M41T00AUD clock operation
M41T00AUD
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5
M41T00AUD clock operation
5.1 Clock
registers
The 10-byte Register Map (see
Table 2
) 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 to D0 or
register 00h (seconds register) contain the seconds count in BCD format with values in the
range 0 to 59. Bit D7 is the ST or stop bit, described below, and is not affected by the
timekeeping operation, but users must avoid inadvertently altering it when writing the
seconds register.
Setting the ST 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
on the backup battery. When reset to a 0 the oscillator restarts within one second.
In order to ensure oscillator start-up after the initial power-up, set the ST bit to a 1 then write
it to 0. This sequence enables the "kick start" circuit which aids the oscillator start-up by
temporarily increasing the oscillator current. This will guarantee oscillator start-up under
worst case conditions of voltage and temperature. This feature can be employed anytime
the oscillator is being started but should not occur on subsequent power-ups when the
oscillator is already running.
Bits D6 to D0 of register 01h (Minutes Register) contain the minutes count in BCD format
with values in the range 0 to 59. Bit D7 always reads 0. Writing it has no effect.
Bits D5 to D0 of register 02h (Century/ Hours Register) contain the hours in BCD format with
values in the range 0 to 23. Bits D7 and D6 contain the century enable bit (CEB) and the
century bit (CB). CB provides a one-bit indicator for the century. The user can apply his
preferred convention for defining the meaning of this bit. For example, 0 can mean the
current century, and 1 the next, or the opposite meanings may be used.
When enabled, CB will toggle every 100 years. Setting CEB to a 1 enables CB to toggle at
the turn of the century, either from 0 to 1 or from 1 to 0, depending on its initial state, as
programmed by the user. When CEB is a 0, CB will not toggle.
Bits D2 through D0 of Register 03h (day register) contain the day of the week in BCD format
with values in the range 0 to 7. Bits D3 and D7 will always read 0. Writes to them have no
effect. Bits D6, D5 and D4 will power up in an indeterminate state.
Register 04h contains the date (day of month) in BCD format with values in the range 01 to
31. Bits D7 and D6 always read 0. Writes to them have no effect.
Register 05 h is the Month in BCD format with values in the range 1 to 12. Bits D7, D6 and
D5 always read 0. Writes to them have no effect.
Register 06h is the years in BCD format with values in the range 0 to 99. Writing to any of
the registers 00h to 06h, including the control bits therein, will result in updates to the
counters and resetting of the internal clock divider chain including the 256/512Hz tone
generator. The updates do not occur immediately after the write(s), but occur upon
completion of the current write access. This is described in greater detail in the next section.
Registers 07h and 09h also contain clock control and status information. These registers
can be written at any time without affecting the timekeeping function.
