Selecting right capacitor for oscillator pin used in rtc

[aamirali641989]

Posted on November 29, 2013 at 09:46

I am using rtc & have 32.768Khz cyrstal oscillator (not resonator). I don't have its datasheet. Neither any marking on oscillator.

I have checked AN2867 for capacitor selection i.e CL1 & CL2. But I didn't have datasheet of oscillator. What should be recommended value of capacitor to be used. Currently I am using 10pF. I don't see any problem right now. But I am not sure whether its right?

[Tesla DeLorean]

Posted on November 29, 2013 at 10:16

But I am not sure whether its right?

May be you could characterize the random crystal, then you'd at least have some data points to actually pluck a value out of thin air? Not even sure which STM32 part you have here, the STM32F405/407 is a lot more sensitive to the characteristics of the circuit. Problems would exhibit as a failure to start, taking a long time to start, or not oscillating in a stable fashion. Sticking a scope probe on the board would typically have a dramatic effect to the circuit.