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<div class="col-md-8 post-24343 post type-post status-publish format-standard has-post-thumbnail hentry category-componentsparts category-ee" id="post-24343">
<div class="post-caption">
<h3>January 24, 2012 AT 6:04 am</h3>
<h2 class="storytitle">Choosing the Right Crystal and Caps for your Design</h2>
</div>
<p><img class="img-responsive" src="Choosing%20the%20Right%20Crystal%20and%20Caps%20for%20your%20Design%20%C2%AB%20Adafruit%20Industries%20%E2%80%93%20Makers,%20hackers,%20artists,%20designers%20and%20engineers%21_files/quartcrystalmode.png" alt="" srcset="https://blog.adafruit.com/wp-content/uploads/2012/01/quartcrystalmode.png 427w, https://blog.adafruit.com/wp-content/uploads/2012/01/quartcrystalmode-150x38.png 150w, https://blog.adafruit.com/wp-content/uploads/2012/01/quartcrystalmode-300x77.png 300w" sizes="(max-width: 427px) 100vw, 427px" width="427" height="110"></p>
<p>Learning to design your own PCBs and being able to put together a
schematic to solve a specific problem is both a valuable and rewarding
skill. &nbsp;There are a number of resources out there to help you avoid
common mistakes, but it isnt always obvious to know where the values
of certain common components come from, particularly common parts like
resistors and capacitors. &nbsp;Figuring this out is part of the
learning process, but it isnt always easy to know where to look since
you first need to know exactly the right terms to search for.</p>
<p>One good example that I havent seen a lot of attention paid to is
deciding which crystal to use for your board and which caps go with it.
Most people just copy and paste what they found on some other schematic
(“12MHz + 22pF? … sounds good!”). &nbsp; Unfortunately, if you want to
get accurate and stable results out of the crystal, you need to match
the capacitors to the specific crystal you chose, and it varies from
model to model even with the same manufacturer. &nbsp;&nbsp;Fortunately,
its trivial to calculate the right capacitors for your crystal.<span id="more-24343"></span></p>
<p>A 12MHz crystal that I use quite a bit is the <a href="http://search.digikey.com/us/en/products/NX3225SA-12.000000MHZ/644-1047-2-ND/1128829" target="_blank">NX3225SA-12.000000MHZ</a>
from NDK. &nbsp;Its a good size, stable (+/-15 ppm), and easy to find.
&nbsp;I use the more expensive +/-15 ppm model for better input to the
PLL, but if you dont use the right capacitors along with the crystal
your signal will never be anything remotely close to +/-15ppm and you
may as well buy a much cheaper crystal.</p>
<p>So how do you know which capacitors to use? &nbsp;Easy. &nbsp;Every crystal datasheet lists something called the <strong>Load Capacitance</strong>
(CL). &nbsp;In the case of the crystal above, its 8 pF. &nbsp;C1 and
C2 need to match this Load Capacitance, with the following formula being
the key:</p>
<p><strong>CL = (C1 * C2) / (C1 + C2) + Cstray</strong></p>
<p>C1 and C2 are the two capacitors that you see attached to the crystal, but you might be wondering what <strong>Cstray</strong>
is. &nbsp;Unfortunately, every trace, every lead on your component,
just about everything on your PCB has some stray capacitance. &nbsp;The
total of these values is represented by Cstray. &nbsp;You can usually
guestimate this in the neighbourhood of 2-5pF as long as you follow good
layout practice and keep the trace from the crystal to the pins on the
MCU as short as possible with no vias, etc.</p>
<p>We know CL since its stated in the crystal datasheet (8 pF), and we
know Cstray (say ~5 pF), so all we need to do is test our value for C1
and C2 to make sure that it will match CL taking into account Cstray.
&nbsp;A commonly tossed around rule of thumb is to start with a pair of
capacitors two times the CL of the crystal, which will get you to CL.
&nbsp;In this case, that would be 2*8 pF = 16pF for the capacitors.
