Datasheet

Product data sheet Rev. 6 — 11 July 2013 40 of 86

NXP Semiconductors

PCF2127AT

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 35

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 returns 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 takes

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 38. Specification of t

w(rst)

WD_CD[1:0] TF[1:0] t

w(rst)

11 00 244 s

01 15.625 ms

10 15.625 ms

11 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

0203

00 01

03XX

01 03 02 01 03 02

duration of first timer period after

enable may range from n−1 to n+1

01 03