Intel mcs-48 manual Oh-++-+, Priority Cell Simplified Logic Diagram, Level Triggered Mode

8259A

LTIM liT

.=EOGE

1 "",LEVEL

,R -+-11>>----'1..

.:~

II I

'1:1

Ii

TO OTHER 'AIORTY CULS

itT

·""ORITY

REIOLVER

CONTRDL

LOGIC

REOUEST

LATCH

o a~~-+---,~uo.~-+~--4~ NON·

MASKED

LATCHREO

c a o oH-++-+

NOTES

1.MAlTER CLEAR ACTIVE ONLY DURING rcw,

2.FIIEEZE/II ACTIVE DURING iJiITJ AND POLL SEQUENCES ONLY

3.TRUTH TAiLE FOR D·LATCH

OPEPIATION

fOLLOW

HOLD

Priority Cell - Simplified Logic Diagram

LEVEL TRIGGERED MODE

This mode is programmed using bit 3 in ICW1.

If LTIM ='1',an interrupt request wilrbe recognized by a 'high'level on IA Input, and there is no need for an edge detection. The interrupt request must be removed before the EOI command Is Issued or the CPU Interrupt is enabled to prevent a second interrupt from occurring.

The above figure shows a conce~::Jal circuit to give the reader an understanding of the level sensitive and edge sensitive input circuitry of the 8259A. Be sure to note that the request latch is a transparent D type latch.

READING THE 8258A STATUS

The Input status of several internal registers can be read to update the user Information on the system. The following registers can be read by Issuing a suitable OCW3 and reading with AD.

Interrupt Mask Register: 8-blt register whose content specifies the Interrupt request lines being masked. acknowledged. The highest request leyel is reset from the IAA when an Interrupt Is acknowledged. (Not affected by IMA.)

In-Service Register (ISR): 8-bit register which contains the priority levels that are being serviced. The .ISA is updated when an End of Interrupt command is Issued.

Interrupt Mask Register: 8-bit register which. contains the interrupt request lines which are masked.

The IAA can be read when, prior to the AD pulse, a WR pulse is Issued with OCW3 (EAIS= 1, RIS=O.)

The ISA can be read in a similar mode when EAIS= 1, AIS = 1 in the OCW3.

There is no need to write an OCW3 before every status read operation, as long as the status read corresponds with the previous one; I.e., the 8259A "remembers" whether the IRA or ISA has been previously selected by the OCW3. This is not true when poll is used.

After Initialization the 8259A is set to IAA.

For reading the IMA, no OCW3 is needed. The output data bus will contain the IMA whenever AD is actiYe and AO= 1.

Polling overrides status read when P= 1, EAIS= 1 in OCW3.

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