M41T00AUD datasheet(16/42 Pages) STMICROELECTRONICS

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M41T00AUD clock operation

M41T00AUD

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Doc ID 13480 Rev 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

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.

31. Bits D7 and D6 always read 0. Writes to them have no effect.

D5 always read 0. Writes to them have no effect.

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 256/512 Hz 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.

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