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stm32 internal crystal accuracy
Posted: 2017/4/25 18:47:34 Read: 13 Cofite Electronics

The STM32 perpetual calendar was initially debugged, using the internal LSI crystal as the source of the RTC crystal, and found two problems:

First: Because the frequency of the LSI crystal is around 40KHZ, it will actually fluctuate between 30 ~ 60KHZ, resulting in inaccurate timing. Basically there will be a deviation of about 1 minute in one hour.

Second: Due to the internal crystal of the LSI, when the power is off, it is not in the backup power supply area. The external 3.3V battery is connected to VBAT and only supplies power to the backup power supply area. After the system is powered off, the battery can only maintain The back-up RTC count did not power the LSI crystal (see the figure below), which caused the RTC to always remain for the time before the power was turned off. Although you can continue to run after power on again. But it needs to be recalibrated.

Finally, an external 32.768KHZ external LSE crystal oscillator was used as the source of the RTC crystal. During the debugging process, it was found that the problem was the same as before using LSI.

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