Specifications
4.4.1
Interrupt Controller
The
interrupt controller within the V40 provides a superset
of
the functionality
of
the 8259A
interrupt
controller
normally
used
in
a
standard
PC.
and
is software compatible
with
that
device. Six
of
the seven interrupt sources
are
connected directly
to
the corresponding signals
on
the
board's
PC
Expansion
Bus.
In
addition, they
are
normally assigned
to
the
board's
onboard
I/O
interfaces
in
the following manner, which maintains compatibility with
the
PC:
•
IRQ7
-- Parallel printer
•
IRQ6
--Floppy
disk controller
•
IRQ5
-- Available (SCSI controller option)
•
IRQ4
--
PC
serial
port
•
IRQ3
-- Available
(PC
serial port option)
•
IRQ2
-- Available
•
IRQ1--
Keyboard
• IRQO -- Hard-wired (within the V40) to counter/timer channel 0
As
indicated, interrupt input IRQO
is
hard-wired internally to
counter/timer
channel 0, which
corresponds to the PC's real-time clock interrupt. IRQ5, though available for the on-board
SCSI bus controller,
is
not
currently used.
4.4.2
Countermmers
The
counter/timer
controller
(CTC)
within
the
V40 provides
the
same
functionality
as
the
8253
CTC
device normally used
in
a standard PC, and is software compatible with
that
device.
The
7.16
MHz
master
system clock is divided
by6
withinASIC1,
resulting
in
a 1.193
MHz
timebase clock which is provided to the V40's "TCLK" input for use
by
the CTC.
This
pro-
vides a PC-compatible timebase for use
by
each
CTC
channel.
The
V40's three
counter/timers
are
normally utilized
by
the board's ROM-BIOS as follows:
•
CTCO
--
Generates 18.2 ·interrupts per second via IRQO.
Used
by
DOS
to
maintain the
system's real time clock.
•
CTC1--
Used
for
baud
rate
generation
to
support
the
V40's internal serial port.
(On
a
PC
this
channel is used for
DRAM
refresh
~)
• CTC2 -- Generates tones for the speaker.
The
output
of
timer channel 2 is connected
to
ASIC2
and
acts
as
the
basic tone source for the Little
Board/PC's
sound circuits.
In
a
standard
PC,
timer
channel
1 is
used
to
trigger
DMA
channel
0 for
DRAM
refresh
cycles. Since
the
Little
Board/PC
has its own refresh control unit, this timer channel
is
avail-
able for the internal serial port's
baud
rate
generator.
4-5