DocID027266 Rev 2

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Internal RC oscillator calibration

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Figure 3 below shows MSI behavior at range 6 (4194304 Hz) versus MSICAL[7:0].

Figure 3. MSI trimming behavior

Note the negative steps when the two MSB bits change from “00” to “01”, from “01” to “10”

and from “10” to “11”. The change of the six LSBs leads to fine-tuning of around 0.5% per

step.

For the HSI48 oscillator, the calibration value is loaded in HSI48CAL[7:0] bits after reset. Six

trimming bits TRIM [5:0] (in CRS control register) are used for fine-tuning. The default

trimming value is 32. An increase/decrease in this trimming value causes an

increase/decrease in HSI48 frequency.

The HSI48 oscillator is fine-tuned in steps of 1% (around 67 kHz).

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Writing a trimming value, in the range of 33 to 63, increases the HSI48 frequency.

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Writing a trimming value, in the range of 0 to 31, decreases the HSI48 frequency.

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Writing a trimming value, equal to 32, causes the HSI48 frequency to keep its default

value.

Figure 4 shows the HSI48 oscillator behavior versus the calibration value. The HSI48

oscillator frequency increases with the calibration value (calibration value = default

HSI48CAL[7:0] + TRIM[5:0]