Interrupt Controller (8259A)
Edge-Triggered Mode
In the edge-triggered mode, the 8259A recognizes only interrupts that
are generated by an inactive (low) to active (high) transition on an IR
input. The edge-triggered mode incorporates an edge-lockout method
of operation. After the rising edge of an interrupt request and
acknowledgment of the request, the positive level of the IR input does
not generate further interrupts on this level. You need not worry about
quickly removing the request after acknowledgment for fear of
generating further interrupts, as might be the case in the level-
triggered mode. Before another interrupt can be generated, IR input
must return to the inactive state.
Because of its edge-lockout operation, the edge-triggered mode is best
used when repetitious interrupt generation is not desired. It is also
useful in systems in which the interrupt request is a pulse (this should
be in the form of a negative pulse to the 8259A). Another advantage
is that it can be used with the automatic EOI mode without the
cautions needed in the level-triggered mode. In most cases, the edge-
triggered mode simplifies operation since the duration of the interrupt
request at a positive level is not usually a factor.
Interrupt Status
You can interrogate the status of the 8259A through software control.
This allows the internal interrupt registers to be read, which may
prove useful for interrupt control during service routines. It also
provides for a modified status poll method of device monitoring by
using the poll command, which makes status of internal IR inputs
available to you via software control. The poll command offers an
alternative to the interrupt vector method, especially for those cases
requiring more than 64 interrupts.
The contents of each 8-bit interrupt register (IRR, ISR, and IMR) can
be read to update the present status of the 8259A. This is a versatile
tool in the decision-making process of a service routine since it gives
you more control over interrupt operations.
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Содержание ZT 8809A
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