April 2015

DocID027266 Rev 2

1/30

1

AN4631

Application note

How to calibrate an STM32L0xx internal RC oscillator

Introduction

The STM32L0xx microcontrollers have two internal RC oscillators that can be selected as the system

clock source. These are known as the HSI16 (High-Speed Internal) and MSI (Multi-Speed Internal)

oscillators. The HSI16 oscillator has a typical frequency of 16 MHz. The MSI oscillator is a lower

speed, low-power clock source.

The STM32L0xx microcontrollers have two secondary clock sources.

• LSI: 37 kHz (Low-Speed Internal)

• HSI48: 48MHz (High-Speed Internal) that can be used directly for USB and for random number

generator (RNG).

The operating temperature has an impact on the accuracy of the RC oscillators. At 25 °C, the HSI16

oscillator has an accuracy of ±1% typically, the MSI oscillator has an accuracy of ± 0.5% and the

HSI48 oscillator has an accuracy of ±4%. But in the temperature range of -40°C to 105 °C, the

accuracy decreases. To compensate for the influence of temperature on internal RC oscillators

accuracy, the STM32L0xx microcontrollers have built-in features to allow you to calibrate the HSI16,

MSI and HSI48 oscillators and measure the MSI and LSI (Low-Speed Internal) oscillator frequencies.

This application note focuses on how to calibrate internal RC oscillators: HSI16, MSI and HSI48.

Three methods are presented: the first method is based on finding the frequency with the minimum

error, the second one consists in finding the maximum allowed frequency error and the third one

implements a table of premeasured values and then only searches in it to for the appropriate change.

All three are implemented by providing an accurate reference signal.

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

capture.

Note:

1

STM32L0xx refers to Ultra Low Power Access line, USB line and USB and LCD line devices in this

document.

2

Ultra Low Power Access line devices are STM32L051xx microcontrollers where the Flash memory

density ranges between 32 and 64 Kbytes.

3

Ultra Low Power USB line devices are STM32L052xx and STM32L062xx microcontrollers where the

Flash memory density ranges between 32 and 64 Kbytes.

4

Ultra Low Power USB and LCD line devices are STM32L053xx and STM32L063xx microcontrollers

where the Flash memory density ranges between 32 and 64 Kbytes.

Table 1. Applicable products and software

Type

Part Number

Microcontrollers

STM32L051C6, STM32L051C8, STM32L051K6, STM32L051K8, STM32L051R6, STM32L051R8,

STM32L052C6, STM32L052C8, STM32L052K6, STM32L052K8, STM32L052R6, STM32L052R8,

STM32L053C6, STM32L053C8, STM32L053R6, STM32L053R8

STM32L062K8

STM32L063C8, STM32L063R8

Software

X-CUBE-RC-CALIB

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