NXP Semiconductors PCF2123, PCF85x3 Century and leap year, Daylight Saving Time, Century tracking

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

11. Century and leap year, Daylight Saving Time

For details on how to implement century tracking and year / leap year tracking, please refer to the datasheets of the respective RTC since register set up differs from type to type. The product comparison in Table 2 shows which parts include century, year and leap year tracking.

11.1 Century tracking

The PCF8563 and PCA8565 contain an 8-bit year register which holds the current year coded in BCD format. These two RTCs further contain a century flag which is toggled when the year counter proceeds from 99 to 00.

PCF8583 and PCF8593 have a four year calendar only and no provision to deal with century change. Also the PCF8573 has no provision to deal with century change.

PCF2123 and PCA2125 contain an 8-bit year register which holds the current year coded in BCD format. There is no century flag. Therefore application firmware needs to deal with century change.

11.2 Year and leap year tracking

A leap year (or intercalary year) is a year containing one extra day in order to keep the calendar year synchronised with the astronomical or seasonal year. Adding an extra day to the calendar every four years compensates for the fact that a solar year is almost six hours longer than 365 days. However, the duration of a solar year is slightly less than

365.25days and therefore some exceptions to this rule are required. Years that are evenly divisible by 100 are not leap years unless they are also evenly divisible by 400. For example, 1600 and 2000 were leap years, but 2100, 2200 and 2300 will not be.

The PCF8563, PCA8565, PCF2123 and PCA2125 all contain an 8-bit year register which can hold values from 00 to 99 in BCD format. These real time clocks compensate for leap years by adding a 29th day to February if the year counter contains a value which is exactly divisible by 4, including the year 00. Therefore in the year 2100 these RTCs add one day to February, where they shouldn’t because it is not a leap year. Until then however leap year correction is correct and automatic.

PCF8583 and PCF8593 have a four year calendar only, which includes leap year tracking. The application firmware needs to deal with keeping track of the actual year.

The PCF8573 has a time counter which counts minutes, hours, days, and months, however, no years. It provides a calendar function in which firmware needs to track the years and which needs to be corrected once every four years to allow for leap year.

11.3 Daylight Saving Time (DST)

There is no provision to deal with day light saving time. Since DST is not implemented equally worldwide and can change often, it is usually better not to implement DST in the RTC but to have the application firmware deal with it. Therefore customers whose applications depend on proper adjustment to DST are advised to design their products such that firmware handles DST changes.

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	25 of 52