M41T00 clock operation
M41T00
3
M41T00 clock operation
The eight byte clock register (see Table 5) 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 (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 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 the
clock calibration section). 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 start-up 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. Provision has been
made to ensure 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:
Note: This 250 ms delay affects only the clock register update and does not alter the actual
clock time.
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STMICROELECTRONICS [ ST ]
