Conclusions

AN3101

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DocID16612 Rev 6

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Conclusions

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

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

This application note gives two routines:

•

one for the high speed internal clock calibration: How to tune the HSI clock to 16 MHz?

•

one for the low speed internal clock measurement: How to get the exact LSI frequency

value?

Several frequency sources can be used to calibrate the HSI oscillator: the LSE crystal, the

AC line, etc. Whatever the reference frequency source, the HSI 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 HSI frequency

measurement. The error is computed as the absolute value of the typical HSI frequency

value and the measured one for each HSITRIMR register configuration. From this, the

calibration value is calculated and then programmed in the HSITRIMR register.

The second section of this application note focuses on the LSI clock measurement. The

internal connection between the LSI clock and channel 1 of Timer 2 provided in

STM8L05xxx/15xxx, STM8L162xx and STM8AL31xx/3Lxx microcontrollers is used to get

the LSI frequency value. The Timer 2 is clocked using the HSI clock and Timer 2 channel 1

is configured in input capture mode. So the time between two consecutive rising edges of

LSI clock represents an entire period.