Conclusion

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DocID027266 Rev 2

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Conclusion

Even if internal RC oscillators are factory-calibrated, the user should calibrate them in the

operating environment if a high-accuracy clock is required in the application.

This application note provides two routines:

•

Multi- and High-Speed Internal oscillator calibration: how to fine-tune the oscillator

to the typical value

•

Multi- and Low-Speed Internal oscillator measurement: how to get the “exact”

LSI/MSI frequency value

Several frequency sources can be used to calibrate the internal RC oscillators (HSI16,

HSI48 and MSI): LSE crystal, AC line, etc. Whatever the reference frequency source, the

internal oscillator calibration principle is the same: a reference signal must be provided to be

measured by a timer. The higher the accuracy of the reference signal frequency, the better

the accuracy of the internal oscillator frequency measurement. The error is computed as the

absolute value of the typical frequency value and the measured one for each trimming

value. From this, the calibration value is calculated and then programmed in the trimming

bits.

The second section of this application note discussed the measurement of LSI and MSI

oscillators. The internal connection between internal oscillators and embedded timers in the

STM32L0xx microcontroller family is used for this purpose. The timer is clocked using the

system clock source and configured in Input capture mode. The captured time between two

consecutive rising edges of internal oscillator represents an entire period.