AN4631 datasheet(7/30 Pages) STMICROELECTRONICS

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Internal RC oscillator calibration

The frequency of the internal RC oscillators may vary from one chip to another due to

manufacturing process variations. For this reason, MSI and HSI16 RC oscillators are

factory-calibrated by ST to have a 1% accuracy at TA = 25 °C. After reset, the factory

calibration value is automatically loaded in the internal calibration bits.

The frequency of the internal RC oscillators can be fine-tuned to achieve better accuracy

with wider temperature and supply voltage ranges. The trimming bits are used for this

purpose.

For the HSI16 oscillator, the calibration value is loaded in HSI16CAL[7:0] bits after reset.

Five trimming bits HSI16TRIM[4:0] are used for fine-tuning. The default trimming value is

16. An increase/decrease in this trimming value causes an increase/decrease in HSI16

frequency. The HSI16 oscillator is fine-tuned in steps of 0.5% (around 80 kHz).

•

Writing a trimming value in the range of 17 to 31 increases the HSI16 frequency.

•

Writing a trimming value in the range of 0 to 15 decreases the HSI16 frequency.

•

Writing a trimming value equal to 16 causes the HSI16 frequency to keep its default

value.

The graph below shows HSI16 oscillator behavior versus calibration value. The HSI16

oscillator frequency increases with calibration value (calibration value = default

HSI16CAL[7:0] + HSI16TRIM[4:0]), except at modulo 16. At these calibration values, the

negative steps can reach three times the positive steps.

Figure 2. HSI16 oscillator trimming characteristics

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