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HT1382

I2C/3-Wire Real Time Clock

Holtek Semiconductor 

HT1382

I2C/3-Wire Real Time Clock

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, I2C or 3-wire. The

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.

Normal Mode (VDD) to Battery Backup Mode (VBAT)

To switch from the VDD to VBAT Mode , both of the following conditions must be valid:

V < V -V and V < V DD

BAT BATHYS

DD

COMP

Battery Backup Mode (VBAT) to Normal Mode (VDD)

To switch from the VBAT to VDD Mode, one of the following conditions must be valid:

V > V + V or V > V + V DD

BAT

BATHYS

DD

COMP

COMPHYS

The power control situation is illustrated graphically below:

VDD

VCOMP

VBAT

Battery

Backup

Mode

2.55V

2.0V

VBAT-VBATHYS

VBAT+VBATHYS

Note: Battery switchover when VBAT < VCOMP

VDD

VBAT

VCOMP

Battery Backup

Mode

3.0V

2.55V

VCOMP

VCOMP+VCOMPHYS

Note: Battery switchover when VBAT > VCOMP

Low Power Mode

In normal mode, the HT1382 switched into battery backup mode when the VDD 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 VDD to VBAT without requiring VDD to drop below V . COMP The power switchover

circuit is disabled and less power is used while operating from VDD. Low Power Mode is activated via

the LPM bit.

Low Power Mode is useful when VDD is normally higher than V . BAT The device will switch from VDD to

VBAT when VDD drops below V , BAT with about 40mVof hysteresis to prevent any switchback of VDD after

switchover. In a system with VDD=5V and VBAT=3V, Low Power Mode can be used. However, it is not

recommended to use Low Power Mode in VDD=3.3V±10%, VBAT³3V.

Rev. 1.40

9

May 27, 2011

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