
3-16
PC/AT Peripheral Registers
3.2.7
Real-Time Clock
I
Counts seconds, minutes, and hours of the day
I
Counts days of the week, date, month, and year
I
Binary or BCD representation of time, calendar, and alarm
I
12–24-hr clock with
A
.
M
. and
P
.
M
. in 12-hr mode
I
Daylight-savings-time option
I
Automatic end-of-month recognition
I
Automatic leap-year compensation
I
14 bytes of clock and control registers
I
114 bytes of general-purpose RAM
I
Three interrupts are separately software maskable and testable
I
Time-of-day alarm (once per second to once per day)
I
Periodic interrupt rates from 122 ms to 500 ms
I
End-of-clock update cycle
The real-time clock (RTC) block in the élanSC310 microcontroller is compatible with the
Motorola MC146818A device used in PC/AT systems. This block consists of a time-of-day
clock with an alarm and a 100-yr calendar. The clock/calendar can be represented in
binary or BCD, has a programmable periodic interrupt, and has 114 bytes of static user
RAM. The RTC block is powered from the same power as the core logic.
The block also has 10 registers for time, calendar, and alarm data and four general-pur-
pose registers. The interrupt signal from the RTC is an active-Low signal and is routed to
interrupt-request line 8.
3.2.7.1
Addressing
The RTC is accessed using 8-bit-wide I/O cycles at addresses 70h and 71h. The contents
of the RTC registers and its 114 bytes of RAM are accessed in an indexed fashion.
Table 3-12
Real-Time Clock
Port
R/W
Register Name
Description
070h
W
RTC Index Address
Index (pointer) to the data in the RTC
071h
R/W
RTC Index Data
Data stored in the RTC
Note:
Bit 7 of the NMI/RTC Index register, I/O address 70h, is the ENMI control bit. If this bit is set, NMIs are
masked. Bits 6–0 of this register are used to access the RTC registers and RAM.