MA28155
FUNCTIONAL DESCRIPTION
The MA28155 is a programmable peripheral interface
(PPI) device designed for use with MAS281. Its function is that
of a general purpose l/O component to interface peripheral
equipment to the microcomputer system bus. The functional
configuration of the MA28155 is programmed by the system
software so that, normally, no external logic is necessary to
interface peripheral devices or structures.
Data Bus Buffer
This 3-state, bidirectional, 8-bit buffer is used to interface
the MA28155 to the system data bus. Data is transmitted or
received by the buffer upon execution of input or output
instructions by the CPU. Control words and status information
are also transferred through the data bus buffer.
Read/Write and Control Logic
The function of this block is to manage all of the internal
and external transfers of both Data and Control Status words.
It accepts inputs from the CPU Address and Control busses
and in turn, issues commands to both of the Control Groups.
Reset (RESET)
A high on this input clears the control register and all ports
(A,B,C) are set to the input mode.
Chip Select (CSN)
A low on this input pin enables the communication between
the MA28155 and the CPU.
Read/Write Select (RD/WN)
A high on RD/WN indicates a CPU read from the MA28155
and a low indicates a CPU data or control word write to the
MA28155. The RD/WN line is active only when DSN is low.
Data Strobe (DSN)
This input indicates that a data transfer is taking place.
During a CPU write operation the MA28155 reads data from
the bus on the rising edge of DSN. During a read operation the
MA28155 outputs data to the bus while
DSN
is low. Data is
valid on the rising edge of DSN.
Port Select O and Port Select 1 (AO and A1 )
These input signals, in conjunction with the DSN and RD/
WN inputs, control the selection of one of the three ports of the
control word registers. They are normally connected to the
least significant bits of the address bus.
Basic Operation
A1 A0
DSN
RD/WN
CSN
WN
0
0
1
0
0
1
1
x
1
x
0
1
0
0
1
0
1
x
1
x
0
0
0
0
0
0
0
x
0
1
1
1
1
0
0
0
0
x
1
x
0
0
0
0
0
1
1
1
0
0
READ
PORT A
→
DATA BUS
PORT B
→
DATA BUS
PORT C
→
DATA BUS
WRITE
DATA BUS
→
PORT A
DATA BUS
→
PORT B
DATA BUS
→
PORT C
DATA BUS
→
CONTROL
DISABLE
DATA BUS
→
TRI-STATE
ILLEGAL CONDITION
DATA BUS
→
TRI-STATE
Table 1: Basic Operation
OPERATIONAL DESCRIPTION
Mode Selection
There are three basic modes of operation, which can be
selected by the system software:
Mode 0. Basic Input/Output
Mode 1. Strobed Input/Output
Mode 2 Bi-directional Bus
When the reset input goes high all ports will be set to the
input mode (i.e. all 24 lines will be in the high impedance state)
After the reset is removed the MA28155 can remain in the
input mode with no additional initialisation required.
During the execution of the system program any of the
other modes may be selected using a single output instruction.
This allows a single MA28155 to service a variety of peripheral
devices with a single software maintenance routine.
The modes for Port A and Port B can be separately
defined, while Port C is divided into two portions as required by
the Port A and Port B definitions. All of the output registers,
including the status register, will be reset whenever the mode
is changed.
Modes may be combined so that their functional definition
can be tailored to almost any l/O structure. For instance; Group
B can be programmed in Mode 0 whilst Group A could be
simultaneously programmed in Mode 1.
2/20
DYNEX [ Dynex Semiconductor ]
