AN10652 pdf datasheet

Real-time clocks (RTC) have been around for a long time and many electronic systems
include one for functions like keeping calendar time, tariff switching, time stamping or
waking up a system periodically to initiate certain actions, for example doing some
measurements. Not all applications need the same level of accuracy, but when high
accuracy is necessary like in applications that rely on real time data where accuracy
better than 5 ppm is needed, a standard RTC does not fulfil these. Time keeping can
only be as accurate as its reference. Several environmental factors like humidity,
vibration and pressure can influence RTC-accuracy but it is mainly the inferior
characteristics of a quartz crystal over temperature which result in deviations if
temperature is changing. Many techniques have been used to improve the timekeeping
accuracy achieved using 32.768 kHz crystals. This application note describes how the
use of an external temperature sensor, placed in a location which will be at or near the
same temperature as the PCF8563 RTC and its crystal can improve accuracy
considerably. If the application is already using a temperature sensor, all that is needed
is some extra firmware without adding to the Bill Of Material (BOM).