DocID022913 Rev 2

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AN4067

Low-speed internal oscillator measurement

26

Figure 15. Timing diagram of an internal RC oscillator measurement

The internal oscillator frequency value is computed as shown by the following formula:

internal oscillator frequency = HSI_Value / Capture

where:



HSI_Value is the HSI frequency value: typical value is 8 MHz,



Capture represents an entire period of internal RC oscillator LSI: IC1ReadValue2 -

IC1ReadValue1.

As users can conclude from the formula above, the frequency measurement accuracy

depends on the HSI frequency accuracy. Consequently, if a reference signal is available,

users can run the internal RC oscillator calibration routine described in Section 3: Internal

RC oscillator calibration before performing the internal RC oscillator measurement

procedure.

The input capture prescaler can be used for better measurement accuracy so the formula

above becomes:

LSI_Frequency = InputCapturePrescaler * HSI_Value / Capture_Value.

Note:

If the HSE clock is available it is highly recommended to use it as the reference clock

instead of HSI, to benefit from its higher accuracy.

4.2

Description of the internal oscillator measurement firmware

The internal oscillator measurement firmware provided with this application note includes

one C source files:



LSIMeasurement.c performing LSI frequency measurement using LSI_FreqMeasure()

function

The internal RC oscillator LSI is measured for a predefined number of periods. Then it

returns the average value to minimize the error of the measured frequency.

Users can change this parameter (number of LSI periods) in the lsi_measurement.h file:

#define LSI_PERIOD_NUMBERS 10