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M41T01

Clock operation

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Clock operation

The eight byte clock register (see Table 2 on 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. Seconds, minutes,

and hours are contained within the first three registers. Bits D6 and D7 of clock register 2

(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 3 contain the Day (day of week). Registers 4, 5 and 6 contain the Date (day of

month), Month and Years. The final register is the Control register (this is described in

Section 3.1: Clock calibration). Bit D7 of register 0 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.

Note:

In order to guarantee oscillator startup after the initial power-up, set the ST bit to a '1,' then

reset this bit to a '0.' This sequence enables a “kick start” circuit which aids the oscillator

start-up during worst case conditions of voltage and temperature.

The seven clock registers may be read one byte at a time, or in a sequential block. The

control register (address location 7) may be accessed independently. A provision has been

made to assure that a clock update does not occur while any of the seven clock addresses

are being read. If a clock address is being read, an update of the clock registers will be

delayed by 250 ms to allow the READ to be completed before the update occurs. This will

prevent a transition of data during the READ.

Note:

This 250 ms delay affects only the clock register update and does not alter the actual clock

time.