Clock operations

M48T35 M48T35Y

12/29

3

Clock operations

3.1

Reading the clock

read to prevent reading data in transition. 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 7FF8h.

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 the 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. Updating is within a second after the bit is reset to a '0.'

3.2

Setting the clock

Bit D7 of the Control Register 7FF8h is the WRITE Bit. Setting the WRITE Bit to a '1,' like

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

WRITE Bit to a '0' then transfers the values of all time registers 7FF9h-7FFFh to the actual

TIMEKEEPER counters and allows normal operation to resume. The FT Bit and the bits

marked as '0' in Table 5 must be written to '0' to allow for normal TIMEKEEPER and RAM

operation. After the WRITE Bit is reset, the next clock update will occur within one second.

information on Century Rollover.

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 the MSB of the seconds register. Setting it to a '1' stops the

oscillator. The M48T35/Y is shipped from STMicroelectronics with the STOP Bit set to a '1.'

When reset to a '0,' the M48T35/Y oscillator starts within 1 second.