X1227
When there is a match, an alarm flag is set.The occur-
rence of an alarm can be determined by polling the
AL0 and AL1 bits or by enabling the IRQ output, using
it as hardware flag.
STATUS REGISTER (SR)
The Status Register is located in the CCR Memory
Map at address 003Fh. This is a volatile register only
and is used to control the WEL and RWEL write
enable latches, read two power status and two alarm
bits. This register is separate from both the array and
the Clock/Control Registers (CCR).
The alarm enable bits are located in the MSB of the
particular register. When all enable bits are set to ‘0’,
there are no alarms.
– The user can set the X1227 to alarm every Wednes-
day at 8:00 AM by setting the EDWn*, the EHRn*
and EMNn* enable bits to ‘1’ and setting the DWAn*,
HRAn* and MNAn* Alarm registers to 8:00 AM
Wednesday.
Table 2. Status Register (SR)
Addr
003Fh BAT AL1 AL0
Default
7
6
5
4
3
2
1
0
0
0
0
0
RWEL WEL RTCF
0
0
0
0
0
1
– A daily alarm for 9:30PM results when the EHRn*
and EMNn* enable bits are set to ‘1’ and the HRAn*
and MNAn* registers are set to 9:30 PM.
BAT: Battery Supply—Volatile
This bit set to “1” indicates that the device is operating
from V , not V . It is a read-only bit and is set/
*n = 0 for Alarm 0: N = 1 for Alarm 1
BACK
CC
reset by hardware (X1227 internally). Once the device
REAL TIME CLOCK REGISTERS
begins operating from V , the device sets this bit to
CC
“0”.
Clock/Calendar Registers (SC, MN, HR, DT, MO,
YR)
AL1, AL3: Alarm bits—Volatile
These registers depict BCD representations of the
time. As such, SC (Seconds) and MN (Minutes) range
from 00 to 59, HR (Hour) is 1 to 12 with an AM or PM
indicator (H21 bit) or 0 to 23 (with MIL=1), DT (Date) is
1 to 31, MO (Month) is 1 to 12, YR (Year) is 0 to 99.
These bits announce if either alarm 0 or alarm 1 match
the real time clock. If there is a match, the respective
bit is set to ‘1’. The falling edge of the last data bit in a
SR Read operation resets the flags. Note: Only the AL
bits that are set when an SR read starts will be reset.
An alarm bit that is set by an alarm occurring during an
SR read operation will remain set after the read opera-
tion is complete.
Date of the Week Register (DW)
This register provides a Day of the Week status and
uses three bits DY2 to DY0 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’.
RWEL: Register Write Enable Latch—Volatile
This bit is a volatile latch that powers up in the LOW
(disabled) state. The RWEL bit must be set to “1” prior
to any writes to the Clock/Control Registers. Writes to
RWEL bit do not cause a nonvolatile write cycle, so the
device is ready for the next operation immediately after
the stop condition. A write to the CCR requires both
the RWEL and WEL bits to be set in a specific
sequence.
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 H21 bit functions as an AM/PM indi-
cator with a ‘1’ representing PM. The clock defaults to
standard time with H21=0.
WEL: Write Enable Latch—Volatile
The WEL bit controls the access to the CCR and
memory array during a write operation. This bit is a
volatile latch that powers up in the LOW (disabled)
state. While the WEL bit is LOW, writes to the CCR or
any array address will be ignored (no acknowledge will
be issued after the Data Byte). The WEL bit is set by
writing a “1” to the WEL bit and zeroes to the other bits
of the Status Register. Once set, WEL remains set
until either reset to 0 (by writing a “0” to the WEL bit
and zeroes to the other bits of the Status Register) or
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,
the year 2100 is not. The X1227 does not correct for
the leap year in the year 2100.
Characteristics subject to change without notice. 5 of 28
REV 1.1.20 1/13/03
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