DocID17369 Rev 2

9/13

AN3193

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 STM32L1xx offers several options to measure the internal oscillators.

Although HSI and MSI are factory trimmed (except for the STM32L100 Value Line devices),

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 (HSI) 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.