Zilog Z86193 - Manual (Page 57)

Z8
®
CPU
User Manual
UM001604-0108 Input/Output Ports
50
Read/Write Operations
In byte input or byte output mode, the port is accessed as General-Purpose Register P1
(
01h
). The port is written by specifying P1 as an instruction's destination register. Writing
to the port causes data to be stored in the port's output register.
The port is read by specifying P1 as the source register of an instruction. When an output
is read, data on the external pins is returned. Under normal loading conditions, this is
equivalent to reading the output register. However, if Port 1 outputs are defined as open-
drain, the data returned is the value forced on the output by the external system. This may
not be the same as the data in the output register. When Port 1 is defined as an input, read-
ing also returns data on the external pins. However, inputs under handshake control return
data latched into the input register via the input strobe.
Figure 35. Port 1 Configuration with TTL Level Shifter
OEN
Port 1
(I/O or AD7–AD0)
8
Z8
(P33 and P34)
PIN
OUT
IN
TTL Level Shifter
Handshake Controls
DAV1 and RDY1