82C59A
treats the RD following a “poll write” operation as an INTA. In both the edge and level triggered modes the IR inputs
After initialization, the 82C59A is set to lRR.
must remain high until after the falling edge of the first INTA.
If the IR input goes low before this time a DEFAULT lR7 will
occur when the CPU acknowledges the interrupt. This can
be a useful safeguard for detecting interrupts caused by spu-
rious noise glitches on the IR inputs. To implement this fea-
ture the lR7 routine is used for “clean up” simply executing a
return instruction, thus, ignoring the interrupt. If lR7 is
needed for other purposes a default lR7 can still be detected
by reading the ISR. A normal lR7 interrupt will set the corre-
sponding ISR bit, a default IR7 won’t. If a default IR7 routine
occurs during a normal lR7 routine, however, the ISR will
remain set. In this case it is necessary to keep track of
whether or not the IR7 routine was previously entered. If
another lR7 occurs it is a default.
For reading the lMR, no OCW3 is needed. The output data bus
will contain the lMR whenever RD is active and A0 = 1 (OCW1).
Polling overrides status read when P = 1, RR = 1 in OCW3.
Edge and Level Triggered Modes
This mode is programmed using bit 3 in lCW1.
If LTlM = “0”, an interrupt request will be recognized by a low to
high transition on an IR input. The IR input can remain high
without generating another interrupt.
If LTIM = “1”, an interrupt request will be recognized by a “high”
level on an IR input, and there is no need for an edge detection.
The interrupt request must be removed before the EOI com-
mand is issued or the CPU interrupt is enabled to prevent a
second interrupt from occurring.
In power sensitive applications, it is advisable to place the
82C59A in the edge-triggered mode with the IR lines nor-
mally high. This will minimize the current through the internal
pull-up resistors on the IR pins.
The priority cell diagram shows a conceptual circuit of the level
sensitive and edge sensitive input circuitry of the 82C59A. Be
sure to note that the request latch is a transparent D type latch.
80C86/88/286
8080/85
IR
INT
80C86/88/286
INTA
LATCH
ARM
(NOTE 1)
8080/85
LATCH
ARM
LATCH
ARM
(NOTE 1)
(NOTE 1)
EARLIEST IR
CAN BE
REMOVED
NOTE:
1. Edge triggered mode only.
FIGURE 10. IR TRIGGERING TIMING REQUIREMENTS
The Special Fully Nested Mode
This mode will be used in the case of a big system where
cascading is used, and the priority has to be conserved
within each slave. In this case the special fully nested mode
will be programmed to the master (using lCW4). This mode
is similar to the normal nested mode with the following
exceptions:
one from that slave. This is done by sending a non-spe-
cific End of Interrupt (EOI) command to the slave and
then reading its In-Service register and checking for zero.
If it is empty, a non-specified EOI can be sent to the mas-
ter, too. If not, no EOI should be sent.
Buffered Mode
a. When an interrupt request from a certain slave is in ser-
vice, this slave is not locked out from the master’s priority
logic and further interrupt requests from higher priority
IRs within the slave will be recognized by the master and
will initiate interrupts to the processor. (In the normal
nested mode a slave is masked out when its request is in
service and no higher requests from the same slave can
be serviced.
When the 82C59A is used in a large system where bus driv-
ing buffers are required on the data bus and the cascading
mode is used, there exists the problem of enabling buffers
The buffered mode will structure the 82C59A to send an
enable signal on SP/EN to enable the buffers. In this mode,
whenever the 82C59A’s data bus outputs are enabled, the
SP/EN output becomes active.
b. When exiting the Interrupt Service routine the software
has to check whether the interrupt serviced was the only
4-13
INTERSIL [ Intersil ]
