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AN2822 Datasheet(PDF) 5 Page - STMicroelectronics

Part # AN2822
Description  STM8S and STM8A high speed internal oscillator calibration
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2822 Datasheet(HTML) 5 Page - STMicroelectronics

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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
oscillator as a system clock source. This oscillator has a nominal frequency, fHSI nominal, of
16 MHz which is factory calibrated at an ambient temperature of TA = 25 °C and a supply
voltage of VCC = 5 V. This accuracy is specified in the datasheet for each device.
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
number (a) can vary from -4 (100b) to +3 (011b). This number is added to the factory
calibration value.
Note:
An increase in this trimming value causes a decrease in HSI frequency.
The frequency change per step is approximately 1% (b).
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%


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