AN2678 Datasheet, PDF (1/16 Pages) STMicroelectronics – Extremely accurate timekeeping over

AN2678
Application note
Extremely accurate timekeeping over
temperature using adaptive calibration
Introduction
Typical real-time clocks use common 32,768 Hz watch crystals. These are readily available
and relatively inexpensive, but they suffer a loss of accuracy when operated over wide
temperature ranges. However, the ultra-low power characteristics of 32 KHz oscillators
make them ideal, and necessary, for battery backed applications.
Conversely, the higher speed, AT-cut crystals used with microprocessors have low drift over
a wide temperature range, and can thus provide high accuracy, but their oscillators are not
suitable for backup since they will draw much more current at the frequencies common with
AT-cut crystals.
The purpose of this application note is to show how, using a combination of these crystal
characteristics, users can get high accuracy over a wide temperature range using ST's
M41T82-83-93 series of RTCs. This is accomplished by first measuring the frequency and
then using both the analog and digital calibration features of these devices.
Figure 1. AT versus watch crystal - typical characteristics
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TEMP (˚C)
Watch crystal
AT cut crystal
ai13944
November 2008
Rev 2
1/16
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