NXP Semiconductors LPC1751, User Manual, Page: 540 / 808

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UM10360_0 © NXP B.V. 2009. All rights reserved.
User manual Rev. 00.06 — 5 June 2009 540 of 808
NXP Semiconductors
UM10360
Chapter 27: LPC17xx Real-Time Clock (RTC) and backup registers
[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.
6.4.2 Calibration register (CALIBRATION - address 0x4002 4040)
The following register is used to calibrate the time counter.
Table 499. Time Counter relationships and values
Counter Size Enabled by Minimum value Maximum value
Second 6 Clk1 (see
Figure 27–129 )
0 59
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 leap year)
Month 4 Day of Month 1 12
Year 12 Month or day of Year 0 4095
Table 500. Time Counter registers
Name Size Description Access Address
SEC 6 Seconds value in the range of 0 to 59 R/W 0x4002 4020
MIN 6 Minutes value in the range of 0 to 59 R/W 0x4002 4024
HOUR 5 Hours value in the range of 0 to 23 R/W 0x4002 4028
DOM 5 Day of month value in the range of 1 to 28, 29, 30,
or 31 (depending on the month and whether it is a
leap year).
[1]
R/W 0x4002 402C
DOW 3 Day of week value in the range of 0 to 6
[1]
R/W 0x4002 4030
DOY 9 Day of year value in the range of 1 to 365 (366 for
leap years)
[1]
R/W 0x4002 4034
MONTH 4 Month value in the range of 1 to 12 R/W 0x4002 4038
YEAR 12 Year value in the range of 0 to 4095 R/W 0x4002 403C