M41ST84W_08 Datasheet, PDF (17/34 Pages) STMicroelectronics – 3.0/3.3 V I2C serial RTC with 44 bytes of NVRAM and supervisory functions

M41ST84W
3
Clock operation
Clock operation
Note:
The eight byte clock register (see Table 3 on page 18) 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 seconds, seconds, minutes, and hours are contained within the first four registers.
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 (depending 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 contains the STOP bit (ST). Setting this bit to a '1' will cause
the oscillator to stop. If the device is expected to spend a significant amount of time on the
shelf, the oscillator may be stopped to reduce current 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 register (address location 08h) may be accessed independently. 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 address is being read, an update of the clock registers will be
halted. This will prevent a transition of data during the READ.
3.1
Power-down time-stamp
When a power failure occurs, the Halt update bit (HT) will automatically be set to a '1.' This
will prevent 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 timekeeper registers with the current time. for more information, see
application note AN1572.
3.2
TIMEKEEPER^® registers
The M41ST84W offers 12 additional internal registers which contain the Alarm, watchdog,
flag, square wave and control data. These registers 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 internal functions except that
they are updated periodically 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 either 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
registers store data in binary format.
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