January 2015

DocID16612 Rev 6

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AN3101

Application note

STM8L05xxx/15xxx, STM8L162xx and STM8AL31xx/3Lxx

internal RC oscillator calibration

Introduction

The STM8L05xxx/15xxx, STM8L162xx and STM8AL31xx/3Lxx microcontrollers offer the

possibility of using internal RC oscillators HSI (High-speed internal factory trimmed

oscillator of 16 MHz, typically) or LSI (Low-speed internal low-consumption oscillator of

38 kHz, typically) as clock sources.

The operating temperature and voltage have an impact on RC accuracy. At 3 V and 25 °C,

the HSI has an accuracy of ±1% typically, but in the full temperature and voltage ranges, the

accuracy of the HSI frequency decreases to -5% / +5% (STM8L05xxx), -4.5% / +3%

(STM8L15xxx and STM8L162xx) and -4% / +4% (STM8AL31xx/3Lxx).

To compensate for the influence of temperature and voltage on internal RC oscillator

accuracy, the STM8L05xxx/15xxx, STM8L162xx and STM8AL31xx/3Lxx microcontrollers

provide the capability of calibrating the HSI clock and of measuring the LSI clock.

This application note gives two methods of calibrating the HSI internal RC oscillator. The

first method is based on finding the frequency with the minimum error and the second one

consists in finding the maximum allowed frequency error. Both are implemented by

providing an accurate reference source.

The measurement of the LSI clock is performed by connecting the LSI oscillator to a timer

input capture. Depending on the measured value, peripheral registers are updated to meet

user requirements.

This application note applies to the products listed in Table 1.

Table 1. Applicable products

Type

Product lines

Microcontrollers

– STM8L051/052 Value Line

– STM8L151/152

– STM8L162

– STM8AL31

– STM8AL3L

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