AN4067
Application note
Calibrating STM32F0x1, STM32F0x2 and STM32F0x8 lines
internal RC oscillators
Introduction
Microcontrollers of STM32F0x1, STM32F0x2 and STM32F0x8 product lines, named
STM32F0xx in this document, have internalRC oscillators that can be selected as the
system clock source, and a High speed internal RC oscillator dedicated to ADC. These are
known as the HSI (High-speed Internal 8 MHz), HSI14 (High-speed Internal 14 MHz) and
HSI48 (High-speed Internal 48 MHz) oscillators.
The operating temperature has an impact on the accuracy of the RC oscillators. At 25 °C,
the HSI, HSI14 and HSI48 oscillators have an accuracy of ±1% typically, but in the
temperature range of -40 to 105 °C, the accuracy decreases.
To compensate for the influence of temperature on internal RC oscillators accuracy, the
STM32F0xx microcontrollers have built-in features to allow users to calibrate the HSI and
HSI14 oscillators and measure the LSI (Low-speed internal) oscillator frequency.
This application note focuses on how to calibrate internal RC oscillators: HSI, HSI14 and
HSI48. Three methods are presented:
Method 1 consists in finding the frequency with the minimum error.
Method 2 consists in finding the maximum allowed frequency error.
Method 3 consists in preparing the table of calibration values that is later used for fast
calibration.
All three methods 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.
Table 1 lists the microcontrollers and software concerned by this application note.
Table 1. Applicable products and software
Type
February 2015
Part number and product line
Microcontrollers
STM32F0x1 line, STM32F0x2 line and STM32F0x8 line.
Software
STSW-STM32132
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Contents
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Contents
1
Applicable STM32F0xx product families . . . . . . . . . . . . . . . . . . . . . . . . . 5
2
STM32F0xx system clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3
Internal RC oscillator calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1
3.2
3.3
Calibration principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1
HSI calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.2
HSI14 calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.3
HSI48 calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Hardware implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
3.2.1
Case where LSE is used as the reference frequency to measure HSI . 11
3.2.2
Case where another source is used as the reference frequency to
measure HSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2.3
Measure the HSI14 frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Description of the internal oscillator calibration firmware . . . . . . . . . . . . . 13
3.3.1
Internal oscillator calibration with minimum error . . . . . . . . . . . . . . . . . . 14
3.3.2
HSI calibration with fixed error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.3.3
Internal oscillator frequency measurement . . . . . . . . . . . . . . . . . . . . . . 18
3.3.4
Internal oscillator calibration using calibration curve . . . . . . . . . . . . . . . 20
3.4
Recommendations on the use of the calibration library . . . . . . . . . . . . . . 20
3.5
Calibration process performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.5.1
4
Duration of the calibration process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Low-speed internal oscillator measurement . . . . . . . . . . . . . . . . . . . . 22
4.1
Measurement principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2
Description of the internal oscillator measurement firmware . . . . . . . . . . 23
5
Internal oscillator calibration/measurement example description . . . 24
6
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
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List of tables
List of tables
Table 1.
Table 2.
Table 3.
Applicable products and software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Applicable STM32F0xx product families. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
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List of figures
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List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
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Simplified clock tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
HSI oscillator trimming characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
HSI14 oscillator trimming characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
HSI48 oscillator trimming capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Timing diagram of internal oscillator calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Timing diagram of internal oscillator calibration HSI14. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Hardware connection using LSE as the reference frequency. . . . . . . . . . . . . . . . . . . . . . . 12
Hardware connection using external reference frequency . . . . . . . . . . . . . . . . . . . . . . . . . 12
Hardware connection For HSI14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Internal oscillator calibration: finding the minimum frequency error . . . . . . . . . . . . . . . . . . 15
“Spring loop” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
HSI calibration flowchart: maximum allowed frequency error . . . . . . . . . . . . . . . . . . . . . . . 17
Internal oscillator frequency measurement flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
LSI measurement configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Timing diagram of an internal RC oscillator measurement . . . . . . . . . . . . . . . . . . . . . . . . . 23
Internal oscillator calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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Applicable STM32F0xx product families
Applicable STM32F0xx product families
Table 2 provides the list of STM32F0xx product families and respective memory size to
which this application note applies.
