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6.0 Expansion Past 64 Interrupts
In some instances, it may be desirable to expand the number
of available interrupts in a system past the maximum of 64
imposed when using cascaded 82C59As. The easiest way
to accomplish this is through the use of the Poll command
with the 82C59A. Figure 18 illustrates one example of how
this expansion can be accomplished. Notice that we are
using two 3 - to -8 decoders to address up to 16 82C59As.
Selection of which decoder is active takes place using the
OE pin driven by AD5 from the CPU’s address bus.
With this type of interrupt structure, we are not using the INT
and
INTA lines from our processor (80C88 for this example).
Because of this, no interrupts will break execution of the
system software. Therefore, it is the task of the software to
poll the various 82C59As in the system to see if any
interrupts are pending. Once it has been established which
interrupt requires servicing, the software can take
appropriate action.
There are disadvantages to using the poll mode for the
systems interrupt structure: (1) the overhead of polling each
of the 82C59As reduces the systems efficiency, and (2)
realtime interrupt servicing cannot be guaranteed.
There are several advantages to using the poll mode in this
manner: (1) there can be more than 64 priority interrupts in
the system, and (2) memory in the system is freed because
no interrupt vector table is required.
FIGURE 18. EXPANDING PAST 64 INTERRUPTS
A, B, C
Y0
DECODER
Y1
AD2 - AD4
Y7
OE
AD5
A0
CS
82C59A
8
3
IR0-IR7
A, B, C
Y0
DECODER
Y1
Y7
OE
3
A0
CS
82C59A
8
IR0-IR7
A0
CS
82C59A
8
IR0-IR7
A0
CS
82C59A
8
IR0-IR7
A0
CS
82C59A
8
IR0-IR7
AD0
Application Note 109
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