PCF8523TSSLASH1 datasheet(15/75 Pages) NXP

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PCF8523

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Product data sheet

Rev. 4 — 5 July 2012

15 of 74

NXP Semiconductors

PCF8523

Real-Time Clock (RTC) and calendar

8.5 Power management functions

The PCF8523 has two power supply pins:

• VDD - the main power supply input pin

• VBAT - the battery backup input pin

The PCF8523 has two power management functions implemented:

• Battery switch-over function

• Battery low detection function

The power management functions are controlled by the control bits PM[2:0] in register

Control_3 (02h):

[1]

When the battery switch-over function is disabled, the PCF8523 works only with the power supply VDD.

[2]

When the battery switch-over function is disabled, the PCF8523 works only with the power supply VDD.

VBAT must be put to ground and the battery low detection function is disabled.

[3]

Default value.

8.5.1 Standby mode

When the device is first powered up from the battery (VBAT) but without a main supply

(VDD), the PCF8523 automatically enters the standby mode. In standby mode, the

PCF8523 does not draw any power from the backup battery until the device is powered up

from the main power supply VDD. Thereafter, the device switches over to battery backup

mode whenever the main power supply VDD is lost.

It is also possible to enter into standby mode when the chip is already supplied by the

main power supply VDD and a backup battery is connected. To enter the standby mode,

the power management control bits PM[2:0] have to be set logic 111. Then the main

power supply VDD must be removed. As a result of it, the PCF8523 enters the standby

mode and does not draw any current from the backup battery before it is powered up

again from main supply VDD.

Table 11.

Power management function control bits

PM[2:0]

Function

000

battery switch-over function is enabled in standard mode;

battery low detection function is enabled

001

battery switch-over function is enabled in direct switching mode;

battery low detection function is enabled

010,011[1]

battery switch-over function is disabled - only one power supply (VDD);

battery low detection function is enabled

100

battery switch-over function is enabled in standard mode;

battery low detection function is disabled

101

battery switch-over function is enabled in direct switching mode;

battery low detection function is disabled

110

not allowed

111[2][3]

battery switch-over function is disabled - only one power supply (VDD);

battery low detection function is disabled

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