CMWX1ZZABZ | sketches using the Arduino core

(false statement about LSE removed)

How can I calibrate the RTC using TXCO?

TXCO can supply HSE, and you can derive the RTC clock from HSE through a series of prescalers.

Section 8.5 in the User Manual says,

When an accurate external-high-speed clock is needed by the STM32, the TCXO_OUT clock pin is supplied by the module pin PH0_OSC_IN by closing SB13.

First, close SB13 with a drop of tin.

PH0_OSC_IN is the HSE bypass clock source. Set HSEBYP then HSEON in RCC->CR to have an accurate 32 MHz HSE clock. But wait, first you have to set RTCPRE to 3 in order to prescale it by 16 for the RTC module. Select this as the RTC clock in RCC->CSR.

Now the RCC generates a 2 MHz clock for the RTC (f_RTCCLK = 2000000). You scale it further down to 1 Hz (f_{CK_SPRE} = 1) with the RTC->PRER register. It has two bitfields, PREDIV_A can go from 0 to 127, PREDIV_S from 0 to 32767. Solving the integer equation

f_{CK_SPRE} = f_RTCCLK / ((PREDIV_S + 1) × (PREDIV_A + 1))

with the above constraints gives