Microprocessors & Microcontrollers: Interrupt controller 8259; The 8259 is known as the Programmable Interrupt Controller (PIC) microprocessor. In 8085 and 8086 there are five hardware interrupts and two hardware interrupts respectively. Bu adding 8259, we can increase the interrupt handling capability. This chip combines the multi-interrupt input source to single interrupt output. This provides 8-interrupts from IR0 to IR7. Let us see some features of this microprocessor.
This chip is designed for 8085 and 8086.
It can be programmed either in edge triggered, or in level triggered mode
We can mask individual bits of Interrupt Request Register.
By cascading 8259 chips, we can increase interrupts up to 64 interrupt lines
Clock cycle is not needed.
8259 microprocessor can be programmed according to given interrupts condition and it can be provided either with level or edge-triggered interrupt level.
It can be programmed to either work in 8085 or in 8086 microprocessors.
Individual interrupt bits can be masked.
By cascading Nine 8259’s in Master-Slave Configuration we can handle up to 64 interrupt pins.
It contains 3 registers commonly known as ISR, IRR, IMR & there is 1 priority resolver (PR).
Interrupt Request Register (IRR): It stores those bits which are requested for their interrupt services.
Interrupt Service Register (ISR): It stores the interrupt levels which is currently being served.
Interrupt Mask Register (IMR): It stores interrupt levels that have to be masked. These interrupt levels are already accepted by the 8259 microprocessor.
Priority Resolver (PR): It examines all the 3 registers and sets the priority of interrupts and sets the interrupt levels in ISR which has the highest priority and the rest of the interrupt bit is IRR which is already accepted.
SP/EN (low active pin): If its value is 1 it works in master mode & if its value=e is 0 then it works in slave mode.
Cascade Buffer: It is used to cascade more number of Programmable Interrupt Controller to increase the interrupts handling capability up to 64 levels.
Advantages:
Interrupt management: The 8259 microprocessor is a specialized chip that is dedicated to managing interrupts, which can help to improve system performance and reduce the workload on the main CPU.
Programmability: The 8259 microprocessor is programmable, which means that it can be customized to handle specific types of interrupts and to prioritize different interrupt requests.
Compatibility: The 8259 microprocessor is compatible with a wide range of microprocessors, making it a popular choice for interrupt management in many different systems.
Multiple interrupt inputs: The 8259 microprocessor can handle multiple interrupt inputs, which makes it a useful peripheral for managing complex systems with multiple devices.
Ease of use: The 8259 microprocessor includes simple interface pins and registers, making it relatively easy to use and program.
2. INTEL 8259
• Intel 8259 is a Programmable Interrupt
Controller (PIC).
• There are 5 hardware interrupts and 2
hardware interrupts in Intel 8085 and Intel
8086 microprocessors respectively. But by
connecting Intel 8259 with these
microprocessors, we can increase their
interrupt handling capability.
• This chip combines the multi-interrupt input
source to single interrupt output. This
provides 8-interrupts from IR0 to IR7.
INTERRUPT
CONTROLLER
8259
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3. FEATURES OF INTEL
8259 PIC
INTERRUPT
CONTROLLER
8259
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• Intel 8259 is designed for Intel 8085 and Intel 8086
microprocessor.
• It can be programmed either in level triggered or in edge triggered
interrupt level.
• We can mask individual bits of interrupt request register.
• We can increase interrupt handling capability upto 64 interrupt
level by cascading further 8259 PICs.
• Clock cycle is not required.
6. ABOUT EACH BLOCK
Block : Description
Data Bus Buffer
:
This block is used to communicate between 8259 and 8085/8086 by acting as buffer. It
takes the control word from 8085/8086 and send it to the 8259. It can send maximum 8-bit
at a time.
R/W Control
Logic :
This block works when the value of pin CS is 0. This block is used to flow the data
depending upon the inputs of read(RD) and write(WR).
Control Logic : It controls the functionality of each block. It has pin called INTR. This is connected to other
microprocessors for taking the interrupt request. The INT pin is used to give the output.
Interrupt
Request
Register :
It stores all interrupt level that are requesting for interrupt service.
