Small Surface Mount Xstals for PIC?

[aaea]

Hi

I have been using 12MHz Xstals that have a package HC49 (through hole) with my PIC projects.
I was wondering if there are any smaller, surface mount Xstals that I can use with my PIC18LF4550?

Also, will I need to put capacitors to ground on the Xstal?

Thank you in advance.
a.

[PCM programmer]

Search at Digikey. They have a good selection menu, with photos.
http://www.digikey.com/

There are many different packages available. Here's an example:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=XC736CT-ND

[aaea]

Hi PCM Programmer

Thank you.
I was thinking along the lines of:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=644-1047-1-ND
or even smaller?

Can I use Xtals that are Ceramic?
What is the difference between quarts and ceramic Xstals?

Thank you
a.

[PCM programmer]

The crystal is quartz. The package is ceramic.

If you look at the specs in the column on the lower left side of the
Digikey link you'll see it has 8 pf load capacitance. I've never used a
crystal like that with a PIC. I only use 18 to 20 pf load crystals.

[aaea]

Hi PCM Programmer

Thank you for your reply.
Can you please explain the role of these small caps on the Xstal?
In the past I had projects which had no caps on the Xstal- and things seemed to work.

Thank you
a.

[Ttelmah]

Every crystal, has a required loading capacitance.
If you don't have this present, then the accuracy _will_ be degraded, reducing the 'point' of having a crystal at all.
The pins of the PIC themselves present some capacitance, and you also have capacitance from the board.
Now, the loading seen by the crystal, is the two capacitors added, and the parasitics, _in series_. So a crystal expecting say 18pF, would require total capacitance at each end to be 36pF (2*36pF capacitors in series = 18pF). Then if the board capacitance is perhaps 4pF, this would want 32pF capacitors (32pF, in parallel with 4pF, gives 36pF).
Error in the load capacitance, tends to drift the frequency 'off spec'. A small error, won't cause much problem, but larger percentage errors increase the error. If you know your PCB material & track lengths, you can make a very good estimate of the capacitances from these. The data sheet will specify pin capacitances for the PIC package. You can then calculate the capacitors that should be used, for any particular crystal.
Your project without capacitors, probably bought the crystal accuracy 'down' to more like that from perhaps a ceramic resonator, rather than a good crystal....

Best Wishes

[aaea]

Hi Ttelmah

Thank you for a thorough explanation. I have a lot to think about!

Have you had any experience with the small surface mount Xtals of the quartz variety?

All the best
a.

[PICoHolic]

You may check those too:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=535-9100-2-ND

[Ttelmah]

Have used any number of small SMD crystals. Most though are from Europe, for prototypes, or far Eastern suppliers for quantity.
A lot depends on what you actually 'need'. Accuracy costs money, and when specifying for large quantitites, you must to specify what is actually needed, rather than just choosing a 'nice' part. For high quality parts, have used Petermann Technik, for GPS, and Bluetooth, to give the required accuracies. For really small sizes, look for the AOR ACU3225....

Best Wishes

[SherpaDoug]

How much accuracy do you need? If you just need reliable RS232 comms you can usually get away with a cheaper ceramic resonator like this:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=490-1220-1-ND

Note that for these resonators the loading caps are built in, which saves a few pennies more.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[Guest]

The BIG issue I have always had with these little SMT resonators (yup, pun intended) is that they can't be washed and most manufacturing lines wash the boards. The Cerlock parts listed above say:

"Please contact us in case you need washable component."

Bummer!

HTH - Steve H.