Nxp Semiconductors UM10237 用户手册
UM10237_2
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 02 — 19 December 2008
656 of 792
NXP Semiconductors
UM10237
Chapter 26: LPC24XX Real-Time Clock (RTC) and battery RAM
7.
Alarm register group
The alarm registers are shown in
. The values in these registers are
compared with the time counters. If all the unmasked (See
) alarm registers match their corresponding
time counters then an interrupt is generated. The interrupt is cleared when a one is written
to bit one of the Interrupt Location Register (ILR[1]).
to bit one of the Interrupt Location Register (ILR[1]).
8.
Alarm output
The RTC includes an alarm output pin that reflects both the alarm comparisons and
interrupts from the RTC. This pin is in the RTC power domain, and therefore it is available
during all power saving modes as long as power is supplied to VBAT. Since the Alarm pin
combines the alarm and interrupt functions of the RTC, either a specific time/date/etc. or a
periodic interval can be provided to the outside world. For example, a time of day alarm
could be used to tell external circuitry to turn on power to the LPC2400 in order to wake up
from Power-down mode.
interrupts from the RTC. This pin is in the RTC power domain, and therefore it is available
during all power saving modes as long as power is supplied to VBAT. Since the Alarm pin
combines the alarm and interrupt functions of the RTC, either a specific time/date/etc. or a
periodic interval can be provided to the outside world. For example, a time of day alarm
could be used to tell external circuitry to turn on power to the LPC2400 in order to wake up
from Power-down mode.
9.
RTC usage notes
The RTC may be clocked by either the 32.786 kHz RTC oscillator, or by the APB
peripheral clock (PCLK) after adjustment by the reference clock divider.
peripheral clock (PCLK) after adjustment by the reference clock divider.
If the RTC is used, VBAT must be connected to either pin V
DD(3V3)
or an independent
power supply (external battery). Otherwise, VBAT should be left floating. No provision is
made in the LPC2400 to retain RTC status upon the VBAT power loss, or to maintain time
incrementation if the clock source is lost, interrupted, or altered.
made in the LPC2400 to retain RTC status upon the VBAT power loss, or to maintain time
incrementation if the clock source is lost, interrupted, or altered.
Since the RTC operates using one of two available clocks (the APB clock (PCLK) or the
32 kHz signal coming from the RTCX1-2pins), any interruption of the selected clock will
cause the time to drift away from the time value it would have provided otherwise. The
variance could be to actual clock time if the RTC was initialized to that, or simply an error
in elapsed time since the RTC was activated.
32 kHz signal coming from the RTCX1-2pins), any interruption of the selected clock will
cause the time to drift away from the time value it would have provided otherwise. The
variance could be to actual clock time if the RTC was initialized to that, or simply an error
in elapsed time since the RTC was activated.
While the signal from RTCX1-2 pins can be used to supply the RTC clock at anytime,
selecting the PCLK as the RTC clock and entering the Power Down mode will cause a
lapse in the time update. Also, feeding the RTC with the PCLK and altering this timebase
selecting the PCLK as the RTC clock and entering the Power Down mode will cause a
lapse in the time update. Also, feeding the RTC with the PCLK and altering this timebase
Table 579. Alarm registers
Name
Size
Description
Access
Address
ALSEC
6
Alarm value for Seconds
R/W
0xE002 4060
ALMIN
6
Alarm value for Minutes
R/W
0xE002 4064
ALHOUR
5
Alarm value for Hours
R/W
0xE002 4068
ALDOM
5
Alarm value for Day of Month
R/W
0xE002 406C
ALDOW
3
Alarm value for Day of Week
R/W
0xE002 4070
ALDOY
9
Alarm value for Day of Year
R/W
0xE002 4074
ALMON
4
Alarm value for Months
R/W
0xE002 4078
ALYEAR
12
Alarm value for Years
R/W
0xE002 407C