ISL12022
14
FN6659.3
November 22, 2011
Real Time Clock Registers
Addresses [00h to 06h]
RTC REGISTERS (SC, MN, HR, DT, MO, YR, DW)
These registers depict BCD representations of the time. As such,
SC (Seconds) and MN (Minutes) range from 0 to 59, HR (Hour)
can either be a 12-hour or 24-hour mode, DT (Date) is 1 to 31,
MO (Month) is 1 to 12, YR (Year) is 0 to 99, and DW (Day of the
Week) is 0 to 6.
The DW register provides a Day of the Week status and uses three
bits DW2 to DW0 to represent the seven days of the week. The
counter advances in the cycle 0-1-2-3-4-5-6-0-1-2-…
The assignment of a numerical value to a specific day of the
week is arbitrary and may be decided by the system software
designer. The default value is defined as “0”.
24 HOUR TIME
If the MIL bit of the HR register is “1”, the RTC uses a
24-hour format. If the MIL bit is “0”, the RTC uses a 12-hour
format and HR21 bit functions as an AM/PM indicator with a “1”
representing PM. The clock defaults to 12-hour format time with
HR21 = “0”.
LEAP YEARS
Leap years add the day February 29 and are defined as those years
that are divisible by 4. Years divisible by 100 are not leap years,
unless they are also divisible by 400. This means that the year 2000
is a leap year and the year 2100 is not. The ISL12022 does not
correct for the leap year in the year 2100.
Control and Status Registers
(CSR)
Addresses [07h to 0Fh]
The Control and Status Registers consist of the Status Register,
Interrupt and Alarm Register, Analog Trimming and Digital
Trimming Registers.
Status Register (SR)
The Status Register is located in the memory map at address
07h. This is a volatile register that provides either control or
status of RTC failure (RTCF), Battery Level Monitor (LBAT85,
LBAT75), alarm trigger, Daylight Savings Time, crystal oscillator
enable and temperature conversion in progress bit.
BUSY BIT (BUSY)
Busy Bit indicates temperature sensing is in progress. In this
mode, Alpha, Beta and ITRO registers are disabled and cannot be
accessed.
OSCILLATOR FAIL BIT (OSCF)
Oscillator Fail Bit indicates that the oscillator has failed. The
oscillator frequency is either zero or very far from the desired
32.768kHz due to failure, PC board contamination or mechanical
issues.
DAYLIGHT SAVINGS TIME CHANGE BIT (DSTADJ)
DSTADJ is the Daylight Savings Time Adjusted Bit. It indicates the
daylight saving time forward adjustment has happened. If a DST
Forward event happens, DSTADJ will be set to “1”. The DSTADJ bit
will stay high after the DSTFD event happens, and will be reset to
“0” when the DST Reverse event happens. It is read-only and
cannot be written. Setting time during a DST forward period will
not set this bit to “1”.
The DSTE bit must be enabled when the RTC time is more than
one hour before the DST Forward or DST Reverse event time
setting, or the DST event correction will not happen.
DSTADJ is reset to “0” upon power-up. It will reset to ”0” when the
DSTE bit in Register 15h is set to “0” (DST disabled), but no time
adjustment will happen.
ALARM BIT (ALM)
This bit announces if the alarm matches the real time clock. If
there is a match, the respective bit is set to “1”. This bit can be
manually reset to “0” by the user or automatically reset by
enabling the auto-reset bit (see ARST bit). A write to this bit in the
SR can only set it to “0”, not “1”. An alarm bit that is set by an
alarm occurring during an SR read operation will remain set after
the read operation is complete.
LOW VDD INDICATOR BIT (LVDD)
This bit indicates when VDD has dropped below the pre-selected
trip level (Brownout Mode). The trip points for the brownout levels
are selected by three bits: VDD Trip2, VDD Trip1 and VDD Trip0 in
PWR_ VDD registers. The LVDD detection is only enabled in VDD
mode and the detection happens in real time. The LVDD bit is set
whenever the VDD has dropped below the pre-selected trip level,
and self clears whenever the VDD is above the pre-selected trip
level.
LOW BATTERY INDICATOR 85% BIT (LBAT85)
In Normal Mode (VDD), this bit indicates when the battery level
has dropped below the pre-selected trip levels. The trip points are
selected by three bits: VB85Tp2, VB85Tp1 and VB85Tp0 in the
PWR_VBAT registers. The LBAT85 detection happens
automatically once every minute when seconds register reaches
59. The detection can also be manually triggered by setting the
TSE bit in BETA register to “1”. The LBAT85 bit is set when the
VBAT has dropped below the pre-selected trip level, and will self
clear when the VBAT is above the pre-selected trip level at the
next detection cycle either by manual or automatic trigger.
In Battery Mode (VBAT), this bit indicates the device has entered
into battery mode by polling once every 10 minutes. The LBAT85
detection happens automatically once when the minute register
reaches x9h or x0h minutes.
TABLE 2. STATUS REGISTER (SR)
ADDR
7
6
5
4
3
2
1
0
07h
BUSY OSCF DSTDJ ALM LVDD LBAT85 LBAT75 RTCF