Internal RC oscillator calibration

AN4067

16/27

DocID022913 Rev 2

HSI_CalibrateMinError()

(refer to ErrorStatus

HSI14_CalibrateFixedError

(uint32_t MaxAllowedError, uint32_t* Freq)), but it

searches for the frequency that has an error (in absolute value) lower than or equal to

MaxAllowedError

.



If it finds this frequency, it stops searching and configures the trimming bits

HSITRIM[4:0] according to this frequency and returns SUCCESS, meaning that the

calibration operation has succeeded.



then sets the trimming bits HSITRIM[4:0] to the default calibration value and returns

ERROR, meaning that the calibration has failed and did not find any frequency with an

error lower than or equal to MaxAllowedError.



The frequency measurement starts with HSTRIM = 16. The HSITRIM value is

computed in loops to find the next value. That is, the HSITRIM value starts from 16,

then goes to the next value to the left, then to the next to the right, then to the second to

the left and so on until it reaches 31, forming a “spring loop” (as shown in Figure 11).

This algorithm is based on the fact that the probability of finding the frequency that has

the minimum error increases when the HSITRIM[4:0] value tends to 16. This algorithm

is implemented so as to minimize the time consumed by the calibration process.

Figure 11. “Spring loop”

The second parameter is used to get the frequency (in Hertz) after calibration in the form of

an unsigned 32-bit integer (unit32_t).

In case of an HSI14 calibration, the HSI14_CalibrateFixedError() function should be used

instead of HSI_CalibrateFixedError().