Alternative to FC-135R 32.768 kHz Crystal

[lpcware]

Content originally posted in LPCWare by jhd on Tue Nov 24 01:03:38 MST 2015
Hello,

In DigiKey no stock for FC-135R X1A00014100003xx.

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=184534335&uq=635839265192216943

Is this another reference valid at alternative?

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=184533349&uq=635839272552837565

70 kOhm.


Regards

[lpcware]

Content originally posted in LPCWare by IanB on Wed Nov 25 09:23:27 MST 2015
Yes. It'll be all right!

[lpcware]

Content originally posted in LPCWare by jhd on Wed Nov 25 07:33:13 MST 2015
Hello,

Thanks for yours replies,

For now I'll leave it like that.

Best regards.

[lpcware]

Content originally posted in LPCWare by IanB on Wed Nov 25 03:29:32 MST 2015
is it oscillating? The odds are it is - either put an oscilloscope on XTALOUT (not XTALIN because the probe capacitance will stop it working).
Or use the processor to check if the counter is counting.

If it is oscillating, how accurate do you need it?
If you need it REALLY accuracy, then make the cap on XTALIN a trimmer, and set it with a frequency counter. You never really do know the value of Cstray
If you don't need it REALLY accurate, at a guess it's already good enough.

I built a similar circuit, and I had just two 15pF capacitors left, and one fell on the floor never to be seen again.
It still worked without the capacitor in XTALOUT.

I keep 15pF capacitors in stock and use then on crystal oscillators without ever bothering to check if it is the right value, and they have all worked.

[lpcware]

Content originally posted in LPCWare by lpcmunich on Wed Nov 25 03:22:27 MST 2015
Hi Javier,
The external load capacitors you calculated/connected (Cd=18pF and Cg=22pF) seems to be okay.
In the equation you are referring, the stray capacitance is sum of XTAL pin (pad) and parasitic (track) capacitors.
For simplifying the calculations, you can safely assume it (Cstary) in the range of 3-5pF.

[lpcware]

Content originally posted in LPCWare by jhd on Tue Nov 24 14:21:33 MST 2015
Hello,

Thanks for yours replies,

Yes is a NXP LPC54102 LQFP-64

This is the diagram in pdf format  https://github.com/hexagon-robotics/AIRAM/blob/master/Misc/airam-autopilot-diagram.pdf

I used ABS07 crystal that is similar to FC-135 (without R) http://media.digikey.com/pdf/Data%20Sheets/Epson%20PDFs/FC-135_135R_Brief~.pdf

ABS07 datasheet --> http://www.abracon.com/Resonators/ABS07.pdf

I have two smd capacitors added but I'm not sure that the calculation is correct. Cg = 18pF  and Cd = 22pF

ABS07 has CL = 12.5 pF  C0 (shunt capacitance) = 0.9 ~ 1.2pF but I don't know wich is the Cstray value.

https://www.lpcware.com/content/faq/how-calculate-value-crystal-load-capacitors

How can I know?

Regards

[lpcware]

Content originally posted in LPCWare by lpcmunich on Tue Nov 24 03:57:20 MST 2015
Hi Javier,
If you are using this 32.768kHz crystal with LPC5410x, it's indeed not that critical.
FC1610AN is a good alternative, but any other crystal with CL in the range of 6 to 12.5pF should be fine.
Accordingly, you need to select appropriate external load capacitors (Cd/Cg = 2CL-4)
Have a look here, http://www.nxp.com/documents/report/R_10077.pdf

[lpcware]

Content originally posted in LPCWare by IanB on Tue Nov 24 01:52:21 MST 2015
In my experience, ANY 32.768kHz crystal will work. It's just driven by a Pierce oscillator circuit, nothing fancy (I'm assuming that you're connecting it to one of the LPC micro controllers, otherwise you wouldn't be writing on this forum)

Make sure you get the load capacitance somewhere close - it doesn't seem to be too critical. That's two capacitors of TWICE the value specified in the "load capacitance", that's because the crystal sees the two capacitors in series, so see half the value - and I don't think that's too critical either.

I use
http://www.rapidonline.com/Electronic-Components/AEL-X32K768L101-32-768kHz-12-5pF-3x8mm-Radial-Cylin...

and I loaded it with 15pF caps, because I didn't have 22pF in stock and it worked OK.