Table 2. Applicable STM32F0xx product families
Note:
STM32F0xx family
Flash memory size
STM32F030
up to 256 Kbytes
STM32F031
up to 32 Kbytes
STM32F038
up to 32 Kbytes
STM32F042
up to 32 Kbytes
STM32F048
up to 32 Kbytes
STM32F051
up to 64 Kbytes
STM32F058
up to 32 Kbytes
STM32F070
up to 128 Kbytes
STM32F071
up to 128 Kbytes
STM32F072
up to 128 Kbytes
STM32F078
up to 128 Kbytes
STM32F091
up to 256 Kbytes
STM32F098
up to 256 Kbytes
All the information related to HSI48 is only applicable to the following product families:
STM32F042, STM32F048, STM32F071, STM32F072, STM32F078, STM32F091 and
STM32F098.
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STM32F0xx system clock
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STM32F0xx system clock
The STM32F0xx microcontroller family has various clock sources that can be used to drive
the system clock:
An 8-MHz High-speed internal (HSI) RC oscillator clock
A 4- to 32-MHz High-speed external (HSE) oscillator clock
A Phase-locked loop (PLL) that is clocked by HSI or HSE or HSI48 oscillators
A 48-MHz High-speed internal (HSI48) RC oscillator clock
Typically, the High-speed internal (HSI) RC oscillator has a frequency of 8 MHz or 48 MHz
(HSI48) and consumes 80 µA (HSI) or 312 µA (HSI48).
The internal RC oscillator (HSI) has the advantage of providing a low-cost clock source (no
external components required). It also has a faster startup time and a lower power
consumption than the external oscillator. The HSI oscillator can be calibrated to improve its
accuracy. But even with calibration, the internal RC oscillator frequency is less accurate
than the frequency of an external crystal oscillator or a ceramic resonator (tens of ppm).
Note:
The internal 48 MHz RC oscillator is mainly used to provide a high precision clock to the
USB peripheral by means of a special Clock Recovery System (CRS) circuitry, which uses
the USB SOF signal or the LSE or an external signal to automatically adjust the oscillator
frequency on-the-fly, through a very small steps.
Figure 1. Simplified clock tree
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The STM32F0xx devices also have three secondary clock sources (that cannot be used as
system clock sources):
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A 40 kHz Low-Speed Internal (LSI) RC which is designed to drive the independent
watchdog and optionally the Real time clock (RTC). The LSI oscillator cannot be
calibrated, but can be measured to evaluate frequency deviations (due to temperature
and voltage changes).
A 32.768 kHz Low-speed external crystal (LSE crystal) which optionally drives the Real
time clock (RTC)
A 14 MHz high speed internal RC (HSI14) dedicated to ADC.
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Internal RC oscillator calibration
Internal RC oscillator calibration
The frequency of the internal RC oscillators may vary from one chip to another due to
manufacturing process variations. For this reason, HSI and HSI14 RC oscillators are
factory-calibrated by STMicroelectronics to have a 1% accuracy at TA = 25 °C. HSI48
oscillator is factory calibrated to have ~3% accuracy at TA = 25 °C.After reset, the factory
calibration value is automatically loaded in the internal calibration bits.
The frequency of the internal RC oscillators can be fine-tuned to achieve a better accuracy
with wider temperature and supply voltage ranges. The trimming bits are used for this
purpose.
For the HSI oscillator, the calibration value is loaded in HSICAL[7:0] bits after reset. Five
trimming bits HSITRIM[4:0] are used for fine-tuning. The default trimming value is 16. An
increase/decrease in this trimming value causes an increase/decrease in HSI frequency.
The HSI oscillator is fine-tuned in steps of 1% (around 40 kHz).