&nbsp;Unfortunately, this ignores the stray capacitance (Cstray), and
wont give you the best results. &nbsp;Even if you are guestimating
Cstray you will get better results taking it into account, but it means
just using a capacitor 2* CL wont work:</p>
<p>CL = 8 pF<br>
C1, C2 = 16 pF<br>
Cstray = 5pF</p>
<p>(16 pF * 16 pF)/(16 pF + 16 pF) + 5 pF= 13 pF</p>
<p>13pF is much too high to get the best results from your crystal.
&nbsp;If instead of 16pF, we take a lower 6 pF value this will give much
better results:</p>
<p>(6pF * 6pF)/(6 pF + 6 pF) + 5 pF =8 pF</p>
<p>If you think the stray capacitance is a bit lower, say 3pF, you might try a 10pF capacitor:</p>
<p>(10 pF * 10 pF)/(10 pF + 10 pF) + 3 pF = 8pF</p>
<p>Theres an obvious tradeoff between choosing a standard capacitor
value, and having a good idea of what your Cstray is, but the above
formula should at least explain HOW to determine what value those
capacitors should be relative to your crystal. &nbsp;Even using a
ballpark Cstray plud good layout should give you far more accurate
results than just copying and pasting a value you saw on another
schematic unless youre using the exact same crystal model (which is
unlikely since they are rarely even listed in the schematic).</p>
<p>A better rule of thumb is:&nbsp;<strong>C1, C2 = 2*CL 2*Cstray</strong></p>
<h2>Further Reading</h2>
<p><a href="http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/APPLICATION_NOTE/CD00221665.pdf" target="_blank">AN2867 Oscillator design guide</a> (ST Microcontrollers)</p>
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<h2 id="comments">7 Comments</h2>
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<li class="comment even thread-even depth-1" id="comment-27362">
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<img alt="" src="Choosing%20the%20Right%20Crystal%20and%20Caps%20for%20your%20Design%20%C2%AB%20Adafruit%20Industries%20%E2%80%93%20Makers,%20hackers,%20artists,%20designers%20and%20engineers%21_files/73ab95e980246179ebb4e522410ab7ec.jpeg" srcset="https://secure.gravatar.com/avatar/73ab95e980246179ebb4e522410ab7ec?s=160&amp;d=mm&amp;r=g 2x" class="avatar avatar-80 photo" originals="80" src-orig="https://secure.gravatar.com/avatar/73ab95e980246179ebb4e522410ab7ec?s=80&amp;d=mm&amp;r=g" scale="1.3953487873077393" width="80" height="80"> <div class="comment-author">mikeselectricstuff</div>
<div class="comment-datetime">
<a href="https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/#comment-27362">January 24, 2012 at 6:44 am</a>
</div>
</div>
<p>A handy tip I got from a Microchip guy a while ago is that a happy
oscillator has a similar amplitude on each side of the crystal. You
ideally need to use a low capacitance probe (e.g. x100) to avoid the
probe affecting things too much.</p>
</li>
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-27365">
<div class="comment-meta">
<img alt="" src="Choosing%20the%20Right%20Crystal%20and%20Caps%20for%20your%20Design%20%C2%AB%20Adafruit%20Industries%20%E2%80%93%20Makers,%20hackers,%20artists,%20designers%20and%20engineers%21_files/23f3f21928a7ee5ad6363d00e8809c1f.jpeg" srcset="https://secure.gravatar.com/avatar/23f3f21928a7ee5ad6363d00e8809c1f?s=160&amp;d=mm&amp;r=g 2x" class="avatar avatar-80 photo" originals="80" src-orig="https://secure.gravatar.com/avatar/23f3f21928a7ee5ad6363d00e8809c1f?s=80&amp;d=mm&amp;r=g" scale="1.3953487873077393" width="80" height="80"> <div class="comment-author">chuckz</div>
<div class="comment-datetime">
<a href="https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/#comment-27365">January 24, 2012 at 9:31 am</a>
</div>
</div>
<p>This is very helpful information. Thank you.</p>
</li>
<li class="comment even thread-even depth-1" id="comment-27378">
