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intet
82C5sA
DESCRIPTION
82C55AFUNCTIONAL
General
peripheral
interface
The82C55Ais a programmable
sysfor usein Intelmicrocomputer
devicedesigned
purpose
l/O
general
is
that
of
a
tems.lts function
to the
equipment
to interfaceperipheral
component
configu'
systembus.The functional
microcomputer
by the system
rationof the 82C55Ais programmed
softwareso that normallyno externallogicis necessaryto interfaceperipheraldevicesor structures.
DataBue Buffer
butferis usedto inter'
This3-statebidirectionalS-bit
face the 82C55Ato the systemdata bus' Data is
transmitted
or receivedby the bufferuponexecution
by the CPU.Gontrol
of inputor outputinstructions
words and statusinformationare also transferred
throughthe databusbutfer.
Read/Wrlteand ControlLoglc
The functionof this block is to manageall of the
internaland externaltransfersof both Data and
Gontrolor Statuswords.lt acceptsinputsfromthe
andin turn,issues
CPUAddressandControlbusses
commandsto bothof the ControlGroups.
GroupA and GroupB Controlg
The functionalconfigurationof each port is programmedby the systemssottware.In ossence,the
OPU"outputs"a controlwordto the 82C55A.The
suchas "mode",
controlwordcontainsinformation
lhe func'
"bit set", "bit r€sot",etc.,that initializes
of the 82C55A.
tionalconfiguration
Each of the Controlblocks (GroupA and Group B)
accepts"commands"from the Read/WriteControl
Logic, receives "control words" from the internal
data bus and issuesthe propercommandsto its associatedports.
ControlGroupA - PortA and PortC upper{C7-O4l
ControlGroupB - Port B and PortC lower(C3-C0)
The control word register can be both written and
read as shown in the addressdecode table in the
pin descriptions.Figure6 shows the control word
format for both Read and Write operations.When
the controlwordis read,bit D7 will alwaysbe a logic
"1", as this impliescontrolword mode information.
Ports A, B, and C
The 82C55Acontainsthree8-bitports(A, B, and C).
All can be configuredin a wide varietyof functional
characteristicsby the system softwarebut each has
its own special features or "personality" to further
enhancethe powerand flexibilityof the 82C55A.
Port A. One 8-bit data output latch/butter and one
I'bit input latch butfer. Both "pull-up" and "pull'
down" bus hold devicesare presenton Port A.
Port B. One 8-bit data input/outputlatch/butfer'
Only "pull-up"bus hold devicesare presenton Port
B.
Port C. One 8-bit data output latch/bufferand one
8-bit data input bufler (no latch for input).This port
can be divided into two 4-bit ports under the mode
control. Each 4-bit port contains a 4'bit latch and it
can be used for the control signaloutputsand status
signalinputsin conjunctionwith portsA and B. Only
"pull-up" bus hold devicesare presenton Port C'
for
See Figure4 for th€ bus-holdcircuitconfiguration
Port A, B, and C.
3-126