Writing a trimming value, in the range of 17 to 31, increases the HSI frequency.
Writing a trimming value, in the range of 0 to 15, decreases the HSI frequency.
Writing a trimming value, equal to 16, causes the HSI frequency to keep its default
value.
Figure 2 shows the HSI oscillator behavior versus the calibration value. The HSI oscillator
frequency increases with the calibration value (calibration value = default HSICAL[7:0] +
HSITRIM[4:0]), except at modulo 16. At these calibration values, the negative steps can
reach three times the positive steps.
Figure 2. HSI oscillator trimming characteristics
For the HSI14 (14 MHz) oscillator, The calibration value is loaded in HSI14CAL[7:0] bits
after reset. Five trimming bits HSI14TRIM[4:0] are used for fine-tuning. The default trimming
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value is 16. An increase/decrease in this trimming value causes an increase/decrease in the
HSI14 frequency. The HSI14 oscillator is fine-tuned in steps of 1% (around 50 kHz).
Writing a trimming value, in the range of 17 to 31, increases the HSI14 frequency.
Writing a trimming value, in the range of 0 to 15, decreases the HSI14 frequency.
Writing a trimming value, equal to 16, causes the HSI14 frequency to keep its default
value.
Figure 3 shows the HSI14 oscillator behavior versus the calibration value. The HSI14
oscillator frequency increases with the calibration value (calibration value = default
HSI14CAL[7:0] + HSI14TRIM[4:0]), except at modulo 16. At these calibration values, the
negative steps can reach three times the positive steps.
Figure 3. HSI14 oscillator trimming characteristics
For the HSI48 oscillator, the calibration value is loaded in HSI48CAL[7:0] bits after reset. Six
trimming bits TRIM [5:0] (in CRS control register) are used for fine-tuning. The default
trimming value is 32. An increase/decrease in this trimming value causes an
increase/decrease in HSI48 frequency.
The HSI48 oscillator is fine-tuned in steps of 1% (around 67 kHz).
Writing a trimming value, in the range of 33 to 63, increases the HSI48 frequency.
Writing a trimming value, in the range of 0 to 31, decreases the HSI48 frequency.
Writing a trimming value, equal to 32, causes the HSI48 frequency to keep its default
value.
Figure 4 shows the HSI48 oscillator behavior versus the calibration value. The HSI48
oscillator frequency increases with the calibration value (calibration value = default
HSI48CAL[7:0] + TRIM[5:0]
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Internal RC oscillator calibration
Figure 4. HSI48 oscillator trimming capability
3.1
Calibration principle
3.1.1
HSI calibration
The calibration principle consists in:
1.
Setting the HSI as the system clock source
2.
Measuring the internal RC oscillator (HSI) frequency for each trimming value
3.
Computing the frequency error for each trimming value
4.
Setting the trimming bits with the optimum value (corresponding to the lowest
frequency error).
The internal oscillator frequency is not measured directly but is computed from the number
of clock pulses counted using a timer compared with the typical value. To do this, a very
accurate reference frequency must be available such as the LSE frequency provided by the
external 32.768 kHz crystal or the 50 Hz/60 Hz of the external signal (refer to Section 3.2.1
and Section 3.2.2).
Figure 5 shows how the reference signal period is measured in number of timer counts.
Figure 5. Timing diagram of internal oscillator calibration
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After enabling the timer counter, when the first rising edge of the reference signal occurs,
the timer counter value is captured and stored in IC1ReadValue1. At the second rising edge,
the timer counter is captured again and stored in IC1ReadValue2. The elapsed time
between two consecutive rising edges (IC1ReadValue2 - IC1ReadValue1) represents an
entire period of the reference signal.
Since the timer counter is clocked by the system clock (internal RC oscillator HSI), the real
frequency generated by the internal RC oscillator versus the reference signal is given by:
Measuredfrequency = IC1ReadValue2 – IC1ReadValue1 referencefrequency
The error (in Hz) is computed as the absolute value of the difference between the measured
frequency and the typical value.
