NXP Semiconductors PCA2125, PCF85x3, PCF2123, PCA8565 user manual Introduction

NXP Semiconductors	UM10301
	User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

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 I2C 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.

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	3 of 52