AN2604 - Application note

RTC calibration basics

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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:

, where:

●

To = 25 °C ± 5 °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)

Instead of this crude analog method, STM32F10xxx products use a digital calibration

feature that gives the user software control over the calibration procedure, and make it user-

friendly.

Figure 1.

Typical crystal accuracy plotted against temperature

acc

k

T

To

–

()

2

×

=

–100

–80

–60

–40

–20

0

20

40

–30

–20

–10

0

10

20

30

40

50

60

70

80

Temperature (˚C)

Accuracy (ppm)

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