<div class="comment-meta">
<img alt="" src="Choosing%20the%20Right%20Crystal%20and%20Caps%20for%20your%20Design%20%C2%AB%20Adafruit%20Industries%20%E2%80%93%20Makers,%20hackers,%20artists,%20designers%20and%20engineers%21_files/67abb62c9dd2fcb955963ac94ea5d321.jpeg" srcset="https://secure.gravatar.com/avatar/67abb62c9dd2fcb955963ac94ea5d321?s=160&amp;d=mm&amp;r=g 2x" class="avatar avatar-80 photo" originals="80" src-orig="https://secure.gravatar.com/avatar/67abb62c9dd2fcb955963ac94ea5d321?s=80&amp;d=mm&amp;r=g" scale="1.3953487873077393" width="80" height="80"> <div class="comment-author">Georg</div>
<div class="comment-datetime">
<a href="https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/#comment-27378">January 24, 2012 at 4:12 pm</a>
</div>
</div>
<p>I have a further question dont you have to take the parasitic
input capacity of the pins of the driven chip into account aswell?</p>
</li>
<li class="comment byuser comment-author-kevin bypostauthor odd alt thread-odd thread-alt depth-1" id="comment-27381">
<div class="comment-meta">
<img alt="" src="Choosing%20the%20Right%20Crystal%20and%20Caps%20for%20your%20Design%20%C2%AB%20Adafruit%20Industries%20%E2%80%93%20Makers,%20hackers,%20artists,%20designers%20and%20engineers%21_files/daa9b954b88210084e3687db33644adb.jpeg" srcset="https://secure.gravatar.com/avatar/daa9b954b88210084e3687db33644adb?s=160&amp;d=mm&amp;r=g 2x" class="avatar avatar-80 photo" originals="80" src-orig="https://secure.gravatar.com/avatar/daa9b954b88210084e3687db33644adb?s=80&amp;d=mm&amp;r=g" scale="1.3953487873077393" width="80" height="80"> <div class="comment-author"><a href="http://www.adafruit.com/" rel="external nofollow" class="url">Kevin</a></div>
<div class="comment-datetime">
<a href="https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/#comment-27381">January 24, 2012 at 5:16 pm</a>
</div>
</div>
<p>Georg: That would be included in the stray capacitance … its
basically a sum of everything between the output of the crystal and the
input to the PLL inside the chip, including the parasitic capacitance of
any pins or leads on any discrete components, etc. You can look in the
datasheets to add all the little bits up, but ~5pF is a pretty good rule
of thumb.</p>
</li>
<li class="comment even thread-even depth-1" id="comment-27394">
<div class="comment-meta">
<img alt="" src="Choosing%20the%20Right%20Crystal%20and%20Caps%20for%20your%20Design%20%C2%AB%20Adafruit%20Industries%20%E2%80%93%20Makers,%20hackers,%20artists,%20designers%20and%20engineers%21_files/67abb62c9dd2fcb955963ac94ea5d321.jpeg" srcset="https://secure.gravatar.com/avatar/67abb62c9dd2fcb955963ac94ea5d321?s=160&amp;d=mm&amp;r=g 2x" class="avatar avatar-80 photo" originals="80" src-orig="https://secure.gravatar.com/avatar/67abb62c9dd2fcb955963ac94ea5d321?s=80&amp;d=mm&amp;r=g" scale="1.3953487873077393" width="80" height="80"> <div class="comment-author">GEorg</div>
<div class="comment-datetime">
<a href="https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/#comment-27394">January 25, 2012 at 8:33 am</a>
</div>
</div>
<p>thanks for this Information i also found out that quite a number of
datasheets dont give information about the capacity of the XIN Pins ..</p>
</li>
<li class="comment byuser comment-author-kevin bypostauthor odd alt thread-odd thread-alt depth-1" id="comment-27399">
<div class="comment-meta">
<img alt="" src="Choosing%20the%20Right%20Crystal%20and%20Caps%20for%20your%20Design%20%C2%AB%20Adafruit%20Industries%20%E2%80%93%20Makers,%20hackers,%20artists,%20designers%20and%20engineers%21_files/daa9b954b88210084e3687db33644adb.jpeg" srcset="https://secure.gravatar.com/avatar/daa9b954b88210084e3687db33644adb?s=160&amp;d=mm&amp;r=g 2x" class="avatar avatar-80 photo" originals="80" src-orig="https://secure.gravatar.com/avatar/daa9b954b88210084e3687db33644adb?s=80&amp;d=mm&amp;r=g" scale="1.3953487873077393" width="80" height="80"> <div class="comment-author"><a href="http://www.adafruit.com/" rel="external nofollow" class="url">Kevin</a></div>
