UM10237_2 © NXP B.V. 2008. All rights reserved.
User manual Rev. 02 — 19 December 2008 655 o f 792
NXP Semiconductors UM10237
Chapter 26: LPC24XX Real-Time Clock (RTC) and battery RAM
6.4 Time Counter Group

The time value consists of the eight counters shown in Table26–577 and Table 26–578.

These counters can be read or written at the locations shown in Table26–578.

[1] These values are simply incremented at the appropriate intervals and reset at the defined overflow point.
They are not calculated and must be correctly initialized in order to be meaningful.

6.4.1 Leap year calculation

The RTC does a simple bit comparison to see if the two lowest order bits of the year

counter are zero. If true, then the RTC considers that year a leap year. The RTC considers

all years evenly divisible by 4 as leap years. This algorithm is accurate from the year 1901

through the year 2099, but fails for the year 2100, which is not a leap year. The only effect

of leap year on the RTC is to alter the length of the month of February for the month, day

of month, and year counters.

Table 576. Consolid ated Time register 2 (CTIME2 - address 0xE002401C) bit description
Bit Symbol Description Reset
value
11:0 Day of Year Day of year value in the range of 1 to 365 (366 for leap years). NA
31:12 - Reserved, user software should not write ones to reserved bits.
The value read from a reserved bit is not defined. NA
Table 577. Time Counter rela tio nships and values)
Counter Size Enabled by Minimum value Maximum value
Second 6 Clk1 (see
Figure 26–134)059
Minute 6 Second 0 59
Hour 5 Minute 0 23
Day of Month 5 Hour 1 28, 29, 30 or 31
Day of Week 3 Hour 0 6
Day of Year 9 Hour 1 365 or 366 (for lea p year)
Month 4 Day of Month 1 12
Year 12 Month or day of Year 0 4095
Table 578. Time Counter registers
Name Size Description Access Address
SEC 6 Seconds value in the range of 0 to 59 R/W 0xE002 4020
MIN 6 Minutes value in the range of 0 to 59 R/W 0xE002 4024
HOUR 5 Hours val ue in the range of 0 to 23 R/W 0xE002 4028
DOM 5 Day of month value in the rang e of 1 to 28, 29, 30,
or 31 (depending on the month and whether it is a
leap year).[1]
R/W 0xE002402C
DOW 3 Day of week value in the range of 0 to 6[1] R/W 0xE002 4030
DOY 9 Day of year value in the range of 1 to 365 (366 for
leap years)[1] R/W 0xE002403 4
MONTH 4 Month value in the range of 1 to 12 R/W 0xE002 4038
YEAR 12 Year value in the range of 0 to 4095 R/W 0xE0 02403C