Notes On Using Rtc - Fujitsu F2MC-8FX Hardware Manual

27.6

Notes on using RTC

This section contains notes for using the real time clock.
Behaviour of power-save mode
In power-saving mode (RTCCRL:PS=1), the RTC runs on the subclock. While in non-power-saving mode
(RTCCRL:PS=0), the RTC on the resource clock. As the RTC runs on the subclock in power-save mode,
the resource clock can be stopped. In the event when the resource clock is stopped in power-saving mode,
all user accessible registers (except RTCCRL:PS) are not writable. The read values are valid only for
RTCCRL, RTCCRH:MOC/DC/HC/MC, MICR, HOUR, HRCR, DYCR, MOCR and FFTR. Other registers
are indeterminate.
Resource clock and subclock frequency
There is an limitation on the frequency of resource clock operating in non-power saving mode
(RTCCRL:PS=0). As resource clock is used as sampling clock for both timing and control logic, it must not
be slower than the subclock in non-power saving mode. resource clock must be at least 4 times the
frequency of subclock.
CHAPTER 27 REAL TIME CLOCK
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