M41T256YMT7F datasheet(13/27 Pages) STMICROELECTRONICS

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M41T256Y

CLOCK OPERATION

Year, Month, and Date are contained in the last

three registers of the TIMEKEEPER® Register

Map (see Table 3., page 14). Bits D0 through D2

of the next register contain the Day (day of week).

Finally, there are the registers containing the Sec-

onds, Minutes, and Hours, respectively. The first

clock register is the Control Register (this is de-

scribed in the Clock Calibration section).

The nine Clock Registers may be read one byte at

a time, or in a sequential block. The Control Reg-

ister (Address location 7FF8h) may be accessed

independently. Provision has been made to as-

sure 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 transi-

tion of data during the read.

Reading the Clock

The

nine

byte

clock

(see

Table

3., page 14) 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 trans-

fer of clock data will be halted whenever the ad-

dress 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 dis-

turbing the clock itself.

Setting the Clock

Bit D7 of the Control Register (7FF8h) is the Write

Clock Bit. Setting the Write Clock Bit to a '1' will al-

low 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 Regis-

ter 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 cas-

es, the clock data will be undisturbed and will re-

tain their previous values.

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 Reg-

ister. Setting it to '1' stops the oscillator. Setting it

to '0' restarts the oscillator in approximately one

second.

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