The second approach is better suited to a manu-

facturing environment, and involves the use of

some test equipment. When the Frequency Test

(FT) bit, the seventh-most significant bit in the Day

Register, is set to a ’1’, and the oscillator is running

at 32,768 Hz, the LSB (DQ0) of the Seconds Reg-

ister will toggle at 512 Hz. Any deviation from 512

Hz indicates the degree and direction of oscillator

frequency shift at the test temperature. For exam-

ple, a reading of 512.01024 Hz would indicate a

+20 ppm oscillator frequency error, requiring a

-10(WR001010) to be loaded into the Calibration

Byte for correction. Note that setting or changing

the Calibration Byte does not affect the Frequency

test output frequency. The device must be selected

and address 7F9h must be held constant when

reading the 512 Hz on DQ0.

The FT bit must be set using the same methodused

to set the clock, using the Write bit. The LSB of the

Seconds Register is monitored by holding the

M48T02/12 in an extended read of the Seconds

Register, without having the Read bit set. The FT

bit MUST be reset to ’0’ for normal clock operations

to resume.

For more information on calibration, see the Appli-

cation Note AN924 ”TIMEKEEPER Calibration”.

POWER SUPPLY DECOUPLING and UNDER-

SHOOT PROTECTION

switching, can produce voltage fluctuations, result-

be reduced if capacitors are used to store energy,

AI02169

VCC

0.1

µF

DEVICE

VCC

VSS

Figure 12. Supply Voltage Protection

the bypass capacitors will be releasedas low going

spikes are generated or energy will be absorbed

when overshoots occur. A bypass capacitor value

of 0.1

µF (as shown in Figure 12) is recommended

in order to provide the needed filtering.

In addition to transients that are caused by normal

SRAM operation, power cycling can generate

negative spikes can cause data corruption in the

SRAM while in battery backup mode. To protect

from these voltage spikes, it is recommeded to

1N5817 is recommended for through hole and

MBRS120T3 is recommended for surface mount.

CLOCK OPERATION (cont’d)

12/15

M48T02, M48T12