High speed internal oscillator calibration

AN3101

18/24

DocID16612 Rev 6

2.5

Calibration process performance

2.5.1

Duration of the calibration process

The duration of the calibration process depends on:

•

the used reference frequency;

•

the number of measured periods per frequency;

•

the number of measured frequencies during the calibration process.

Once peripherals are configured and ready, the duration of the calibration process is

approximated by:

Where:

–

NPeriod is the number of times the frequency is measured for each HSITRIM

configuration (the same frequency)

–

NFreq is the number of measured frequencies (number of HSITRIM values used

for the frequency measurement)

–

divider)

In the case of the calibration process with a minimum frequency error

(HSI_CalibrateMinError()) the number of NFreq is equal to 21. If the LSE oscillator is used

and the selected number of measured periods configured by the user is 10, the calibration

lasts around 113 ms.

In the case of the calibration process with a maximum allowed error

(HSI_CalibrateFixedError()), NFreq changes from chip to chip. NFreq also depends on the

maximum allowed error that is selected. The higher the selected allowed error, the more

NFreq will tend to 1. The lower the selected allowed error, the more NFreq will tend to 21.

So, the duration of the calibration process with a maximum allowed error is lower than or

equal to the duration of calibration when using the minimum frequency error process.

Note:

The factor 2 in the CalibDuration formula above is due to the fact that there is no

synchronization between reference signal and timer start counting.

CalibDuration

2x

NPeriod 1

+

() NFreq

×

---------------------------------------------------------------

=

CalibDuration

2

11

30

×

4096

--------------------

×

113ms

==