PCF8523USLASH12AASLASH1 datasheet(18/75 Pages) NXP

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PCF8523

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Product data sheet

Rev. 4 — 5 July 2012

18 of 74

NXP Semiconductors

PCF8523

Real-Time Clock (RTC) and calendar

8.5.2.2

Direct switching mode

If VDD > VBAT the internal power supply is VDD.

If VDD < VBAT the internal power supply is VBAT.

The direct switching mode is useful in systems where VDD is higher than VBAT at all times

(for example, VDD = 5 V, VBAT = 3.5 V). If the VDD and VBAT values are similar (for

example, VDD = 3.3 V, VBAT  3.0 V), the direct switching mode is not recommended. In

direct switching mode, the power consumption is reduced compared to the standard mode

because the monitoring of VDD and Vth(sw)bat is not performed.

8.5.2.3

Battery switch-over disabled, only one power supply (VDD)

When the battery switch-over function is disabled:

• The power supply is applied on the VDD pin

• The VBAT pin must be connected to ground

• The battery flag (BSF) is always logic 0

8.5.3 Battery low detection function

The PCF8523 has a battery low detection circuit, which monitors the status of the battery

VBAT.

Generation of interrupts from the battery low detection is controlled via bit BLIE (register

Control_3). If BLIE is enabled, the INT1 follows the status of bit BLF (register Control_3).

When VBAT drops below the threshold value Vth(bat)low (typically 2.5 V), the BLF flag

(register Control_3) is set to indicate that the battery is low and that it must be replaced.

Monitoring of the battery voltage also occurs during battery operation.

Fig 10. Battery switch-over behavior in direct switching mode and with bit BSIE set

logic 1 (enabled)

013aaa322

internal power supply (= VBBS)

cleared via interface

backup battery operation

BSF

Vth(sw)bat

(= 2.5 V)

VDD (= 0 V)

VBAT

VDD

VBBS

INT1

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