Take the 2-minute tour ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.

I need precise low-ppm crystal for my circuit with 8-bit ATMEGA (AT90USB1286). Currently 18pF TXC HC-49 (11.5x5mm) is used. I need also minimize size to reduce potential EMC and PCB. TXC - 7M/B (5x3.2/3.2x2.5nn) seems as potential alternative. But they are 10pF crystals.

Atmega datasheets do not mention XTAL pin capacitance, there is general Capacitance for each I/O Pin as high as 10pF (with exclamation that is not accurate and not dependend on socket TQFP/QFN). Does 10pF value relate also for XTAL pins? And does it implies that the low load capacity crystal cannot be used because of formula \$C_x = 2 \times (C_{xtal}-C_{pin})-C_{stray}\$? SMD 18pF alternative e.g. Abracon ABM3B (5x3.2mm).

BTW: Does make difference if the crystal has GND pin (4 pins or metal case) or is in 2 pin ceramic socket e.g. Abracon ABM7 (18pF). AFAIR strong EMC recommendation is to connect metal case with GND.

Related posts: MCU crystal capacitor selection, Choosing a crystal and load capacitors for micro controller

share|improve this question

1 Answer 1

up vote 2 down vote accepted

Crystal pins are not I/O pins, and 10pF is a guaranteed maximum not a nominal capacitance.

Usually 5pF will suffice for a crystal pin plus stray capacitance with reasonable layout (as short traces as reasonable, possibly over a ground plane). However, the AT90USB1286 datasheet does not recommend load capacitors of less than 12pF so 10pF total would seem to be marginal (you'd get 11pF if the input+stray was 5pF).

http://www.atmel.com/Images/doc7593.pdf

If you don't care about the exact frequency, 12pF or even a bit higher would probably be okay with a "10pF crystal", it would just shift the nominal oscillation frequency very slightly. Otherwise you can pick a different crystal.

share|improve this answer
    
I try to achieve accurate (16MHz) clock, i.e. use low-ppm (10ppm) crystals. Maybe capacitance non-matching adds more error than I gain with more accurate crystal, i.e. answer is avoiding 10pF crystal. You're right, recommendation is 12-22pF and probably both conditions should be satisfied. –  TMa Sep 1 at 15:44
    
Keep in mind that you're not going to be able to predict the capacitance to better than a couple of pF, but choosing to go higher will give a systemic shift. –  Spehro Pefhany Sep 1 at 15:45
    
P.S. If you're really interested in a guaranteed accurate frequency, I would suggest considering a packaged oscillator rather than a crystal. –  Spehro Pefhany Sep 1 at 16:09

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.