Real Time Clock Plus Cmos Ram; Real-Time Clock - HP Vectra Reference Manual

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
l l
•
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