Cascade Buffer : To increase number of interrupt pin, we can cascade more number of pins, by using
cascade buffer. When we are going to increase the interrupt capability, CSA lines are used
to control multiple interrupts.
Interrupt Mask
Register :
It stores interrupt level that will be masked, by storing the masking bits of interrupt level.
Priority Resolver
:
It checks all three registers, and set the priority of the interrupts. Interrupt with the highest
priority is set in the ISR register. It also reset the interrupt level which is already been
serviced in the IRR.
INTERRUPT
CONTROLLER
8259
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7. OPERATION MODE OF 8259:
1. Fully nested mode : It is the default mode of operation of 8259. Here, IR0 has the highest priority and IR7 has the
lowest priority. When any interrupt requests occurs then the highest priority interrupt request is serviced first and
its vector address is placed on data bus and its corresponding bit in ISR register is set until the processor
executes the EOI command before returning the interrupt service routine until the falling of the last INTA’. When
the ISR bit is set for an interrupt, then all the equal and lower priority interrupts are masked, but a higher level
interrupt request can occur and which will be acknowledged only if the microprocessor interrupt enables flag IF=
1. It is suitable for a single 8259 configuration.
2. Special fully nested mode (SFNM) : This mode is used by master 8259 in a cascaded mode. Its priority
structure is fixed and is the same as fully nested mode (i.e. IR0 has the highest priority and IR7 has the lowest
priority). In a special fully nested mode, the master will only serve higher priority interrupt from a slave, whose
another interrupt is currently in service.
3. Special mask mode(SMM) : In SMM, 8259 enables interrupts of all levels (lower or higher) except the one that is
currently in service. Because we are especially masking the request of the priority level of interrupt, which is the
same as the current interrupt priority level, therefore it is called special mask mode.
4. Poll mode :
Here the INT pin of 8259 is not used, so, 8259 cannot interrupt the µp. Instead, the µP will provide a poll
command to 8259 using OCW3. In response, 8259 provides a poll word to the µP. The poll word indicates the
highest priority interrupt which needs service from µP. Thereafter, the µP services the interrupt.
INTERRUPT CONTROLLER 8259
INTERRUPT CONTROLLER 8259 7
8. 5. EOI – (End Of Interrupt) : When the microprocessor acknowledges an interrupt
request by sending the first INTA signal, the 8259 sets the corresponding bit in
the In
Service Register (ISR). This starts the service of the interrupt. When this bit in
the ISR is
cleared, then it is known as the end of interrupt (EOI).
EOI Modes:
1. Normal EOI Mode – Here, an EOI command is compulsory. The EOI
command is
written by the programmer at the end of the ISR. It makes 8259 to reset the bit
from
ISR. Further EOI command is of two types :
i) Non Specific EOI Command : Here the programmer doesn’t specify the
bit
number to be reset in the ISR. 8259 itself resets the highest priority bit
from
ISR.
ii) Specific EOI Command : Here the programmer determines the bit
number
to be reset from ISR.
2. Automatic EOI mode (AEOI) – In this mode, the EOI command is not
required.
Instead, 8259 will itself clear the corresponding bit from ISR at the end of the
2nd
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INTERRUPT
CONTROLLER
8259
10. THE INTEL 8259A IS INDEED A REVISED
VERSION OF THE ORIGINAL INTEL 8259
PROGRAMMABLE INTERRUPT
CONTROLLER (PIC). THE 8259A WAS
INTRODUCED TO ADDRESS SOME
LIMITATIONS AND IMPROVE UPON THE
FUNCTIONALITY OF THE ORIGINAL 8259.
• The 8259A is a programmable interrupt controller (PIC) that
works with Intel microprocessors 8085, 8086, and 8088.
• The 8259 works with 8-bit processors, while the 8259A works
with 8-bit and 16-bit processors.
INTERRUPT
CONTROLLER
8259
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11. HOW IS 8259A
DIFFERENT FROM 8259
• Improved cascade support for handling
more interrupts.
• Refined initialization process for more
setup.
• Maintains compatibility with 8259.
• May perform improved performance and
reliability.
INTERRUPT
CONTROLLER
8259
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