M41ST84Y, M41ST84W
12/31
CLOCK OPERATION
The eight byte clock register (see Table
3., page 13) is used to both set the clock and to
read the date and time from the clock, in a binary
coded decimal format. Tenths/Hundredths of Sec-
onds, Seconds, Minutes, and Hours are contained
within the first four registers.
Note: A WRITE to any clock register will result in
the Tenths/Hundredths of Seconds being reset to
“00,” and Tenths/Hundredths of Seconds cannot
be written to any value other than “00.”
Bits D6 and D7 of Clock Register 03h (Century/
Hours Register) contain the CENTURY ENABLE
Bit (CEB) and the CENTURY Bit (CB). Setting
CEB to a '1' will cause CB to toggle, either from '0'
to '1' or from '1' to '0' at the turn of the century (de-
pending upon its initial state). If CEB is set to a '0,'
CB will not toggle. Bits D0 through D2 of Register
04h contain the Day (day of week). Registers 05h,
06h, and 07h contain the Date (day of month),
Month and Years. The ninth clock register is the
Control Register (this is described in the Clock
Calibration section). Bit D7 of Register 01h con-
tains the STOP Bit (ST). Setting this bit to a '1' will
cause the oscillator to stop. If the device is expect-
ed to spend a significant amount of time on the
shelf, the oscillator may be stopped to reduce cur-
rent drain. When reset to a '0' the oscillator restarts
within one second.
The eight clock registers may be read one byte at
a time, or in a sequential block. The Control Reg-
ister (Address location 08h) may be accessed in-
dependently. Provision has been made to assure
that a clock update does not occur while any of the
eight clock addresses are being read. If a clock ad-
dress is being read, an update of the clock regis-
ters will be halted. This will prevent a transition of
data during the READ.
Power-down Time-Stamp
When a power failure occurs, the Halt Update Bit
(HT) will automatically be set to a '1.' This will pre-
vent the clock from updating the TIMEKEEPER
®
registers, and will allow the user to read the exact
time of the power-down event. Resetting the HT
Bit to a '0' will allow the clock to update the TIME-
KEEPER registers with the current time. For more
information, see Application Note AN1572.
TIMEKEEPER
®
Registers
The M41ST84Y/W offers 12 additional internal
registers which contain the Alarm, Watchdog,
Flag, Square Wave and Control data. These reg-
isters are memory locations which contain external
(user accessible) and internal copies of the data
(usually referred to as BiPORT
™
TIMEKEEPER
cells). The external copies are independent of in-
ternal functions except that they are updated peri-
odically by the simultaneous transfer of the
incremented internal copy. The internal divider (or
clock) chain will be reset upon the completion of a
WRITE to any clock address.
The system-to-user transfer of clock data will be
halted whenever the address being read is a clock
address (00h to 07h). The update will resume ei-
ther due to a Stop Condition or when the pointer
increments to a non-clock or RAM address.
TIMEKEEPER and Alarm Registers store data in
BCD. Control, Watchdog and Square Wave Reg-
isters store data in Binary Format.
