Technical data

Time-of-Year Clock
Overview
The time-of-year (TOY) clock consists of an MC146818BM CMOS
watch chip that keeps the date and time of day and contains 50
bytes of general purpose RAM storage. This chip includes a time
base oscillator and a lithium battery on-chip. The battery powers
the chip with logic and oscillator while system power is off.
Battery Backup
A lithium battery within the watch chip supplies power to the
watch chip and its time base oscillator while system power is off.
The battery maintains the clock operation and the data stored in
the 50 bytes of RAM for a minimum of 10 years before it becomes
exhausted.
Watch Chip
Registers
The watch chip contains 64 8-bit registers. Ten of these contain
date and time data, four are control and status registers, and the
remaining 50 provide general purpose RAM storage. The registers
occupy 64 consecutive longwords of address space as shown in the
next table.
Each register is accessed as bits <9:2> of a longword. Bits <31:10>
and <1:0> are ignored on writing and undefined on reading.
WARNING
Because each register spans two bytes on the system
bus, only word or longword access instructions may
be used to manipulate these registers. The effects
of using byte access instructions are undefined. In
particular, instructions for modifying bits such as
BBSS, BBSC, BBCC, and BBCS cannot be used; they
generate byte-access read-modify-write cycles that will
corrupt the portion of the register that is not in the byte
being accessed.
1–30