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 37 of 78
NXP Semiconductors
PCF8523
Real-Time Clock (RTC) and calendar
At the end of every countdown, the timer sets the countdown timer flag CTAF (register
Control_2). CTAF 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 T_A. 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.
If a new value of T_A is written before the end of the actual timer-period, this value takes
immediate effect. It is not recommended to change T_A without first disabling the counter
by setting TAC[1:0] = 00 (register Tmr_CLKOUT_ctrl). The update of T_A 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 T_A will be correctly stored
and correctly loaded on subsequent timer-periods.
Loading the counter with 0 effectively stops the timer.
When starting the countdown timer for the first time, only the first period does not have a
fixed duration. The amount of inaccuracy for the first timer-period depends on the chosen
source clock, see Table 41
.
The generation of interrupts from the countdown timer is controlled via the CTAIE bit
(register Control_2).
In this example, it is assumed that the countdown timer flag (CTAF) is cleared before the next
countdown period expires and that the interrupt output is set to pulse mode.
Fig 20. General countdown timer behavior
Table 41. First period delay for timer counter value T_A
Timer source clock Minimum timer-period Maximum timer-period
4.096 kHz T_A T_A + 1
64 Hz T_A T_A + 1
1 Hz (T_A  1) +
1
⁄
64 Hz
T_A +
1
⁄
64 Hz
1
⁄
60
Hz (T_A  1) +
1
⁄
64 Hz
T_A +
1
⁄
64 Hz
1
⁄
3600
Hz (T_A  1) +
1
⁄
64 Hz
T_A +
1
⁄
64 Hz
DDD
FRXQWGRZQYDOXH7B$
WLPHUVRXUFHFORFN
FRXQWGRZQFRXQWHU
:'&'>@
&7$)
,17

 
;;
;;
     
7B$
GXUDWLRQRIILUVWWLPHUSHULRGDIWHU
HQDEOHPD\UDQJHIURP7B$WR7B$
7B$

