Internal RC oscillator calibration

AN4067

18/27

DocID022913 Rev 2

3.3.3

Internal oscillator frequency measurement

The internal oscillator frequency measurement is performed by Timer 14 capture interrupt.

In the timer TIM14 ISR, an entire period of internal oscillator frequency is computed. The

number of periods to be measured for each trimming value is configurable by the user in the

InternOscCalibration.h file, as follows:

#define NUMBER_OF_LOOPS 50

The averaging method is used to minimize frequency error measurements. So, if the

computed.

Users can easily configure the frequency of the reference source. It is defined in the

InternOscCalibration.h header file, as follows:



If the LSE clock is used as the reference frequency, uncomment the line below to make

sure the LSE is configured and internally connected to Timer 14 channel 1:

#define USE_REFERENCE_LSE



If the reference frequency is an external signal equal to 50 Hz, then comment the line

above and define the reference frequency as shown below:

#define REFERENCE_FREQUENCY (uint32_t)50 /* The reference

frequency value in Hz */

The computation of the frequency measurements does not depend on the duty cycle of the

source reference signal. It depends on its frequency since the capture 1 interrupt is

configured to occur on every rising edge of the reference signal (refer to Figure 5).

Note:

Figure 3.3.4 provides the frequency measurement algorithm.