Élan™SC520 Microcontroller User’s Manual 20-1
CHAPTER
20
REAL-TIME CLOCK
20.1 OVERVIEW
The real-time clock (RTC) included on the ÉlanSC520 microcontroller is compatible with
the MC146818A device used in PC/AT systems. The RTC consists of a time-of-day clock
with alarm and a 100-year calendar. The clock/calendar has a programmable periodic
interrupt and 114 bytes of static user RAM. The clock/calendar can be represented in either
binary or binary-coded decimal (BCD).
The RTC includes the following features:
■ PC/AT-compatible
■ Counts seconds, minutes, and hours of the day
■ Counts day of the week, date, month, and year
■ Binary or BCD representation of time, calendar, and alarm
■ 12- or 24-hour clock, with AM and PM indicator in 12-hour mode
■ Daylight saving time option
■ Automatic end-of-month recognition
■ Automatic leap year compensation
■ 14 bytes of clock and control registers
■ 114 bytes of general-purpose RAM
■ Three interrupt sources separately maskable with corresponding status bits
■ Time-of-day alarm is programmable to occur from once-per-second to once-per-day
■ Periodic interrupts can be programmed to occur at rates from 122 µs to 500 ms
■ Update-ended interrupt provides cycle status
■ Internal RTC reset signal can perform a reset on power-up
The RTC has its own power pin and reset separate from the rest of the other core supplies.
When the chip is powered off, the RTC can remain powered up and in full functional mode,
maintaining time, calendar, and user RAM data.
The RTC includes registers for time, calendar, and alarm data and four control/status
registers. The RTC Status D (RTCSTAD) register (RTC index 0Dh) has a status bit
(RTC_VRT) that indicates the validity of the contents of the RAM, time registers and the
calendar. The RTC_VRT bit is set based on the assertion of the internal RTC reset.
The RTC interrupt request is connected internally to the programmable interrupt controller
block.
20.2 BLOCK DIAGRAM
Figure 20-1 shows a block diagram of the real-time clock.
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