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STM32F2xx RTC: LSE Jitter problem

Question asked by wild.andreas on Nov 18, 2014
Latest reply on Nov 20, 2014 by Clive One
Hello to all,

For calibration I use Timer 5 Capture-Channel4 to measure the LSE Clock on a STM32F207 CPU.
However the result was variing +- 20 Hz.
To deviate external crystal issues I routed LSI to the Timer 5 capture circuit.
I still discovered about the same jitter in the frequency.
In order to check my Timer5 measurement I changed the capture input to PA3 GPIO which was driven by an external stable frequency. The reading of the Timer 5 result was perfectly stable now!
So I gradually doubted the stability of the 32KHz LSE in general.
To verify this I routed the LSE frequency to MCO1 (PA8). Indeed with a scope I could see the jitter of the 32Khz signal!

Has anybody an idea why this signal can be so unstable, even though there is no PLL involved?

Thanks a lot for your comments!