Hence, the internal oscillator frequency error is expressed as:
Error Hz = Measuredfrequency – typicalvalue
After calculating the error for each trimming value, the algorithm determines the optimum
trimming value (that corresponds to the nearest frequency to typical value) to be
programmed in the trimming bits (refer to Section 3.3: Description of the internal oscillator
calibration firmware for more details).
3.1.2
HSI14 calibration
The calibration principle consists in:
1.
Setting the PLL, clocked by the HSI (after calibrating the HSI), as the system clock
source.
2.
Measuring the internal RC oscillator (HSI14) frequency for each trimming value.
3.
Computing the frequency error for each trimming value.
4.
Finally, setting the trimming bits with the optimum value (corresponding to the lowest
frequency error).
Figure 6 shows how the reference signal period is measured in number of timer counts.
Figure 6. Timing diagram of internal oscillator calibration HSI14
After enabling the timer counter, when the first rising edge of the reference signal occurs,
the timer counter value is captured and stored in IC1ReadValue1. At the second rising edge,
the timer counter is captured again and stored in IC1ReadValue2. The elapsed time
between two consecutive rising edges (IC1ReadValue2 - IC1ReadValue1) represents an
entire period of the reference signal.
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Internal RC oscillator calibration
The real frequency generated by the internal RC oscillator HSI14 is given by:
Measuredfrequency = SystemClock IC1ReadValue2 – IC1ReadValue1
The error (in Hz) is computed as the absolute value of the difference between the measured
frequency and the typical value.
Hence, the internal oscillator frequency error is expressed as:
Error Hz = Measuredfrequency – typicalvalue
After calculating the error for each trimming value, the algorithm determines the optimum
trimming value (that corresponds to the nearest frequency to the typical value) to be
programmed in the trimming bits (refer to Section 3.3: Description of the internal oscillator
calibration firmware for more details).
Note:
If the HSE clock is available it is highly recommended to use it as the reference clock
instead of HSI, to benefit from its higher accuracy.
In order to improve the accuracy, the timer prescaler can be used to increase the ratio
between HSI14 frequency and the reference frequency.
3.1.3
HSI48 calibration
HSI48 can be calibrated in the same way as HSI or HSI14. However, STM32F0xx product
family implements CRS (Clock recovery system) that is capable of doing automatic
adjustment of the oscillator trimming based on the comparison with a selectable
synchronization signal.
Internally HSI48 implements a 16-bit down/up counter which step by step increments or
decrements of TRIM value until it reaches the expected frequency value based on LSE
reference clock or the USB SOF signal.
The HSI48 calibration using CRS can be run in fully automatic mode. To speed up the
process, the CRS can be used for measurement of the actual error, and set trim value with
pre-calculated value. This process can repeated once or twice as the curve might not be
linear. When requested frequency is reached automatic calibration can be activated for
further smooth calibration, e.g. compensating temperature changes.
3.2
Hardware implementation
3.2.1
Case where LSE is used as the reference frequency to measure HSI
The STM32F0xx offers a useful feature, that is the ability to connect internally and indirectly
the Low-speed external (LSE) oscillator to Timer 14 channel 1. Thus, the LSE clock can be
used as the reference signal for internal oscillator calibration and no additional hardware
connections are required. Only the LSE oscillator should be connected to OSC32_IN and
OSC32_OUT.
Figure 7 shows the hardware connections needed for internal oscillators calibration, using
LSE as an accurate frequency source for calibration.
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Figure 7. Hardware connection using LSE as the reference frequency
1. TI1_RPM: TIM14 input 1 remapping
2. TI1_RPM: Timer input 1
3. IC1PS: Input capture 1 prescaled
Note:
It is also possible to use MCO for this purpose.
3.2.2
Case where another source is used as the reference frequency to
measure HSI
Any signal with an accurate frequency can be used for the internal oscillator calibration, and
the external signal is one of the possibilities.
