DS1644LPM

041697 3/11

DS1644L TRUTH TABLE Table 1

CE

OE

WE

MODE

DQ

POWER

5 VOLTS

10%

X

X

DESELECT

HIGH–Z

STANDBY

5 VOLTS

10%

X

X

X

DESELECT

HIGH–Z

STANDBY

5 VOLTS

± 10%

X

WRITE

DATA IN

ACTIVE

READ

DATA OUT

ACTIVE

READ

HIGH–Z

ACTIVE

<4.5 VOLTS

X

X

X

DESELECT

HIGH–Z

CMOS STANDBY

X

X

X

DESELECT

HIGH–Z

DATA RETENTION

MODE

SETTING THE CLOCK

The eighth bit of the control register is the write bit. Set-

ting the write bit to a one, like the read bit, halts updates

to the DS1644L registers. The user can then load them

with the correct day, date and time data in 24 hour BCD

format. Resetting the write bit to a zero then transfers

those values to the actual clock counters and allows

normal operation to resume.

STOPPING AND STARTING THE CLOCK

OSCILLATOR

The clock oscillator may be stopped at any time. To in-

crease the shelf life, the oscillator can be turned off to

minimize current drain from the battery. The OSC bit is

the MSB for the seconds registers. Setting it to a one

stops the oscillator.

FREQUENCY TEST BIT

Bit 6 of the day byte is the frequency test bit. When the

frequency test bit is set to logic “1” and the oscillator is

running, the LSB of the seconds register will toggle at

512 Hz. When the seconds register is being read, the

DQ0 line will toggle at the 512 Hz frequency as long as

conditions for access remain valid (i.e., CE low, OE low,

and address for seconds register remain valid and

stable).

CLOCK ACCURACY

The DS1644L is guaranteed to keep time accuracy to

within

±1 minute per month at 25°C. The clock is cali-

brated at the factory by Dallas Semiconductor using

special calibration nonvolatile tuning elements. The

DS1644L does not require additional calibration, and

temperature deviations will have a negligible effect in

most applications. For this reason, methods of field

clock calibration are not available and not necessary.

Attempts to calibrate the clock that may be used with

similar device types (MK48T08 family) will not have any

effect even though the DS1644L appears to accept cal-

ibration data.