Internal RC oscillator calibration

AN4067

20/27

DocID022913 Rev 2

3.3.4

Internal oscillator calibration using calibration curve

Both the minimal and fixed error methods can take some time due to the high number of

measurements to run (for example 32 measurement for HSI with the minimal error method).

First, for all trimming values, the difference between the corresponding and the requested

frequency is measured and stored in a table. This uses the same principle as the minimal

error method. When it is necessary to calibrate the internal oscillator function

HSI_CalibrateCurve(), only the actual frequency is measured and the appropriate value to

compensate the difference is taken from the table.

In case of an HSI14 calibration, the HSI14_ GetCurve () and HSI14_CalibrateCurve()

functions should be used instead of HSI_GetCurve() and HSI_CalibrateCurve().

3.4

Recommendations on the use of the calibration library

1.

If the external signal frequency is lower than system clock / 65535, the TIM14 counter

prescaler should be used to support low frequencies.

2.

If the external signal frequency is higher than system clock / 100, TIM14 input capture

prescaler (divider) should be used to support high frequencies.

3.

It is recommended to stop all application activities before the calibration process, and

to restart them after calling the calibration functions.

Therefore, the application has to stop the communications, the ADC measurements

and any other processes (except when using the ADC for the calibration, refer to Step

6. below).

These processes normally use clock configurations that are different from those used

in the calibration process. Otherwise, errors might be introduced in the application:

errors while reading/sending frames, ADC reading errors since the sampling time has

changed, and so on.

4.

The internal RC oscillator calibration firmware uses the following peripherals: Reset

and Clock Control (for trimming internal RC oscillators), Timer 14 (for measuring

internal RC oscillators). Therefore, it is recommended to reconfigure these peripherals

(if used in the application) after running the calibration routine.

5.

Real-time calibration vs. temperature can be used when the ambient temperature

changes noticeably while the application is running. The internal temperature sensor

can be used with the ADC watchdog with two thresholds. Each time an ADC watchdog

interrupt occurs, a new calibration process has to be performed and the two thresholds

are updated according to the current temperature (this feature is not implemented in

the firmware provided with this application note):

Threshold_High = CurrentTemperatureValue + TemperatureOffset

Threshold_Low = CurrentTemperatureValue – TemperatureOffset

6.

It might happen that with change of operation conditions (e.g. surrounding

temperature) the calibration curve can change. From this reason it's recommended to

measure from time to time (or when the condition change) again to keep working with

correct values.