As shown in Figure 8 below, the reference signal should be connected to Timer 14
channel 1.
Figure 8. Hardware connection using external reference frequency
1. TI1_RPM: TIM14 input 1 remapping
2. TI1: Timer input 1
3. IC1PS: Input capture 1 prescaled
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Note:
Internal RC oscillator calibration
When using an external signal as a reference, another timer can be used instead of TIM14.
The firmware provided with this application note uses TIM14.
Refer to the application note AN2868 “Internal RC oscillator (HSI) calibration” for more
details about using the external signal frequency for calibration.
3.2.3
Measure the HSI14 frequency
The STM32F0xx product family offers a useful feature, that is the ability to connect internally
and indirectly the High-speed internal (HSI14) oscillator to Timer 14 channel 1.
Figure 7 shows the hardware connections needed for internal oscillators calibration.
Figure 9. Hardware connection For HSI14
1. TI1_RPM: TIM14 input 1 remapping
2. TI1: Timer input 1
3. IC1PS: Input capture 1 prescaled
3.3
Description of the internal oscillator calibration firmware
The internal RC oscillator calibration firmware provided with this application note includes 4
major functions:
uint32_t HSI_CalibrateMinError(void)
ErrorStatus HSI_CalibrateFixedError(uint32_t MaxAllowedError,
uint32_t* Freq)
uint32_t HSI14_CalibrateMinError(void)
ErrorStatus HSI14_CalibrateFixedError(uint32_t MaxAllowedError,
uint32_t* Freq)
ErrorStatus HSI_CalibrateCurve( uint32_t* Freq );
void HSI_GetCurve( void );
ErrorStatus HSI14_CalibrateCurve(uint32_t* Freq);
void HSI14_GetCurve(void);
uint32_t HSI48_FreqMeasure( void );
uint32_t HSI48_ManualTrimming( void );
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3.3.1
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Internal oscillator calibration with minimum error
The HSI_CalibrateMinError() function calibrates the internal oscillator (HSI) to have
the frequency nearest to the typical value. It measures all frequencies for different trimming
values and provides the trimming value that corresponds to the frequency with the minimum
error. The trimming value thus obtained is programmed in the trimming bits.
After calibration, the HSI_CalibrateMinError() function returns the internal oscillator
frequency value as an unsigned 32-bit integer (uint32_t).
The flowchart in Figure 10 provides the algorithm for this function.
Example
uint32_t InternOscAfterCalib = 0;
{
......
/* Get the internal oscillator (HSI) value after calibration */
InternOscAfterCalib = HSI_CalibrateMinError();
}
In case of an HSI14 calibration, the HSI14_CalibrateMinError() function should be used
instead of HSI_CalibrateMinError().
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Internal RC oscillator calibration
Figure 10. Internal oscillator calibration: finding the minimum frequency error
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1. If the system clock source is HSI, the trimming bits have a 5-bit length and the number of steps is 32.
2. The frequency measurement is detailed in Section 3.3.3.
3.3.2
HSI calibration with fixed error
The HSI_CalibrateFixedError() function is provided to calibrate the HSI oscillator with a
maximum allowed frequency error. It is configured by the user as an absolute value given in
Hertz (the first parameter: MaxAllowedError). This function is the same as
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HSI_CalibrateMinError() (refer to ErrorStatus
HSI14_CalibrateFixedError(uint32_t MaxAllowedError, uint32_t* Freq)), but it
searches for the frequency that has an error (in absolute value) lower than or equal to
MaxAllowedError.
If it finds this frequency, it stops searching and configures the trimming bits
HSITRIM[4:0] according to this frequency and returns SUCCESS, meaning that the
calibration operation has succeeded.
Otherwise, it continues searching for it until the HSITRIM bits = 31 (32nd frequency). It
then sets the trimming bits HSITRIM[4:0] to the default calibration value and returns
ERROR, meaning that the calibration has failed and did not find any frequency with an
error lower than or equal to MaxAllowedError.
