NXP Semiconductors UM10301 PCF2123 - Manual (Page 25)

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 25 of 52
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
365.25 days 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 29
th
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.