Zilog Z16C35 - Manual (Page 155)

6
6
A PPLICATION N
OTE
I
NTERFACING
Z80
®
CPU
S TO THE
Z8500
P
ERIPHERAL
F
AMILY
6
INTRODUCTION
The Z8500 Family consists of universal peripherals that
can interface to a variety of microprocessor systems that
use a non-multiplexed address and data bus. Though
similar to Z80 peripherals, the Z8500 peripherals differ in
the way they respond to I/O and Interrupt Acknowledge
cycles. In addition, the advanced features of the Z8500
peripherals enhance system performance and reduce
processor overhead.
To design an effective interface, the user needs an
understanding of how the Z80 Family interrupt structure
works, and how the Z8500 peripherals interact with this
structure. This application note provides basic information
on the interrupt structures, as well as a discussion of the
hardware and software considerations involved in
interfacing the Z8500 peripherals to the Z80 CPUs.
Discussions center around each of the following situations:
■ Z80A 4 MHz CPU to Z8500 4 MHz peripherals
■ Z80B 6 MHz CPU to Z8500A 6 MHz peripherals
■ Z80H 8 MHz CPU to Z8500 4 MHz peripherals
■ Z80H 8 MHz CPU to Z8500A 6 MHz peripherals
This application note assumes the reader has a strong
working knowledge of the Z8500 peripherals; it is not
intended as a tutorial.
CPU HARDWARE INTERFACING
The hardware interface consists of three basic groups of
signals; data bus, system control, and interrupt control,
described below. For more detailed signal information,
refer to Zilog’s DataBook, Universal Peripherals.
Data Bus Signals
D7-D0. Data Bus (bidirectional tri-state). This bus transfers
data between the CPU and the peripherals.
System Control Signals
AD-A0. Address Select Lines (optional). These lines
select the port and/or control registers.
/CE. Chip Enable (input, active Low). /CE is used to select
the proper peripheral for programming. /CE should be
gated with /IORQ or /MREQ to prevent spurious chip
selects during other machine cycles.
/RD* Read (input, active Low). /RD activates the chip-read
circuitry and gates data from the chip onto the data bus.
/WR* Write (input, active Low). /WR strobes data from the
data bus into the peripheral.
*Chip reset occurs when /RD and /WR are active
simultaneously.
Interrupt Control
/INTACK. Interrupt Acknowledge (input, active Low). This
signal indicates an Interrupt Acknowledge cycle and is
used with /RD to gate the interrupt vector onto the data
bus.
/INT . Interrupt Request (output, open-drain, active Low).
The IUS bit indicates that an interrupt is currently being
serviced by the CPU. The IUS bit is set during an Interrupt
Acknowledge cycle if the IP bit is set and the IEI line is
High. If the IEI line is Low, the IUS bit is not set, and the
device is inhibited from placing its vector onto the data bus.
In the Z80 peripherals, the IUS bit is normally cleared by
decoding the RETI instruction, but can also be cleared by
a software command (SIO). In the Z8500 peripherals, the
IUS bit is cleared only by software commands.
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