PCF8523 datasheet(36/66 Pages) NXP

36 / 66 page

PCF8523

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Product data sheet

Rev. 3 — 30 March 2011

36 of 66

NXP Semiconductors

PCF8523

Real-Time Clock (RTC) and calendar

When the interrupt generation is enabled (CTAIE = 1) and the countdown timer flag CTAF

is set logic 1, an interrupt signal on INT1 will be generated. The interrupt may be

generated as a pulsed signal every countdown period or as a permanently active signal,

which follows the condition of CTAF (register Control_2). The TAM bit (register

Tmr_CLKOUT_ctrl) is used to control this mode selection. The interrupt output may be

disabled with the CTAIE bit (register Control_2).

8.9.3 Timer B

Timer B can only be used as a countdown timer and can be switched on and off by the

TBC bit in register Tmr_CLKOUT_ctrl (0Fh).

The generation of interrupts from the countdown timer is controlled via the CTBIE bit

(register Control_2).

When enabled, it counts down from the software programmed 8 bit binary value n in

timer clock the following events occur (see Figure 20):

• The countdown timer flag CTBF (register Control_2) is set logic 1.

• When the interrupt generation is enabled (CTBIE = 1), interrupt signals on INT1 and

INT2 will be generated.

• The counter automatically reloads.

• The next timer-period starts.

At the end of every countdown, the timer sets the countdown timer flag CTBF (register

Control_2). CTBF may only be cleared by using the interface. Instructions, how to clear a

flag, is given in Section 8.7.5.

When reading the timer, the current countdown value is returned and not the initial

value n. Since it is not possible to freeze the countdown timer counter during read back, it

is recommended to read the register twice and check for consistent results.

In this example, it is assumed that the countdown timer flag (CTBF) is cleared before the next

countdown period expires and that interrupt output is set to pulse mode.

Fig 20. General countdown timer behavior

013aaa329

countdown value, n

timer source clock

countdown counter

WD/CD [1:0]

CTBF

INT1/INT2

duration of first timer period after

enable may range from n

−1 to n+1

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