M48T35_11 Datasheet, PDF (12/28 Pages) STMicroelectronics – 5 V, 256 Kbit (32 Kb x 8) TIMEKEEPER® SRAM

Clock operations
3
Clock operations
M48T35, M48T35Y
3.1
Reading the clock
Updates to the TIMEKEEPER^® registers (see Table 5) should be halted before clock data is
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 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). 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.
See the application note AN923, “TIMEKEEPER^® Rolling Into the 21^st Century” for
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.
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Doc ID 2611 Rev 10