Preliminary Information
X1288
Leap Years
WEL: Write Enable Latch—Volatile
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 X1288 does not correct for
the leap year in the year 2100.
The WEL bit controls the access to the CCR and mem-
ory 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 until the part pow-
ers up again. Writes to WEL bit do not cause a nonvol-
atile write cycle, so the device is ready for the next
operation immediately after the stop condition.
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).
RTCF: Real Time Clock Fail Bit—Volatile
This bit is set to a ‘1’ after a total power failure. This is a
read only bit that is set by hardware (X1288 internally)
when the device powers up after having lost all power
Table 2. Status Register (SR)
Addr
003Fh BAT AL1 AL0
Default
7
6
5
4
3
2
1
0
to the device. The bit is set regardless of whether V
0
0
0
0
RWEL WEL RTCF
CC
or V
is applied first. The loss of only one of the
BACK
0
0
0
0
0
1
supplies does not result in setting the RTCF bit. The
first valid write to the RTC after a complete power fail-
ure (writing one byte is sufficient) resets the RTCF bit
to ‘0’.
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/reset
BACK
CC
by hardware (X1288 internally). Once the device begins
Unused Bits:
operating from V , the device sets this bit to “0”.
CC
This device does not use bits 3 or 4 in the SR, but must
have a zero in these bit positions.The Data Byte output
during a SR read will contain zeros in these bit loca-
tions.
AL1, AL3: Alarm bits—Volatile
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.
CONTROL REGISTERS
The Control Bits and Registers, described under this
section, are nonvolatile.
Block Protect Bits—BP2, BP1, BP3
The Block Protect Bits, BP2, BP1 and BP0, determine
which blocks of the array are write protected. A write to a
protected block of memory is ignored. The block protect
bits will prevent write operations to one of eight segments
of the array.The partitions are described in Table 3.
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.
Watchdog Timer Control Bits—WD1, WD3
The bits WD1 and WD0 control the period of the
Watchdog Timer. See Table 4 for options.
14 of 31
REV 1.1.30 3/24/04
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