Functional Description

The HT1382 is a low power real time clock device which provides full date and time functions.

Communication with the device is provided through two integral serial interfaces, I

device version selects the type of interface. The clock and calendar information is generated in BCD

format and also has alarm features. The calendar is accurate until the year 2099, with automatic leap

year correction.

Basic timing is provided using an external 32768Hz crystal, for which the device includes load

capacitances of 12.5pF. An oscillator compensation function is provided to compensate for crystal

oscillator temperatures. With fully integrated power control circuitry which can detect power failures,

the device can automatically switch to a reserve battery supply when a power failure occurs.

Power Control Function

The internal battery switchover circuit continually monitors the main power supply on the VDD pin

and automatically switches to the backup battery supply when a power failure condition is detected.

In the battery backup mode, the interface is disabled to minimise power consumption. The interface

inputs will not be recognized which prevents extraneous data being written to the device. The interface

outputs are high-impedance. All RTC function are operational when the device is in the battery backup

mode.

To switch from the V

DD

to V

BAT

Mode , both of the following conditions must be valid:

V

DD

<V

BAT

-V

BATHYS

and V

DD

<V

COMP

To switch from the V

BAT

to V

DD

Mode, one of the following conditions must be valid:

V

DD

>V

BAT

+V

BATHYS

or V

DD

>V

COMP

+V

COMPHYS

The power control situation is illustrated graphically below:

Low Power Mode

In normal mode, the HT1382 switched into battery backup mode when the V

DD

power is lost. This will

ensure that the device can accept a wide range of backup voltages from many types of sources while

reliably switching into backup mode. Another mode, called Low Power Mode, is available to allow

direct switching from V

DD

to V

BAT

without requiring V

DD

to drop below V

COMP

. The power switchover

circuit is disabled and less power is used while operating from V

DD

. Low Power Mode is activated via

the LPM bit.

Low Power Mode is useful when V

DD

is normally higher than V

BAT

. The device will switch from V

DD

to

V

BAT

when V

DD

drops below V

BAT

, with about 40mVof hysteresis to prevent any switchback of V

DD

after

switchover. In a system with V

DD

=5V and V

BAT

=3V, Low Power Mode can be used. However, it is not

recommended to use Low Power Mode in V

DD

=3.3V

±10%, V

BAT

³3V.

HT1382

Rev. 1.40

9

May 27, 2011

Battery Backup

Mode

2.55V

3.0V

Battery

Backup

Mode

2.55V

2.0V