ZiLOG Z8 Technical Manual

9.2.1 Read/Write Operations
In the nibble I/O mode, Port 0 is accessed as gen-
eral-purpose register PO (%00). The port is writ-
ten by specifying PO as an instruction's destina-
tion register. Writing the port causes data to be
stored in the port's output register.
The port is read by speci fy ing PO as the source
register of an instruction. When an output nibble
is read, data on the external pins is returned.
Under normal loading conditions this is equivalent
to reading the output register. Reading a nibble
defined as input also returns data on the external
pins. However, input bits under handshake control
return data latched into the input register via
the input strobe.
The Port 0-1 Mode register bits 0
1
0
0
and 0
7
0
6
are
used to configure Port 0 nibbles (Figure 9-3).
The lower nibble (Po
o
-P0
3
) can be defined as
inputs by setting bits 0
1
to 0 and DO to 1, or as
outputs by setting both D1 and DO to O. Likewise,
the upper nibble (P0
4
-P0
7
) can be defined as
inputs by setting bits 07 to 0 and 06 to 1, or as
outputs by.setting both D6 and 07 to O.
9.2.2 Handshake Operation
When used as an I/O port, Port 0 can be placed
under handshake control by programming the Port 3
Mode register bit 02 to 1 (Figure 9-4). In this
configuration, handshake control lines are DAVo
(P3
2
)
and ROYo (P3
5
)
when Port 0 is an input port,
or ROY
o
(P3
2
)
and DAVO (P3
5
)
when Port 0 is an
output port.
Handshake direction is determined by the configu-
ration (input or output) assigned to Port 0' s
upper
nibble, P04-P07~ The lower nibble must have
the same I/O configuration as the upper nibble to
be under handshake control. Figure 9-5 illus-
trates the Port 0 upper and lower nibbles, and the
associated handshake lines of Port 3.
Handshake operation is discussed in detail in Sec-
tion 9.6.
3047-007, 3047-009, 2037-008
I/O Ports
R248 P01M
Port 0-1 Mode Register
(%
F8; Write Only)
P0
4
-P0
7
MODE ~
OUTPUT = 00 ~
INPUT = 01
A
1r
A
15
= 1X
-r PO
O
-P0
3
MODE
L 00 = OUTPUT
01 = INPUT
1X = As-All
Figure 9-3.
Port 0 I/O Operation
R247 P3M
Port 3 Mode Register
(%
F7; Write Only)
L 0 P32 = ,NPUT
1 P32 = DAVO/RDYO
P3s = OUTPUT
P3s
= RDYO/DAVO
Figure 9-4. Port 0 Handshake Operation
}
P04- P07 } PORT 0
POO-P03 (I/O OR AS-A15)
.....- } HANDSHAKE CONTROLS
DAVo AND RDYo
(P32 AND P3s)
Figure 9-5. Port 0
9-3

ZiLOG Z8 Specifications

General

Brand	ZiLOG
Model	Z8
Category	Desktop
Language	English