PCF2127ATSLASH1 datasheet(38/80 Pages) NXP

38 / 80 page

PCF2127A_2

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Product data sheet

Rev. 02 — 7 May 2010

38 of 80

NXP Semiconductors

PCF2127A

Integrated RTC, TCXO and quartz crystal

8.11.4 Countdown timer function

The countdown timer function is controlled by the WD_CD[1:0] bits in register

Watchdg_tim_ctl (see Table 34).

The timer counts down from the software programmed 8 bit binary value n in register

Watchdg_tim_val. When the counter reaches 1

• the countdown timer flag CDTF is set

• the counter automatically reloads

• and the next time period starts

Loading the counter with 0 effectively stops the timer.

Reading the timer will return the actual value of the countdown counter.

If a new value of n is written before the end of the actual timer period, this value will take

immediate effect. It is not recommended to change n without first disabling the counter by

setting WD_CD[1:0] = 00. The update of n is asynchronous to the timer clock. Therefore

changing it on the fly could result in a corrupted value loaded into the countdown counter.

This can result in an undetermined countdown period for the first period. The countdown

value n will, however, be correctly stored and correctly loaded on subsequent timer

periods.

Table 37.

Specification of tw(rst)

WD_CD[1:0]

TF[1:0]

tw(rst)

244

μs

15.625 ms

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

countdown period expires and that INT is set to pulsed mode.

Fig 23. General countdown timer behavior

001aag071

countdown value, n

timer source clock

countdown counter

WD/CD [1:0]

CDTF

INT

duration of first timer period after

enable may range from n

−1 to n+1

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