The frequency measurement starts with HSTRIM = 16. The HSITRIM value is
computed in loops to find the next value. That is, the HSITRIM value starts from 16,
then goes to the next value to the left, then to the next to the right, then to the second to
the left and so on until it reaches 31, forming a “spring loop” (as shown in Figure 11).
This algorithm is based on the fact that the probability of finding the frequency that has
the minimum error increases when the HSITRIM[4:0] value tends to 16. This algorithm
is implemented so as to minimize the time consumed by the calibration process.
Figure 11. “Spring loop”
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The second parameter is used to get the frequency (in Hertz) after calibration in the form of
an unsigned 32-bit integer (unit32_t).
In case of an HSI14 calibration, the HSI14_CalibrateFixedError() function should be used
instead of HSI_CalibrateFixedError().
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Internal RC oscillator calibration
The flowchart in Figure 12 provides the algorithm for this function.
Figure 12. HSI calibration flowchart: maximum allowed frequency error
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1. The frequency measurement is detailed in Section 3.3.3: Internal oscillator frequency measurement.
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Internal RC oscillator calibration
3.3.3
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Internal oscillator frequency measurement
The internal oscillator frequency measurement is performed by Timer 14 capture interrupt.
In the timer TIM14 ISR, an entire period of internal oscillator frequency is computed. The
number of periods to be measured for each trimming value is configurable by the user in the
InternOscCalibration.h file, as follows:
#define NUMBER_OF_LOOPS 50
The averaging method is used to minimize frequency error measurements. So, if the
counter of loops reaches NUMBER_OF_LOOPS, the average of all measured frequencies is
computed.
Users can easily configure the frequency of the reference source. It is defined in the
InternOscCalibration.h header file, as follows:
If the LSE clock is used as the reference frequency, uncomment the line below to make
sure the LSE is configured and internally connected to Timer 14 channel 1:
#define USE_REFERENCE_LSE
If the reference frequency is an external signal equal to 50 Hz, then comment the line
above and define the reference frequency as shown below:
#define REFERENCE_FREQUENCY (uint32_t)50 /* The reference
frequency value in Hz */
The computation of the frequency measurements does not depend on the duty cycle of the
source reference signal. It depends on its frequency since the capture 1 interrupt is
configured to occur on every rising edge of the reference signal (refer to Figure 5).
Note:
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Figure 3.3.4 provides the frequency measurement algorithm.
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Internal RC oscillator calibration
Figure 13. Internal oscillator frequency measurement flowchart
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Internal RC oscillator calibration
3.3.4
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Internal oscillator calibration using calibration curve
Both the minimal and fixed error methods can take some time due to the high number of
measurements to run (for example 32 measurement for HSI with the minimal error method).
First, for all trimming values, the difference between the corresponding and the requested
frequency is measured and stored in a table. This uses the same principle as the minimal
error method. When it is necessary to calibrate the internal oscillator function
HSI_CalibrateCurve(), only the actual frequency is measured and the appropriate value to
compensate the difference is taken from the table.
In case of an HSI14 calibration, the HSI14_ GetCurve () and HSI14_CalibrateCurve()
functions should be used instead of HSI_GetCurve() and HSI_CalibrateCurve().
3.4
Recommendations on the use of the calibration library
1.
If the external signal frequency is lower than system clock / 65535, the TIM14 counter
prescaler should be used to support low frequencies.
2.
If the external signal frequency is higher than system clock / 100, TIM14 input capture
prescaler (divider) should be used to support high frequencies.
3.
It is recommended to stop all application activities before the calibration process, and
to restart them after calling the calibration functions.
Therefore, the application has to stop the communications, the ADC measurements
and any other processes (except when using the ADC for the calibration, refer to Step
6. below).
These processes normally use clock configurations that are different from those used
in the calibration process. Otherwise, errors might be introduced in the application:
errors while reading/sending frames, ADC reading errors since the sampling time has
changed, and so on.
4.
