Data Sheet

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Product data sheet Rev. 05 — 17 July 2007 5 of 32

NXP Semiconductors

PCF8563

Real time clock/calendar

7.2 Timer

The 8-bit countdown timer at address 0Fh is controlled by the timer control register at

address 0Eh. The timer control register determines one of 4 source clock frequencies for

the timer (4096 Hz, 64 Hz, 1 Hz, or

⁄

Hz), and enables or disables the timer. The timer

counts down from a software-loaded 8-bit binary value. At the end of every countdown,

the timer sets the Timer Flag (TF). The TF may only be cleared by software. The

asserted TF can be used to generate an interrupt (INT). The interrupt may be generated

as a pulsed signal every countdown period or as a permanently active signal which

follows the condition of TF. Bit TI/TP is used to control this mode selection. When reading

the timer, the current countdown value is returned.

7.3 Clock output

A programmable square wave is available at pin CLKOUT. Operation is controlled by the

CLKOUT control register at address 0Dh. Frequencies of 32.768 kHz (default), 1024 Hz,

32 Hz and 1 Hz can be generated for use as a system clock, microcontroller clock, input to

a charge pump, or for calibration of the oscillator. CLKOUT is an open-drain output and

enabled at power-on. If disabled it becomes high-impedance.

7.4 Reset

The PCF8563 includes an internal reset circuit which is active whenever the oscillator is

stopped. In the reset state the I

C-bus logic is initialized and all registers are reset

according to Table 25.

7.5 Voltage-low detector

The PCF8563 has an on-chip voltage-low detector (see Figure 7). When V

drops below

low

, bit VL in the seconds register is set to indicate that the integrity of the clock

information is no longer guaranteed. The VL ﬂag can only be cleared by software.

Bit VL is intended to detect the situation when V

is decreasing slowly, for example under

battery operation. Should V

reach V

low

before power is re-asserted then bit VL is set.

This will indicate that the time may be corrupted.

Fig 7. Voltage-low detection

VL set

normal power

operation

period of battery

operation

low

mgr887