8259A PROGRAMMABLE INTERRUPT CONTROLLER

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• The 8259A consist of eight data bus lines from D0-D7

• The data bus is the path over which data are transferred between MPU and 8259A.

• The data can be command words , status information or interrupt type numbers.

• The control signals WR and RD (active low) logic 0, signal the 8259A whether data to be written or read from its internal register.

• The CS (active low) to enable the host interface enable.

• A0 input involved in he selection of internal register that is accessed during read and write operations.

• Interrupt request IR0-IR7 inputs are issued by external devices for service

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• CAS0-CAS2 are known as cascade interface.

• The cascade lines are used to cascaded system where number of 8259A ICs are interconnected in master/slave configuration to expand the number of IR inputs from 8 to as high as 64.

• SP/EN (active low) is used to indicate the current device acts as a master or slave.

• INT is the interrupt request output of 8259A.

• It is applied directly to the INTR input of the 8086.

• Logic 1 is produced at this output whenever the interrupt controller receives a valid request from an interrupting device.

• INTA (active low) is an input pin of the 8259A.

• This pin is used to receive acknowledgement

INTERRUPT REQUEST REGISTER

• Interrupt request register keeps track of which interrupt inputs are asking for service

• If an interrupt input has an interrupt signal on it , then corresponding bit in the interrupt request register will be set.

INTERRUPT MASK REGISTER

• Interrupt mask register is used to disable or enable the individual interrupt inputs.

• Each bit in this register corresponds to the interrupt input with the same number.

IN-SERVICE REGISTER

• The In-service register keep tracks of which interrupt inputs are currently being serviced

• The corresponding bit will be set in the in service register.

PRIORITY RESOLVER

• The priority resolver acts as a “judge” that determines if and when an interrupt request on one of the IR inputs get serviced.

PROGRAMMING THE 8259A

• The way in which the 8259A operates is determined by how the device is programmed.

• Two types of command words are provided for the programming of 8259A.

• ICW (Initialization Command words)

• OCW( Operational Command Words)

ICW

• ICW commands are used to load the internal control registers of the 8259A to define the basic configuration or mode in which it is used.

• There are four command words identified as • ICW1• ICW2• ICW3• ICW4

• OCW are needed for 8086 microprocessor to initiate variation in the basic operating modes defined by the ICW commands.

• OCW1,OCW2,OCW3.

ICW1

ICW1 BITS

• A0=0 and D4=1 bit is set indicates the ICW1.

• D0 bit indicates ICW4 is needed or not.

• 1=ICW4 needed

• 0=no ICW4 needed.

• D1=1 single mode

• D1=0 cascade mode

• LTIM=1 Level Triggered Input

• LTIM=0 Edge Triggered Input

• In 8086 mode D5,D6,D7 are don’t care conditions.

ICW2

• In 8086 system, ICW2 is used to tell the 8259A the type number to send in response to an interrupt signal on IR0 input.

• In response to an interrupt signal on some other IR input , the 8259A will automatically add the number of IR input to the base number and send the result to the 8086 as the type number for that input.

ICW2

ICW3

• ICW3 is needed when 8259A is working in a cascade mode of operation.

• ICW3 is used in a different function based on 8259A is working as master or slave.

• If it is master the bits D7-D0 are labeled as S7-S0.

• If it is slave the device load with a 3-bit identification code ID2,ID1,ID0.

•

ICW4

• ICW4 is used when the device is configured with the 8088 or 8086.

• When PU bit is set to 1 indicates the working with 8086.

• AEOI

• MS

• BUF

• SFNM

OPERATIONAL COMMAND WORDS

• OCW are used to provided for controlling the operation of 8259A.

• The OCW1 is used to access the contents of mask register.

• This permits the selective masking of interrupt inputs.

•

OCW2

• The OCW2 selects the appropriate priority scheme and assigns an IR level scheme that requires a specific interrupt level.

• It is used to reset bit in the in-service register

OCW3

• It permits the reading of the contents of ISR or IRR registers through software.

MODES OF OPERATIONS

• 1>Fully nested mode

• 2>special fully nested mode.

• 3>nonspecific rotating mode.

• 4>specific rotating mode.

• 5>special mask mode.

• 6>polled mode.

INTERFACING 5259