DocID025944 Rev 1

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AN4445

A versatile clock management

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The price of a crystal oscillator may not be neglected in cost sensitive applications. For this

reason, the STM32L0xx offers several options to measure the internal oscillators.

Although HSI16 and MSI are factory trimmed, they can be further trimmed by 0.5% steps

during run time to compensate for frequency deviations due to temperature and voltage

changes. Similarly, manufacturing process deviations of the LSI can be evaluated and

compensated using a higher accuracy clock reference, either internally (HSI16) or externally

(LSE or HSE).

As an example, in an application where a 32.768 kHz crystal is used for the RTC, it is

interesting to use the low power MSI oscillator which can provide a clock frequency of up to

4 MHz for the CPU with a typical consumption of 20 µA. Taking advantage of the high

precision of the LSE crystals (typically a few tens of ppm), it is possible to determine the MSI

frequency with the same resolution, and then to trim it on-the-fly.

2. Accuracy reached after trimming. See product datasheet for low voltage and temperature drift details

3. -10% to +4% drift after initial measurement