PCF8523 datasheet(14/66 Pages) NXP

14 / 66 page

PCF8523

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Product data sheet

Rev. 3 — 30 March 2011

14 of 66

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;

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

[2]

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

from the main power supply VDD. Thereafter, the device will switch 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 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 this the PCF8523 will enter the standby mode

and will not draw any current from the backup battery before it is powered up again from

main supply VDD.

Table 9.

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[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[1][2]

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

battery low detection function is disabled

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