency can usually be related to the 
crystal oscillator frequency by some integer or rational ratio.  That's 
not very helpful, is it?

> And what can be measured in
> the crystal pin when the MCU is in reset or run state?

That depends on the MCU.  Generally, not much, because unless you take 
pains to do it right, attempting to measure oscillator behavior at a 
crystal pin will stop the oscillator.  Getting accurate measurements of 
the oscillator behavior without changing the oscillator behavior 
significantly usually involves board mods.

> 
> Also I would like to know how to read crystal value marked on the
> crystal

That varies with the manufacturer, too.  It may be a part number, it may 
be the design frequency of the crystal -- but it usually doesn't include 
the full information you need to use the crystal in a circuit.

-- 
http://www.wescottdesign.com

jackycheung wrote:
> 
> From my understanding, crystal is used to generate CLK for MCU.
> But what is the relationship between crystal and PLL? And what
> can be measured in the crystal pin when the MCU is in reset or
> run state?
> 
> Also I would like to know how to read crystal value marked on the
> crystal

A PLL requires a standard frequency with which to compare the
generated frequency.  That's the function of the crystal.  For
reading, see the manufacturers data sheet.

-- 
 [mail]: Chuck F (cbfalconer at maineline dot net) 
 [page]: <http://cbfalconer.home.att.net>
            Try the download section.

On Feb 15, 3:31 pm, "jackycheung" <jackymaill...@gmail.com> wrote:
> Dear
>
> I've 2 stupid questions related to crytstal
>
> From my understanding, crystal is used to generate CLK for MCU. But what
> is the relationship between crystal and PLL? And what can be measured in
> the crystal pin when the MCU is in reset or run state?
>
> Also I would like to know how to read crystal value marked on the crystal

The use of crystals is poorly understood by most digital engineers.
It turns out that there are a number of parameters that need to be
matched to the MCU, but most MCUs don't spec those parameters.
Sometimes you can get the values for these parameters from the MCU
makers, if you ask.  Less often they are in the data sheets.

If you want to learn about crystals, go to the web sites of multiple
crystal makers and read all the data sheets and app notes you can
find.  Some are good enough that you can actually learn a few things
from them.  That is how I learned, that and also asking dumb questions
of the crystal makers.  I learned nothing from the MCU makers, but I
have taught a few a bit.

Another approach is to use an oscillator instead of a crystal.  Then
you only have to worry about the power supply voltage, frequency and a
few parameters on stability if it is important to your app.  You also
need to read the MCU data sheet to see how to connect an oscillator
and what range of frequency is acceptable.

Rick

On Sun, 15 Feb 2009 21:54:24 -0800 (PST), rickman <g...@gmail.com>
wrote:

>On Feb 15, 3:31 pm, "jackycheung" <jackymaill...@gmail.com> wrote:

>> From my understanding, crystal is used to generate CLK for MCU. But what
>> is the relationship between crystal and PLL? 

>> Also I would like to know how to read crystal value marked on the crystal
>
>The use of crystals is poorly understood by most digital engineers.
>It turns out that there are a number of parameters that need to be
>matched to the MCU, but most MCUs don't spec those parameters.
>Sometimes you can get the values for these parameters from the MCU
>makers, if you ask.  Less often they are in the data sheets.

Fundamental mode crystals are made for frequencies up to about 20 MHz.
In a typical MCU, the crystal connection is just an internal inverter
and the crystal is connected across the inverter input and output with
some small (about 30 pF) load capacitors from both ends to ground.

In a proper external oscillator circuit, the crystal can be forced to
oscillate at an odd overtone (usually 3rd, 5th or 7th) thus generating
frequencies up to 60, 100 or 140 MHz, by including a frequency
selective element (usually an LC circuit) tuned close to the desired
harmonic. The generated frequency can then be connected to the MCU
oscillator inverter input.

If the MCU requires over 140 MHz clock frequencies, these can not be
generated directly by the crystal, but some form of frequency
multiplication needs to be done. This can be done with a traditional
frequency multiplier chain consisting of non-linear elements and LC
filters, but is much easier to integrate a PLL on to the MCU chip
itself. Assuming an external fundamental mode 20 MHz crystal and an
internal PLL with a divide by 16 divider, a 320  MHz system clock
frequency can be generated.

Paul