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Write short note on 8259A programmable interrupt controller.



Write short note on 8259A programmable interrupt controller.

       8259 A programmable Interrupt Controller : For application where we require multiple interrupt sources, we need to use an external device called priority interrupt controller (PIC).

     By connecting a PIC to the microprocessor we can increase the interrupt handling capacity of the microprocessor, 8259 A is the commonly used priority interrupt controller.

      Cascade PICs System with 8085 : The 8259 A can be easily interconnected to obtain multiple interrupts, Figure 1 below shows 8259 A connected in cascade mode.
      When 8259 A IC's are connected in cascade, one 8259 A is configured as master while all other 8259 A's are configured as slave.
      In Figure 1 8259-1 is in master mode and others are in slave mode.
      Each slave is recognized by a number that is assigned as a part of its initialization.
      The 8085 microprocessor has only one INTR input. Hence only one of 8259 A INT pin is connected to the 8085 INTR pins.
      The INT pins from other 8259s are connected to the IR inputs of the master 8259 A.
      These cascaded 8259s are called as slave. The INTA signal is connected to master as well as slave 8259 A.
      The cascade pins CAS0 - CAS2 are connected from master to corresponding slave pins. For master they function as outputs while for slave these pins function as input.
      The SP/EN signals is tied high for the master. However it is grounded for all slaves.
Address for 8259 A -1
A7
A6
A5
A4
A3
A2
A1
A0
1
1
1
1
0
0
0
X
= F0 or F1 H
For 8259 A - 2
A7
A6
A5
A4
A3
A2
A1
A0
1
1
1
1
0
0
1
X
= F2 H or F3 H
For 8259 A - 3
A7
A6
A5
A4
A3
A2
A1
A0
1
1
1
1
0
1
0
X
= F4 H or F5 H

       Upto 8 PICs can be cascade together to act as a slave unit to master PIC. Thus, a total of 64 peripherals can be connected to the microprocessor.

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