DocID16612 Rev 6

17/24

AN3101

High speed internal oscillator calibration

23

2.3.4

HSI calibration demo description

The demo provided with this application note shows the ability of the firmware to calibrate

the HSI oscillator of the STM8L05xxx/15xxx, STM8L162xx and STM8AL31xx/3Lxx

microcontrollers.

By default, the demo is configured to run the calibration of the HSI oscillator routine using

the LSE source frequency.

To run the calibration process that provides the frequency with minimum error, you have to

comment the following define line in the main.c file. Conversely, to run the calibration

process that finds the frequency with a maximum allowed error, you have to uncomment the

same line:

/* #define USE_HSI_FIXED_ERROR */

To display the HSI signal, connect an oscilloscope probe to the PC4 pin and uncomment the

following line in the main.c file:

/* #define OUTPUT_HSI_ON_CCO_FOR_DEBUG */

2.4

Recommendations on the use of the HSI calibration library

1.

If the reference frequency used for HSI calibration is lower than 250 Hz, the TIM2

counter prescaler can be used to support low reference frequencies.

2.

The TIM2 input capture divider (1, 2, 4 and 8) can be used to support high reference

frequencies.

3.

Frequency measurement accuracy is not guaranteed when using a reference

frequency that exceeds 240 kHz.

4.

It is not recommended to call calibration functions in an interrupt routine since, in this

case, the calibration process may be long (refer to the Section 2.5.1).

5.

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

to restart them after calling the calibration functions. The application therefore has to

stop communications, ADC measurements (see Note: 1) etc. since these processes

are supposed to use clock configurations 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, etc.

Note:

1

Except when using the ADC for the calibration process (refer to 7.).

6.

The HSI calibration firmware uses the following peripherals: Clock (for trimming HSI),

Timer 2 (for measuring HSI), Beeper (for connecting LSE to Timer 2 if LSE method is

used). So, it is recommended to reconfigure these peripherals (if used in the

application) after running HSI calibration routine

7.

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 HSI calibration process has to be performed and the two

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

implemented in this application note):

Threshold_High = CurrentTemperatureValue + TemperatureOffset

Threshold_Low = CurrentTemperatureValue – TemperatureOffset