Configuring Port - ZiLOG Z8 Technical Manual

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Interface lZB6Bl,ZB6B2)
PDo-P07' P2o-P27, P30-P37. I/O Port lines
(inputs/outputs, TTL-compatible). These 24 I/O
lines are divided into 3 8-bit I/O ports that can
be configured under program cont rol for I/O or
memory interface. Individual lines of a port are
denoted by the second digit of the port number.
For example, P3 0 refers to bit 0 of Port 3.
RESET. Reset (input, active low, pin 6).
RESET
initializes the Z8681/82. When
RESET
is
deactivated, program execution begins from
external program location %C for the Z8681 and
location %812 for the Z8682.
If
held Low,
RESET
acts as a register file protect during power-down
and power-up sequences.
XTAl1, XTAl2. Crystal 1, Crystal 2 (oscillator
input and output, pins 3 and 2). These pins
connect a parallel resonant crystal or an external
source to the on-board clock osci llator and buf-
fer.
7.3 CONFIGURING PORT 0
The minimum bus configuration uses Port 1 as a
multiplexed Address/Data port (ADO-AD7) allowing
access to 256 bytes of memory. In this configura-
tion, the eight low order address bits (AO-A7) are
multiplexed with the data (DO-D7).
Port 0 can be programmed to provide either four
additional address lines (A 8 -A11) which increases
the addressable memory to 4K bytes, or eight
additional address lines (A 8 -A 1 5) which increases
the addressable memory to 64K bytes for the Z8681
and 62K bytes for the Z8682. Refer to Chapter 3,
Figures 3-5 and 3-6, for the memory maps.
In the Z8681 , Port
0
lines intended for use as
address lines are automatically configured as
inputs after a Reset. These lines therefore float
and their logic state remains unknown until an
initialization routine configures Port O. In the
Z8682, Port
0
lines are configured as address
lines A 8 -A 15 following a Reset.
7.3.1 lB6B1 Initialization
The initialization routine must reside within the
first 256 bytes of executable code and must be
physically mapped into memory by forcing the port
o
address lines to a known state. Figures 7-3 and
7-4 illustrate how a 4K byte memory space can be
addressed.
PORn
<
ADo-ADT
>
As, OS, RlW
Z8681
POo As
PROGRAM
MCU
MEMORY
112 PORT
o{
POl Ag
(4K BYTES)
P02
Al0
P03 An
<C
Vee
The initialization routine is mapped in the top 256 bytes of program memory. Depending on the
application, the interrupt vectors may need to be written in the first 12 byte locations of program
memory by the initialization routine.
Figure 7-3. Example lB6B1/He.ary Interface
7-2
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