124
An interrupt control word
is
next sent to the port:
0
0
Enable
OR
Active Mask
Interrupts Logic High Follows Interrupt
control
The mask word following the interrupt mode word is:
Do
0
0 0
Selects AS,
A3
and
AO
to be monitored
Now,
if
a sensor puts a high level on line A
5
,
A
3
,
or A
0
,
an interrupt request will be generated. The mask word may
select any combination
of
inputs or outputs to cause an interrupt. For example, if the mask word above had been:
0
0
0
0
then an interrupt request would also occur if bit A
7
(Special Test)
of
the
output
register was set.
Assume
that
the following
port
assignments are
to
be used:
EOH
=Port
A Data
ElH
=Port
B Data
E2H = Port A
Control
E3H
=Port
B Control
All
port
numbers are in hexadecimal notation. This particular assignment
of
port
numbers
is
convenient since A
0
of
the
address bus can be used
as
the Port B/A Select and A
1
of
the address bus can be used
as
the Control/Data Select. The
Chip Enable would be the decode
of
CPU address bits A
7
thru A
2
(1110 00). Note that if only a few peripheral devices
are being used, a
Chip Enable decode may
not
be required since a higher order address bit could be used directly.
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