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M48T35AY, M48T35AV

CLOCK OPERATIONS

Reading the Clock

ble 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 is-

sued.

All of the TIMEKEEPER registers are updated si-

multaneously. A halt will not interrupt an update in

progress. Updating is within a second after the bit

is reset to a '0.'

Setting the Clock

Bit D7 of the Control Register 7FF8h is the WRITE

Bit. Setting the WRITE Bit to a '1,' like the READ

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 op-

eration. After the WRITE Bit is reset, the next clock

update will occur within one second.

Century Rollover.

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 M48T35AY/V is

shipped from STMicroelectronics with the STOP

Bit set to a '1.' When reset to a '0,' the M48T35AY/

V oscillator starts within 1 second.

Table 5. Register Map

Keys: S = SIGN Bit

FT = FREQUENCY TEST Bit (Must be set to '0' upon power

for normal operation)

R = READ Bit

W = WRITE Bit

ST = STOP Bit

0 = Must be set to '0'

CEB = Century Enable Bit

CB = Century Bit

Note: When CEB is set to '1,' CB will toggle from '0' to '1' or from '1' to '0' at the turn of the century (dependent upon the initial value set).

When CEB is set to '0,' CB will not toggle. The WRITE Bit does not need to be set to write to CEB.

Address

Data

Function/Range

BCD Format

D7

D6

D5

D4

D3

D2

D1

D0

7FFFh

10 Years

Year

Year

00-99

7FFEh

0

0

0

10 M.

Month

Month

01-12

7FFDh

0

0

10 Date

Date

Date

01-31

7FFCh

0

FT

CEB

CB

0

Day

Century/Day

00-01/01-07

7FFBh

0

0

10 Hours

Hours

Hours

00-23

7FFAh

0

10 Minutes

Minutes

Minutes

00-59

7FF9h

ST

10 Seconds

Seconds

Seconds

00-59

7FF8h

W

R

S

Calibration

Control