DocID022913 Rev 2

11/27

AN4067

Internal RC oscillator calibration

26

The real frequency generated by the internal RC oscillator HSI14 is given by:

The error (in Hz) is computed as the absolute value of the difference between the measured

frequency and the typical value.

Hence, the internal oscillator frequency error is expressed as:

After calculating the error for each trimming value, the algorithm determines the optimum

trimming value (that corresponds to the nearest frequency to the typical value) to be

programmed in the trimming bits (refer to Section 3.3: Description of the internal oscillator

calibration firmware for more details).

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.

In order to improve the accuracy, the timer prescaler can be used to increase the ratio

between HSI14 frequency and the reference frequency.

3.1.3

HSI48 calibration

HSI48 can be calibrated in the same way as HSI or HSI14. However, STM32F0xx product

family implements CRS (Clock recovery system) that is capable of doing automatic

adjustment of the oscillator trimming based on the comparison with a selectable

synchronization signal.

Internally HSI48 implements a 16-bit down/up counter which step by step increments or

decrements of TRIM value until it reaches the expected frequency value based on LSE

reference clock or the USB SOF signal.

The HSI48 calibration using CRS can be run in fully automatic mode. To speed up the

process, the CRS can be used for measurement of the actual error, and set trim value with

pre-calculated value. This process can repeated once or twice as the curve might not be

linear. When requested frequency is reached automatic calibration can be activated for

further smooth calibration, e.g. compensating temperature changes.

3.2

Hardware implementation

3.2.1

Case where LSE is used as the reference frequency to measure HSI

The STM32F0xx offers a useful feature, that is the ability to connect internally and indirectly

the Low-speed external (LSE) oscillator to Timer 14 channel 1. Thus, the LSE clock can be

used as the reference signal for internal oscillator calibration and no additional hardware

connections are required. Only the LSE oscillator should be connected to OSC32_IN and

OSC32_OUT.

Figure 7 shows the hardware connections needed for internal oscillators calibration, using

LSE as an accurate frequency source for calibration.

Measuredfrequency

SystemClock

 IC1ReadValue2 IC1ReadValue1

–







=

Error Hz



Measuredfrequency

typicalvalue

–

=