LP3925 Datasheet, PDF (38/51 Pages) Texas Instruments – High Performance Power Management Unit for Handset Applications

Real Time Clock
The Real Time Clock (RTC) block is used for time tracking in
any chip condition. It uses 32 kHz crystal oscillator for accu-
rate timekeeping and is supplied either from system supply in
normal condition or from coin battery when the PMU has no
main power. The RTC gets power from V_COIN, with min guar-
anteed operation above 1.9V. This RTC has following fea-
tures:
• Accurate time counting with fine-grained correction for
long-term accuracies;
• Calendar for years 2000 – 2099 with leap year
compensation and automatic weekday calculation;
• Two highly customizable alarms; and
• Data in software-friendly binary format.
FIGURE 20. RTC Functional Block Diagram
30120444
Calendar and alarm data is presented in following format:
Time Unit
Seconds
Minutes
Hours
Day of Month
Month
Year
Weekday
Register Data
000000 - 111011
000000 - 111011
00000 - 10111
00001 - 11111
0001 - 1100
0000000 - 1100011
0000001 - 1000000 (1-hot code)
Represented Values
0 - 59
0 - 59
0 - 23
1 - 31
January - December
2000 - 2099
Monday - Sunday
RTC calendar and alarm registers are user-writable, except
for calendar weekday registers, which are calculated auto-
matically and are read-only. All RTC registers are in RTC
power domain. The registers are zeroed, if RTC is powered
up. As long as RTC is supplied, the alarm registers will hold
the written data and the calendar will keep track of time.
Writing data to a calendar register will initiate a calendar write
sequence, which will last 3ms. During this time the register's
data should not be read, because it may not be accurate.
Alarms allow creating periodical or one-time events. The re-
sult of an alarm event depends on the PMU state. If PMU is
in standby, then alarm can cause PMU to start up. If PMU is
in working mode, then alarm can create an interrupt.
Alarm event happens if the alarm is activated (ALARM AC-
TIVATED bit is 1) and current RTC time matches the time in
all alarm configuration registers. To exclude a time unit value
from matching check, write the unit's IGNORE bit to 1. If
alarm's weekday is not important or not known, then all week-
day bits should be set to 1.
Interrupt or PMU startup is triggered at the start of an alarm
event.
The RTC also has a time counting correction register. This
can slightly change time counting speed to compensate for
oscillator inaccuracies. Correction events happen 7 times in
an hour. The maximum correction range is 241 ppm (corre-
sponds to code 127) and one step size is approximately 1.9
ppm.
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