PCF8523 datasheet(17/66 Pages) NXP

17 / 66 page

PCF8523

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Product data sheet

Rev. 3 — 30 March 2011

17 of 66

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

(e.g. VDD = 5 V, VBAT = 3.5 V). The direct switching mode is not recommended if the VDD

and VBAT values are similar (e.g. VDD = 3.3 V, VBAT  3.0 V). 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 will follow 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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