Datasheet

PCF8523 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 6 — 17 September 2013 18 of 78
NXP Semiconductors
PCF8523
Real-Time Clock (RTC) and calendar
8.5.2.2 Direct switching mode
If V
DD
> V
BAT
the internal power supply is V
DD
.
If V
DD
< V
BAT
the internal power supply is V
BAT
.
The direct switching mode is useful in systems where V
DD
is higher than V
BAT
at all times
(for example, V
DD
= 5 V, V
BAT
= 3.5 V). If the V
DD
and V
BAT
values are similar (for
example, V
DD
= 3.3 V, V
BAT
 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 V
DD
and V
th(sw)bat
is not performed.
8.5.2.3 Battery switch-over disabled, only one power supply (V
DD
)
When the battery switch-over function is disabled:
• The power supply is applied on the V
DD
pin
• The V
BAT
pin must be connected to V
DD
• 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
V
BAT
.
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 V
BAT
drops below the threshold value V
th(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)
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