<div class="comment-datetime">
<a href="https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/#comment-27399">January 25, 2012 at 10:41 am</a>
</div>
</div>
<p>GEorg: Its true that datasheets arent always terribly exhaustive,
but you can usually find something close to what you want. I used the
LPC1343 a lot (an inexpensive 72MHz ARM Cortex M3 chip from NXP), and
they dont list the input capacitance for the XTAL in/out pins — only
ADC inputs are listed at 20pF — but if you look at Table 21 of section
11.2 (XTAL Input) you can at least see a table showing suggested values.
Using a 12MHz crystal (which is more or less the de facto crystal for
LPC MCUs), they show suggested values for a 10pF or 20pF crystal. If
this is all the information you have and accuracy is very important,
youll be better off picking a 10pF cap instead of the 8pF mentionned
above, and then using their suggestion of two 18pF capacitors. You can
work backwards of course to guestimate the input capacitance with those
values. Or … you can also drop the companies a line if its important
since someone almost certainly (should!) know this information, though
it can be hard to get through the corporate firewall if you dont know
someone inside the company already.</p>
</li>
<li class="comment byuser comment-author-kevin bypostauthor even thread-even depth-1" id="comment-27401">
<div class="comment-meta">
<img alt="" src="Choosing%20the%20Right%20Crystal%20and%20Caps%20for%20your%20Design%20%C2%AB%20Adafruit%20Industries%20%E2%80%93%20Makers,%20hackers,%20artists,%20designers%20and%20engineers%21_files/daa9b954b88210084e3687db33644adb.jpeg" srcset="https://secure.gravatar.com/avatar/daa9b954b88210084e3687db33644adb?s=160&amp;d=mm&amp;r=g 2x" class="avatar avatar-80 photo" originals="80" src-orig="https://secure.gravatar.com/avatar/daa9b954b88210084e3687db33644adb?s=80&amp;d=mm&amp;r=g" scale="1.3953487873077393" width="80" height="80"> <div class="comment-author"><a href="http://www.adafruit.com/" rel="external nofollow" class="url">Kevin</a></div>
<div class="comment-datetime">
<a href="https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/#comment-27401">January 25, 2012 at 10:46 am</a>
</div>
</div>
<p>GEorg:</p>
<p>Heres a better example for the LPC1850 another ARM Cortex M3 MCU
Ive been using lately. In this datasheet they list the input
capacitance of the XTAL pins as max 0.8pF (search for Cio p.96 of the 21
Nov 2011 DS):</p>
<p><a href="http://ics.nxp.com/products/lpc1000/datasheet/lpc1810.lpc1820.lpc1830.lpc1850.pdf" rel="nofollow">http://ics.nxp.com/products/lpc1000/datasheet/lpc1810.lpc1820.lpc1830.lpc1850.pdf</a></p>
<p>This is only a simulated value in this case (see note 14) but it should give you a pretty close approximation.</p>
<p>You still need to add in some margin for stray capacitance due to the
trace, parasitic capacitance of the capacitors (use smaller physical
packages if possible), but honestly just using the rule of thumb above
and guessing 2-5pF will already give you far better results that just
copying the capacitor values you saw on some other schematic.</p>
<p>Kevin</p>
</li>
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<p>Sorry, the comment form is closed at this time.</p>
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by Kevin <br><br><a href="https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/#comments">Comments (7)</a> </div>
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