115
4.3 SELECTING AN OPERATING MODE
Port
A
of
the
PIO may be operated in any
of
four
distinct modes: Mode 0
(output
mode), Mode 1
(input
mode),
Mode 2 (bidirectional
mode)
, and Mode 3
(control
mode)
.
Note
that
the
mode
number
s have been selected for mne-
monic significance; i.e.
0
=Out
, 1
=In
, 2
=Bidirectional.
Port
B can operate in any
of
these modes
except
Mode 2.
The mode
of
operation
must
be established
by
writing a
control
word
to
the PIO in
the
following format:
Ds
X X
mode word
signifies mode word
to be set
Do
X=
unused bit
Bits D
7
and D
6
form
the
binary code for the desired
mode
according
to
the
following table:
07
06
Mode
0 0 0
(output)
0
1
1
(input)
0 2 (bidirectional)
3
(control)
Bits D
5
and D
4
are ignored. Bits D
3
-D
0
must
be set
to
1111
to
indicate "
Set
Mode" .
Selecting Mode 0 enables any data
written
to
the
port
output
register
by
the
CPU
to
be
output
to
the
port
data
bus. The
contents
of
the
output
register may be changed
at
a
ny
time
by
the
CPU simply
by
writing a new data word
to
the
port.
Also,
the
current
contents
of
the
output
register may be read
back
to
the Z-80A-CPU at any time
through
execution
of
an
input
instruction.
With Mode
0 active, a
dat
a write from
the
CPU causes
the
Ready handshake line
of
that
port
to
go
high
to
notify
the peripheral
that
data
is
available. This signal remains high
until
a strobe
is
received from
the
peripheral. The rising
edge
of
the
strobe generates an
interrupt
(if
it has
been
enabled) and causes
the
Ready line to
go
inactive. This very
simple handshake
is
similar
to
that
used in
many
peripheral devices.
Selecting Mode
1 puts
the
port
into
the
input
mode.
To
start handshake
operation
,
the
CPU merely performs an
input
read operation from
the
port.
This activates
the
Ready
line
to
the
peripheral
to
signify
that
data should be loaded
into
the
empty
input
register. The peripheral device
then
strobes data
into
the
port
input
register using the strobe line .
Again,
the
rising edge
of
the
strobe causes an
interrupt
request
(if
it
h::rs
been
enabled) and deactivates
the
Ready signal.
Data may be strobed
into
the
input
register regardless
of
the
state
of
the Ready signal
if
care
is
taken
to
prevent a
data
overrun
condition
.
Mode
2
is
a bidirectional
data
transfer mode which uses all four handshake lines. Therefore only
Port
A may be
used for Mode 2 operation. Mode 2
operation
uses the
Port
A handshake signals for
output
control
and
the
Port
B
handshake signals for
input
control.
Thus
,
both
A RDY and B
RDY
may be active simultaneously. The only operation-
al
difference
between
Mode 0 and
the
output
portion
of
Mode 2
is
that
data
from
the
Port
A
output
register
is
allowed
on
to
the
port
data bus only
when
A STB
is
active in
order
to
achieve bidirectional capability.
Mode 3
operation
is
intended
for status
and
control applications and does
not
utilize
the
handshake signals. When
Mode
3
is
selected,
the
next
control word sent
to
the PIO must define which
of
the
port
data bus lines are
to
be inputs
and which are
outputs.
The format
of
the
control
word
is
shown below:
Ds
Do
1/0s
1/0
0
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