Z86C21 Datasheet, PDF (8/35 Pages) Zilog, Inc. – 8K ROM Z8 CMOS MICROCONTROLLER

PIN FUNCTIONS (Continued)
Z86C21 MCU
WITH 8K ROM
Port 1 (P17-P10). Port 1 is an 8-bit, byte programmable,
bidirectional, TTL compatible port. It has multiplexed Ad-
dress (A7-A0) and Data (D7-D0) ports. For Z86C21, these
eight I/O lines can be programmed as Input or Output lines
or can be configured under software control as an ad-
dress/data port for interfacing external memory. When
used as an I/O port, Port 1 can be placed under handshake
control. In this configuration, Port 3 line P33 and P34 are
used as the handshake controls RDY1 and /DAV1.
Memory locations greater than 8192 are referenced through
Port 1. To interface external memory, Port 1 is programmed
for the multiplexed Address/Data mode. If more than 256
external locations are required, Port 0 must output the
additional lines.
Port 1 can be placed in a high-impedance state along with
Port 0, /AS, /DS and R//W, allowing the MCU to share
common resource in multiprocessor and DMA applica-
tions. Data transfers are controlled by assigning P33 as a
Bus Acknowledge input, and P34 as a Bus request output
(Figure 6).
Z86C21
MCU
8
Port 1
(AD7-AD0)
Handshake Controls
/DAV1 and RDY1
(P33 and P34)
OEN
PAD
Out
TTL Level Shifter
In
R ≈ 500 KΩ
Auto Latch
Figure 6. Port 1 Configuration
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