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AN2822 Datasheet(PDF) 5 Page - STMicroelectronics |
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AN2822 Datasheet(HTML) 5 Page - STMicroelectronics |
5 / 19 page 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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