Real
Time
Clock
plus
CMOS
RAM
The
HP
Vectra
PC
maintains
an
accurate real-time clock
with
the
MC146818
RT/CMOS
plus RAM chip
without
processor
interaction.
This
chip
is
designed
to
perform RAM, time, and
calendar functions. The CMOS RAM, timebase, and oscillator
maintain their functions during a
power
failure by using battery
backup. The backup power
is
a lithium battery pack, which
has
an
average life
of
2.3 years. The battery pack
is
located in a bracket
on
the
power supply.
Real-Time
Clock
The real-time clock
(RTC)
uses
a 32.768kHz crystal
as
its timebase.
It counts seconds, minutes, hours, days, and years.
In
addition,
it
keeps track
of
the day
of
the week and provides automatic leap
year compensation.
The
RTC
can
provide
an
interrupt request
(IRQ8)
at either a fixed
interval (i.e., every second),
or
when
a certain
time
has
arrived (in
the alarm clock mode). The
RTC
keeps track
of
the time and day
when the system power
is
off. During power-on reset,
the
system
software reads the current time and date
from
the
RTC
and
converts it into the appropriate number
of
system clock IIticks
ll
•
From this point on, system time
is
kept via system clock ticks
which are generated by Counterrrimer
o.
The
RTC
is
accessed
via
two
ports in the system I/O map:
the
Address Port (70H), and the Data Port
(71
H).
The
RTC
contains
64
bytes
of
memory. The first 14
of
these bytes are
the
Real-Time
Clock registers, the remaining
50 bytes are CMOS RAM
for
storing
system parameters.
In
order
to
access
the
RTC,
the address
of
the
byte
of
RAM (or
clock register)
to
be
accessed
is
output
to
port
70H. The address
(in the range
of
0 -
3FH)
is
placed on bits 0 -
5.
(Bit 6
is
unused
and bit 7
is
the NMI mask.)
After
the desired register
or
RAM byte
has
been selected, data may
be
either read
or
written
to
the
Data
Port,
71
H.
Processor Board
41
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