M48T512Y, M48T512V

Clock operations

Doc ID 5747 Rev 6

11/23

3

Clock operations

3.1

Reading the clock

prevent reading data in transition (see Table 5 on page 12). The BiPORT™ TIMEKEEPER

cells in the RAM array are only data registers and not the actual clock counters, so updating

the registers can be halted without disturbing the clock itself.

Updating is halted when a '1' is written to the READ bit, D6 in the control register (7FFF8h).

As long as a '1' remains in that position, updating is halted. After a halt is issued, the

registers reflect the count; that is, the day, date, and time that were current at the moment

the halt command was issued. All of the TIMEKEEPER registers are updated

simultaneously. A halt will not interrupt an update in progress. The next update occurs 1

second after the READ bit is reset to a '0.'

3.2

Setting the clock

Bit D7 of the control register (7FFF8h) is the WRITE bit. Setting the WRITE bit to a '1,' like

the READ bit, halts updates to the TIMEKEEPER registers. The user can then load them

with the correct day, date, and time data in 24 hour BCD format (see Table 5 on page 12).

Resetting the WRITE bit to a '0' then transfers the values of all time registers 7FFFFh-

7FFF9h to the actual TIMEKEEPER counters and allows normal operation to resume. After

the WRITE bit is reset, the next clock update will occur approximately one second later.

Note:

Upon power-up, both the WRITE bit and the READ bit will be reset to '0.'

3.3

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 is located at bit D7 within 7FFF9h. Setting it to a '1' stops the

oscillator. The M48T512Y/V is shipped from STMicroelectronics with the STOP bit set to a

'1.' When reset to a '0,' the M48T512Y/V oscillator starts after approximately one second.

Note:

It is not necessary to set the WRITE bit when setting or resetting the FREQUENCY TEST

bit (FT) or the STOP bit (ST).