
iiiPeriPheraLModuLes1(systeM):reaL-tiMeCLoCk(rtC)
iii-3-26
ePson
s1C33L17teChniCaLManuaL
iii.3.7Precautions
The contents of all RTC control registers are indeterminate when power is turned on and are not initialized to
specific values by initial reset. Be sure to initialize these registers in software.
While 1 is being carried over to the next-digit counter, the correct counter value may not be read out. Moreover,
attempting to write to the counters or other control registers may corrupt the counter value. Therefore, do not
write to the counters while 1 is being carried over. For the correct method of operation, see Section III.3.3.5,
“Counter Hold and Busy Flag,” and Section III.3.3.6, “Reading from and Writing to Counters in Operation.”
Note that rewriting RTC24H (D4/0x301908) to switch between 12-hour mode and 24-hour mode may corrupt
the count data for hours, days, months, years, or days of the week. Therefore, after changing the RTC24H (D4/
0x301908) setting, be sure to set data in these counters back again.
Avoid the settings below that may cause timekeeping errors.
- Settings exceeding the effective range
Do not set count data exceeding 60 seconds, 60 minutes, 12 or 24 hours, 31 days, 12 months, or 99 years.
- Settings nonexistent in the calendar
Do not set nonexistent dates such as April 31 or February 29, 2006. Even if such settings are made, the
counters operate normally, so that when 1 is carried over from the hour counter to the 1-day counter, the day
counter counts up to the first day of the next month. (For April 31, the day counter counts up to May 1; for
February 29, 2006, the day counter counts up to March 1, 2006.)
The contents of all RTC interrupt control bits are indeterminate when power is turned on, and are not initialized
to specific values by initial reset.
After power-on, be sure to set RTCIEN (D0/0x301904) to 0 (interrupt disabled) for preventing the occurrence of
unwanted RTC interrupts. Also be sure to write 1 to RTCIRQ (D0/0x301900) to reset it.
Immediately after the OSC1 oscillator circuit is activated (as at power-on), a finite time (of about 3 seconds) is
required for OSC1 oscillation to stabilize. Do not let the RTC start counting until this time elapses.