M48T129YPM datasheet(6/22 Pages) STMICROELECTRONICS

6 / 22 page

M48T129Y, M48T129V

6/22

CLOCK OPERATIONS

Reading the Clock

Updates to the TIMEKEEPER registers should be

halted before clock data is read to prevent reading

data in transition. Because the BiPORT TIME-

KEEPER cells in the RAM array are only data reg-

isters, and not the actual clock counters, 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 (1FFF8h). 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 is-

sued. All of the TIMEKEEPER registers are updat-

ed simultaneously. A halt will not interrupt an

update in progress. Updating occurs 1 second af-

ter the READ bit is reset to a ’0’.

Setting the Clock

Bit D7 of the Control Register (1FFF8h) is the

WRITE bit. Setting the WRITE bit to a ’1’, like the

READ bit, halts updates to the TIMEKEEPER reg-

isters. The user can then load them with the cor-

rect day, date, and time data in 24 hour BCD

format (see Table 11).

Resetting the WRITE bit to a ’0’ then transfers the

values of all time registers (1FFFFh-1FFF9h,

1FFF1h) 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 following a power failure,

both the WRITE bit and the READ bit will be reset

to ’0’.

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 1FFF9h. Setting it to

a ’1’ stops the oscillator. When reset to a ’0’, the

M48T129Y/V oscillator starts within 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).

SETTING ALARM CLOCK

Registers 1FFF6h-1FFF2h contain the alarm set-

tings. The alarm can be configured to go off at a

prescribed time on a specific month, date, hour,

minute, or second or repeat every month, day,

hour, minute, or second. It can also be pro-

grammed to go off while the M48T129Y/V is in the

battery back-up to serve as a system wake-up call.

Bits RPT5-RPT1 put the alarm in the repeat mode

of operation. Table 12 shows the possible config-

urations. Codes not listed in the table default to the

once per second mode to quickly alert the user of

an incorrect alarm setting.

Note: User must transition address (or toggle Chip

Enable) to see Flag Bit change.

When the clock information matches the alarm

clock settings based on the match criteria defined

by RPT5-RPT1, the AF (Alarm Flag) is set. If AFE

(Alarm Flag Enable) is also set, the alarm condi-

tion activates the IRQ/FT pin. To disable alarm,

write ’0’ to the Alarm Date register and RPT1-4.

The IRQ/FT output is cleared by a read to the

Flags register as shown in Figure 12. A subse-

quent read of the Flags register will reset the

Alarm Flag (D6; Register 1FFF0h).

The IRQ/FT pin can also be activated in the bat-

tery back-up mode. The IRQ/FT will go low if an

alarm occurs and both ABE (Alarm in Battery

Back-up Mode Enable) and AFE are set. The ABE

and AFE bits are reset during power-up, therefore

an alarm generated during power-up will only set

AF. The user can read the Flag Register at system

boot-up to determine if an alarm was generated

while the M48T129Y/V was in the deselect mode

during power-up. Figure 13 illustrates the back-up

mode alarm timing.

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