DocID022913 Rev 2

7/27

AN4067

Internal RC oscillator calibration

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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, HSI and HSI14 RC oscillators are

calibration value is automatically loaded in the internal calibration bits.

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

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

purpose.

For the HSI oscillator, the calibration value is loaded in HSICAL[7:0] bits after reset. Five

trimming bits HSITRIM[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 HSI frequency.

The HSI oscillator is fine-tuned in steps of 1% (around 40 kHz).



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



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



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

value.

Figure 2 shows the HSI oscillator behavior versus the calibration value. The HSI oscillator

frequency increases with the calibration value (calibration value = default HSICAL[7:0] +

HSITRIM[4:0]), except at modulo 16. At these calibration values, the negative steps can

reach three times the positive steps.

Figure 2. HSI oscillator trimming characteristics

For the HSI14 (14 MHz) oscillator, The calibration value is loaded in HSI14CAL[7:0] bits

after reset. Five trimming bits HSI14TRIM[4:0] are used for fine-tuning. The default trimming