We use the STM32F2 for a project and have problems with the LSE crystal.
We use a Citizen CMR200T crystal (32.768kHz, CL=6pF, Farnell Nr. 1652560) wich has a gain margin of 6.1 (according to the ST application note AN2867) and therefore is suitable.
It worked fine with STM32F205VGT6 Ref Y, now we changed to Ref X and the frequency shifts about 40ppm! There are no information about changes made from Ref Y to Ref X (despite other bootloader version).
With uC Ref Y: f=32.7679kHz (-3ppm)
With uC Ref X: f=32.7694kHz (+43ppm)
The measurement are made with the same layout, crystal and CL1/CL1 of 4.7pF, only the uC has changed. The frequency is measured at PA8 which outputs the clock trough microcontroller clock output (MCO1).
Can anyone help? How can we build a accurate RTC when the frequency shifts from uC to uC?