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M41T256Y Clock operation
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3 Clock operation
Year, month, and date are contained in the last three registers of the TIMEKEEPER
®
register map (see Table 3 on page 18). Bits D0 through D2 of the next register contain the
day (day of week). Finally, there are the registers containing the seconds, minutes, and
hours, respectively. The first clock register is the control register (this is described in the
clock calibration section).
The nine clock registers may be read one byte at a time, or in a sequential block. The control
register (Address location 7FF8h) may be accessed independently. Provision has been
made to assure that a clock update does not occur while any of the nine 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 Reading the clock
The nine 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. The system-to-user
transfer of clock data will be halted whenever the address being read is a clock address
(7FF9h to 7FFFh). The update will resume either due to a stop condition or when the pointer
increments to a RAM address.
This prevents reading data in transition. The TIMEKEEPER
®
cells in the register map are
only data registers and not actual clock counters, so updating the registers can be halted
without disturbing the clock itself.
3.2 Setting the clock
Bit D7 of the control register (7FF8h) is the write clock bit. Setting the write clock bit to a '1'
will allow the user to write the desired day, date, and time data in 24-hour BCD format.
Resetting the write clock bit to a '0' then transfers the values of all time registers (7FF8h-
7FFFh) to the actual clock counters and resets the internal divider (or clock) chain.
Note: The tenths/hundredths of seconds register will automatically be reset to zero when the
WRITE clock bit is set.
Other register bits such as FT, TEB, and ST may be written without setting the WC Bit. In
such cases, the clock data will be undisturbed and will retain their previous values.
3.3 Stopping and starting the oscillator
The oscillator may be stopped at any time. If the device is going to spend a significant
amount of time on the shelf, the oscillator can be turned off to minimize current drain on the
battery. The stop bit (ST) is the most significant bit of the seconds register. Setting it to '1'
stops the oscillator. Setting it to '0' restarts the oscillator in approximately one second.
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