Hi,
Im currently working on a project with an STM32L452 that uses a Temperature Compensated Crystal Oscillator (TCXO) as the Low-Speed External (LSE) clock source. The TCXO is responsible for clocking the RTC. To fully calibrate the TCXO, I have designed the hardware in such a way that I can fine-tune the input capacitance of the oscillation signal line using a trimmer capacitor.
In order to adjust the trimmer capacitor properly, I need to measure the period duration of the signal. My plan is to use a frequency counter for this measurement. Specifically, I would like to connect the frequency counter to pin PB2, which can be configured as RTC_OUT. Unfortunately, I cannot use the default PC13 pin for this purpose due to other design constraints.
According to the reference manual, enabling the RTC_OUT function on PB2 should simply require setting two bits: the COE bit in the RTC->CR register and the RTC_OUT_RMP bit in the RTC->OR register.
Despite following these steps, Im not seeing any output on PB2 when I attempt to measure the signal — the frequency counter shows nothing. I have verified that my test setup is functional because when I connect the frequency counter directly to the oscillation signal from the TCXO, I get a valid period duration reading. However, I assume this method introduces some capacitance from the probe itself, leading to inaccurate measurements.
I wouldd really appreciate any guidance on why PB2 is not outputting the RTC signal and how I might resolve this issue.
Thanks in advance!
