NXP Semiconductors UM10301 PCF2123 - Manual (Page 3)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 3 of 52
1. Introduction
The real time clocks from NXP (previously Philips Semiconductors) have a long tradition
and are used in numerous application fields. Starting from applications like VCR, they
have been used in a wide variety or products like burglar alarm systems, water
sprinklers, (platform) timers, e-metering, time-and-attendance monitoring, building
access control, Point-of-Sale terminals, industrial applications, cars and trucks, telecom
applications such as mobile phones and in gaming machines. In those applications they
are used for functions like keeping calendar time, tariff switching, watch-dog, time
stamping or waking up a system periodically to initiate certain actions, for example
making measurements.
This application note deals with the PCF85x3 family with focus on the PCF8563, and with
the more recent additions to the NXP RTC portfolio PCF2123 and PCA2125. The
PCF2123 is an extremely low power RTC which allows fine tuning of the clock using an
offset register (electronic tuning). PCA2125 is targeted at automotive applications. Where
appropriate, comparisons to other devices are made.
PCF2120 is a low power 32.768 kHz oscillator with two integrated oscillator capacitances
and a CLKOUT pin (32.768 kHz only), but without time, date and configuration registers.
This application note is valid for the PCF2120 as well, particulary information with respect
to oscillator, crystal, crystal and capacitor selection and layout guidelines.
Chapters 2 and 3 describe the features of these RTCs and include a comparison of the
various types. Starting from chapter 4 more technical details are described that need to
be understood in order to achieve succesful application of these real time clocks.
Chapters 4 and 5 deal with the power-on reset and voltage-low detection. Chapters 6
through 10 deal with the heart of the RTC; the oscillator, the crystal, crystal and capacitor
selection, accuracy and oscillator tuning. Chapter 11 contains a description of how
century change, leap years and daylight savings time is handled or needs to be handled
in an application. This is followed by some examples in chapter 12 about how to initialize
the RTC and how to set alarm and timer. Providing backup power when the rest of the
system is not powered is covered in chapter 13. In order to make a reliable and accurate
application it is important that the PCB layout is designed carefully and guidelines to
achieve this are listed in chapter 14. This is followed by some further design tips in
chapters 15 and 16 about partial circuit switch down and low power consumption.
Sometimes a component behaves different from what one may initially expect. This does
not imply that it behaves wrongly, but in order to properly deal with it, it is important to be
aware of such behavior. Chapter 17 describes how inaccurate timer performance can be
avoided. Chapter 18 explains why the RTC will loose time if I
2
C and SPI read and write
operations are not finalized within one second of initiating it.
The application note is concluded with a short chapter on trouble shooting.