ST AN2604 Application note
AN2604
Application note
STM32F101xx and STM32F103xx RTC calibration
Introduction
The real-time clock (RTC) precision is a requirement in most embedded applications, but
due to external environment – temperature change, frequency variation of the crystal that
clocks the RTC – the RTC precision may not be as accurate as expected.
The RTC embedded in the STM32F101xx and STM32F103xx comes with a digital clock
calibration circuit suitable for manufacturing environments, that allows applications to
compensate for crystal and tempe rature v ariations . This applica tion note discusses the R TC
calibration basics and explai ns how RTC calibration can be us ed to improve timekeeping
accuracy.
August 2007 Rev 1 1/14
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Contents AN2604 - Application note
Contents
1 RTC calibration basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Crystal accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Calculating the needed amount of calibration . . . . . . . . . . . . . . . . . . . 10
3 Calculating calibration over a temperature range . . . . . . . . . . . . . . . . 11
4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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AN2604 - Application note List of tables
List of tables
Table 1. Calibration table: compensation values in ppm and seconds per month (3 0 days) . . . . . . . 7
Table 2. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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List of figures AN2604 - Application note
List of figures
Figure 1. Typical crystal accuracy plotted against temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 2. RTC calibration clock output. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 3. Crystal accuracy over a temperature range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
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AN2604 - Application note RTC calibration basics
1 RTC calibration basics
1.1 Crystal accuracy
The term “quartz-accurate” has become a familiar phrase used to describe the accuracy of
many time keeping functions. Quartz oscillators provide an accuracy far superior to that of
other conventional oscillator designs, but they are not perfect. Quartz crystals are sensitive
to temperature variations. Figure 1 shows the relationship between accuracy (acc),
temperature (T) and curvature (K) for a typical 32.768 kHz crystal. The curve follows the
general formula given below:
acc k T To–()
● To = 25 °C ± 5 °C
● K = –0.040 ppm/°C
×=
Note: The variable K is crystal-dependent, the value indicated here is for the crystal mounted on
the STM3210B-EVAL board. Refer to the crystal manufacturer for more details on this
parameter.
The clocks used in most applications require a high degree of accuracy, and there are
several factors involved in achieving this accuracy. Typically most crystals are compensated
for by adjusting the load capacitance of the oscillator. This method, though effective, has
several disadvantages:
1. it requires external components (trim capacitors)
2. it can increase the oscillator current (a major factor in battery-supported applications)
2
, where:
2
Instead of this crude analog method, STM32F10xxx products use a digital calibration
feature that gives the user software control over the calibrati on proced ur e , a nd make it userfriendly.
Figure 1. Typical crystal accuracy plotted against temperature
40
20
Temperature (˚C)
ai14625
–30 –20 –10
0
0
–20
–40
–60
–80
–100
10 20 30 40 50 60 70 80
Accuracy (ppm)
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