The internal RC oscillator calibration firmware uses the following peripherals: Reset
and Clock Control (for trimming internal RC oscillators), Timer 14 (for measuring
internal RC oscillators). Therefore, it is recommended to reconfigure these peripherals
(if used in the application) after running the calibration routine.
5.
Real-time calibration vs. temperature can be used when the ambient temperature
changes noticeably while the application is running. The internal temperature sensor
can be used with the ADC watchdog with two thresholds. Each time an ADC watchdog
interrupt occurs, a new calibration process has to be performed and the two thresholds
are updated according to the current temperature (this feature is not implemented in
the firmware provided with this application note):
Threshold_High = CurrentTemperatureValue + TemperatureOffset
Threshold_Low = CurrentTemperatureValue – TemperatureOffset
6.
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It might happen that with change of operation conditions (e.g. surrounding
temperature) the calibration curve can change. From this reason it's recommended to
measure from time to time (or when the condition change) again to keep working with
correct values.
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Internal RC oscillator calibration
3.5
Calibration process performance
3.5.1
Duration of the calibration process
The duration of the calibration process depends on:
1.
the frequency of the reference signal (prescaled value) “REFERENCE_FREQUENCY”,
2.
the number of measured periods per trimming value “NUMBER_OF_LOOPS”,
3.
the number of measured frequencies during the calibration process “number of steps”.
Once the peripherals are configured and ready (mainly the LSE oscillator), the duration of
the calibration process is approximated by:
duration = (2 x (NUMBER_OF_LOOPS + 1) x number of steps) / REFERENCE_FREQUENCY
If the calibration process is run with a minimum frequency error for an HSI oscillator
(HSI_CalibrateMinError()), the number of steps is equal to 32. If the LSE oscillator is
used as the reference frequency (REFERENCE_FREQUENCY = LSE value / Input capture
prescaler = 32768/8 = 4096 Hz) and the selected number of measured periods is 10, the
calibration consumes approximately:
duration = (2 x 51 x 32) / 4096 = 797 ms
The duration of the calibration process with a maximum allowed error is lower than or equal
to the duration of calibration when using the minimum frequency error process.
Note:
Multiplying by 2 in the duration formula above is due to the fact that there is no
synchronization between the reference signal and the start of counting by the timer.
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Low-speed internal oscillator measurement
4
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Low-speed internal oscillator measurement
The internal LSI RC oscillator is a low-power clock source. In the STM32F0x microcontroller
family, an internal and indirect connection is provided between the internal RC oscillator LSI
and the embedded timer TIM14 to facilitate the measurement procedure.
4.1
Measurement principle
The internal RC oscillator measurement procedure consists in running the timer counter
using the HSI clock, configuring the internal RC oscillator LSI as the clock source for RTC,
configuring the timer in Input capture mode and then connecting the RTC to the timer.
Figure 14 shows the configuration used to perform an LSI measurement.
Figure 14. LSI measurement configuration
After enabling the timer counter, when the first rising edge of the internal oscillator signal to
be measured occurs, the timer counter value is captured and then stored in
IC1ReadValue1. On the second rising edge, the timer counter is captured again and stored
in IC1ReadValue2. The elapsed time between two consecutive rising edges of the clock
represents an entire period. Figure 15 shows the timing diagram of an internal RC oscillator
measurement.
Note:
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MCO can be also used for this purpose.
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Low-speed internal oscillator measurement
Figure 15. Timing diagram of an internal RC oscillator measurement
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The internal oscillator frequency value is computed as shown by the following formula:
internal oscillator frequency = HSI_Value / Capture
where:
HSI_Value is the HSI frequency value: typical value is 8 MHz,
Capture represents an entire period of internal RC oscillator LSI: IC1ReadValue2 IC1ReadValue1.
As users can conclude from the formula above, the frequency measurement accuracy
depends on the HSI frequency accuracy. Consequently, if a reference signal is available,
users can run the internal RC oscillator calibration routine described in Section 3: Internal
RC oscillator calibration before performing the internal RC oscillator measurement
procedure.
