M41T256Y

Clock operation

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3

Clock operation

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.

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.