NXP Semiconductors UM10301 PCF2123 - Manual (Page 45)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 45 of 52
As an example, the desired timer period is 5 seconds. If the timer source clock frequency
is set to 1 Hz and n = 5, the minimum possible timer period will be (5-1) + 15.625 ms =
4.015625 s. The maximum possible timer period will be 5 + 15.625 ms = 5.015625 s. The
resulting timer period will have a duration somewhere within these limits.
Similarly, if the desired timer delay is 1 minute, one option would be to chose the timer
source clock 1/60 Hz and set n = 1. However, then there would be only one timer period
and it has an uncertainty. The duration is not exactly defined. A better way is to select the
1 Hz source clock and set n = 60. There will be an uncertainty in the first period too but
the consecutive 59 periods are exact and the resulting total uncertainty is 60 times
smaller.
The conclusion is that for a given desired delay, minimum uncertainty will be achieved by
choosing a higher setting of n combined with a higher timer source clock.
At the end of every count down 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 signal on pin
INT provided that this mode is enabled. Refer to the relevant datasheet for details on
how the interrupt can be controlled which is done via certain bits in the control registers.
The timer has two operating modes, TI and TP. If the timer interrupt is enabled, the bit
TI/TP determines the operating mode. If bit TI_TP is set to 1, the chosen timer mode is
‘pulsed’. In this mode an interrupt is generated after the timer period elapses. This is
independent of the timer flag and will thus happen every time the timer periode elapses,
periodically. The clearing of the timer flag is only necessary if TI mode is chosen, if the
consecutive interrupt is to happen. In the TI mode the signal remains permanently high
as long as the Timer Flag is active.
18. Timing requirements for I
2
C read and write
Reading to and writing from the time and date registers is an event which is controlled by
the interface bus (I
2
C or SPI) but which is asynchronous to the internal 32.768 kHz clock
of the RTC. It happens at random instants with respect to the automatic update of the
internal registers. Without precautions, two types of read errors could occur when the
time and date registers increment while being read. First, the data could change while a
single register is being read. Second, the data could change during the time between
reading two registers. Assume for example that the clock increments from 09:59:59 to
10:00:00 during a read of the seconds, minutes and hours registers. The time read could
be 10:00:59 and this is incorrect. A similar reasoning applies to writing. Measures must
be taken to prevent such read and write errors. In the NXP RTCs this is realized by
freezing the contents of all counters when one of the RTC registers is read or written.
This is called time counter freeze. Therefore faulty reading of the clock/calendar during a
carry condition is prevented.
Fig 10 shows the block diagram of the PCF8563 with attached to the I
2
C-interface a
watchdog. More details are given in Fig 21 where a block diagram representing some
blocks in the RTC has been drawn. In Fig 22 a sequence of events after a read operation
starts is depicted.