Low-speed quartz resonator load capacitance

[beatle7]

Posted on December 07, 2012 at 13:57

Hello!

In the device datasheet DS6329 v.9 on page 85 I found the following:Caution: To avoid exceeding the maximum value of CL1 and CL2 (15 pF) it is strongly recommended to use a resonator with a load capacitance CL ≤  7 pF. Never use a resonator with a load capacitance of 12.5 pF. This seem to be interesting for AFAIK 12.5 pF load capacitance of 32768 Hz resonators is very common. May anybody tell me what is the cause of this strong limitation? I also didn't find Oscillator Design Guide related to STM32F2xx microcontrollers. Is AN2867 suitable for them? And the last question, are there any crystals provided by Geyer recommendable to use with STM32F2xx?

[Tesla DeLorean]

Posted on December 07, 2012 at 14:21

Well presumably it's due to the drive strength of the pins, and by inference the transistor size, and power consumption. The oscillator is not drive rail-to-rail.

9, 7 and 6 pF are also quite common, and you usually use the rating specified rather than what is most convenient in the part draw. Perhaps you should inquire with Geyer's sales dept. They definitely sell such parts

http://www.geyer-electronic.de/uploads/tx_userartikelfrequenz/GEYER-KX-327NHT_03.pdf

Failure to follow ST guidelines will result in the oscillator failing to start, or starting intermittently.