AN2822

High speed internal oscillator trimming

Doc ID 14983 Rev 2

5/19

1

High speed internal oscillator trimming

1.1

Introduction

The STM8S and STM8A 8-bit microcontrollers can use a high speed internal (HSI) RC

The HSI frequency is calibrated to give reasonable accuracy for the vast majority of

applications. However, the output frequency of the RC oscillator is affected by temperature

and/or voltage drift, while, total accuracy of the HSI frequency is reduced a percentage

given in the datasheet. To compensate for such influences in the application, the STM8S

and STM8A HSI oscillator output frequency can be trimmed by the user runtime calibration

routine to improve frequency accuracy. This is especially important for communication

peripherals.

Figure 1.

STM8S and STM8A HSI trimming principle

Initial calibration data, INIT_TRIM, are stored in the protected part of the internal memory

and loaded onto the internal calibration register after reset.

The frequency of the HSI RC oscillator can be fine-tuned to achieve better accuracy with

wider temperature and supply voltage ranges. The lowest three bits of the CLK_HSITRIMR

register are reserved for this purpose. The number stored in these bits is processed as a

signed number with the two's-complement representation (see Table 1). The trimming

calibration value.

Note:

An increase in this trimming value causes a decrease in HSI frequency.

a.

Some selected STM8S and STM8A devices can be set to 4-bit trimming mode by the HSITRIM option byte,

see device datasheet for details.

RC OSCILLATOR

DECODER

CLK_HSITRIM

HSI Clock Output

+

INIT_TRIM

b.

If the 4-bit trimming mode is set by the HSITRIM option byte, the frequency change between two steps in the

trimming value is approx. 0.5%