STM32F10xxx internal RC oscillator (HSI) calibration.pdf

AN2868

Application note

STM32F10xxx

internal RC oscillator (HSI) calibration

Introduction

The STM32F10xxx microcontrollers offer the possibility of running from an internal RC

oscillator (HSI: high-speed internal oscillator of 8 MHz, typically). At 25 °C, the HSI has an

accuracy of ±1% typically. In the range of –40 to 105 °C, the accuracy value of the RC

frequency increases to the maximum value of ±3%. Temperature therefore has an impact on

RC accuracy.

To compensate for the influence of temperature in the application, the output frequency of

the STM32F10xxx HSI oscillator can be further trimmed by the user runtime calibration

routine to improve the HSI frequency accuracy. This may prove crucial for communication

peripherals.

This application note gives two methods of calibrating the internal RC oscillator: finding the

frequency with the minimum error or finding the maximum allowed frequency error. Both are

implemented by providing an accurate reference source such as an RTC/64 signal or a

mains source signal.

Both methods are based on the same technique: computing of the RC frequency vs. the

reference frequency, computing of the HSI frequency error and setting of the HSITRIM bits

in the RCC_CR register.

February 2009

Rev 1

1/22

www.st.com

Contents

AN2868

Contents

STM32F10xxx’s internal clock: HSI clock . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1

Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

RC calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1

Principle of calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2

Hardware implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.1

Case where RTC/64 is used as the reference frequency: 512 Hz . . . . . . 7

2.2.2

Case where the mains frequency is used as the reference frequency:

50 Hz/60 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Description of the RC calibration library . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1 HSI_FreqMeasure() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 HSI_CalibrateMinError() function . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.3 HSI_CalibrateFixedError() function . . . . . . . . . . . . . . . . . . . . . . . 13

3.4

Calibration demo description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.5

Recommendations on the use of the HSI calibration library . . . . . . . . . . 17

Calibration process performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.1

Accuracy of frequency measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.2

Duration of the calibration process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2/22

AN2868

List of tables

Table 1.

Component values when using the mains frequency as the reference . . . . . . . . . . . . . . . . 8

Table 2.

RC frequency accuracy vs. reference frequency accuracy . . . . . . . . . . . . . . . . . . . . . . . . 18

Table 3.

Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3/22

List of figures

AN2868

List of figures

Figure 1.

Quantification of the reference signal period (RTC signal) . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 2.

Hardware connection using RTC/64 as a source for calibration. . . . . . . . . . . . . . . . . . . . . . 7

Figure 3.

Hardware connection in the AC mains calibration method . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 4.

RC frequency measurement flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 5.

RC calibration flowchart: finding the minimum frequency error . . . . . . . . . . . . . . . . . . . . . 12

Figure 6.

“Spring loop” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 7.

RC calibration flowchart: calibration with the maximum allowed frequency error . . . . . . . . 15

4/22

AN2868

STM32F10xxx’s internal clock: HSI clock

The HSI clock signal is generated from an internal 8 MHz RC oscillator and can be used

directly as a system clock or divided by 2 to be used as a PLL input. The HSI RC oscillator

has the advantage of providing a clock source at low cost (no external components). It also

has a faster startup time than the HSE crystal oscillator. However, even with calibration the

frequency is less accurate than an external crystal oscillator or ceramic resonator. The HSI

signal can also be used as a backup source (auxiliary clock) if the HSE crystal oscillator

fails.

1.1

Calibration

RC oscillator frequencies may vary from one chip to another due to manufacturing process

variations. For this reason, each device is factory-calibrated by ST for 1% accuracy at T A =

25 °C.

After reset, the factory calibration value is loaded into the HSICAL[7:0] bits in the clock

control register RCC_CR.

User calibration is performed by setting the HSITRIM[4:0] bits in the RCC_CR register.

These bits can be programmed to take into account voltage and temperature variations that

affect the frequency of the internal HSI RC oscillator. The default value is 16, which, when

added to the HSICAL value, should trim the HSI to 8 MHz ±1%. The trimming step (F hsitrim )

is around 40 kHz between two consecutive HSICAL steps.

5/22

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