The input capture prescaler can be used for better measurement accuracy so the formula
above becomes:
LSI_Frequency = InputCapturePrescaler * HSI_Value / Capture_Value.
Note:
If the HSE clock is available it is highly recommended to use it as the reference clock
instead of HSI, to benefit from its higher accuracy.
4.2
Description of the internal oscillator measurement firmware
The internal oscillator measurement firmware provided with this application note includes
one C source files:
LSIMeasurement.c performing LSI frequency measurement using LSI_FreqMeasure()
function
The internal RC oscillator LSI is measured for a predefined number of periods. Then it
returns the average value to minimize the error of the measured frequency.
Users can change this parameter (number of LSI periods) in the lsi_measurement.h file:
#define LSI_PERIOD_NUMBERS 10
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Internal oscillator calibration/measurement example description
5
AN4067
Internal oscillator calibration/measurement example
description
The example provided with this application note shows the ability of the firmware to calibrate
the internal RC oscillators (HSI, HSI14 and HSI48) and explains how to use it to measure
the internal RC oscillators (HSI14 and LSI) of the STM32F0xx microcontroller.
In this example:
After system reset, the HSI is selected to be used as the system clock source.
The HSI is calibrated using the LSE oscillator as a reference clock.
When the HSI oscillator has been calibrated, the PLL (clocked by HSI) is configured to
48 MHz and used as the system clock source.
Then, the HSI14 is calibrated according to HSI precision.
HSI48 is measured and calibrated.
Then HSI48 is selected as system clock, CRS peripheral initialized and run.
After that, the LSI frequency is measured.
Finally, the measured frequency of the different oscillators is displayed on the
STM320518-EVAL board's LCD, as shown in Figure 16.
By default, the example uses the minimum error method to calibrate the HSI and HSI14
oscillators.
To run the calibration process that provides the frequency with fixed error, users have to
comment out the following define in the main.c file:
#define CALIBRATION_MIN_ERROR
Figure 16. Internal oscillator calibration
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Conclusion
Conclusion
Even if internal RC oscillators are factory-calibrated, the user should calibrate them in the
operating environment, when a high-accuracy clock is required in the application.
This application note provides two routines:
High-speed internal oscillator (HSI, HSI14 and HSI48) calibration: how to fine-tune
the oscillator to the typical value.
Low-speed internal oscillator measurement: how to get the “exact” LSI frequency
value.
Several frequency sources can be used to calibrate the internal RC oscillator (HSI): LSE
crystal, AC line, etc. Whatever the reference frequency source, the internal oscillator
calibration principle is the same: a reference signal must be provided to be measured by a
timer. The higher the accuracy of the reference signal frequency, the better the accuracy of
the internal oscillator frequency measurement. The error is computed as the absolute value
of the typical frequency value and the measured one for each trimming value. From this, the
calibration value is calculated and then programmed in the trimming bits.
The second section of this application note is about the measurement of LSI oscillator. The
internal connection between internal oscillators and embedded timers in the STM32F0xx
microcontroller family is used for this purpose. The timer is clocked using the system clock
source and configured in the Input capture mode. The captured time between two
consecutive rising edges of internal oscillator represents an entire period.
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Revision history
7
AN4067
Revision history
Table 3. Document revision history
Date
Revision
02-Jul-2012
1
Initial release.
2
Extended the document applicability to all STM32F0x1, STM32F0x2
and STM32F0x8 product lines.
– Added HSI48.
– Added Section 1: Applicable STM32F0xx product families.
– Updated Section 3.3: Description of the internal oscillator
calibration firmware.
– Added Section 3.1.3: HSI48 calibration.
– Added Section 3.3.4: Internal oscillator calibration using calibration
curve.
– Updated Section 3.4: Recommendations on the use of the
calibration library.
– Updated Figure 12: HSI calibration flowchart: maximum allowed
frequency error.
– Updated Figure 13: Internal oscillator frequency measurement
flowchart.
– Updated Figure 16: Internal oscillator calibration.
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Changes
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