INTEL82801 datasheet(77/414 Pages) INTEL

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82801AA and 82801AB Datasheet

5-27

Functional Description

Special Fully Nested Mode

This mode is used in the case of a system where cascading is used, and the priority has to be

conserved within each slave. In this case, the special fully nested mode is programmed to the

master controller. This mode is similar to the fully nested mode with the following exceptions:

• When an interrupt request from a certain slave is in service, this slave is not locked out from

the master's priority logic and further interrupt requests from higher priority interrupts within

the slave is recognized by the master and initiates interrupts to the processor. In the normal

nested mode, a slave is masked out when its request is in service.

• When exiting the Interrupt Service routine, software has to check whether the interrupt

serviced was the only one from that slave. This is done by sending a Non-Specific EOI

command to the slave and then reading its ISR. If it is 0, a non-specific EOI can also be sent to

the master.

Automatic Rotation Mode (Equal Priority Devices)

In some applications, there are a number of interrupting devices of equal priority. Automatic

rotation mode provides for a sequential 8-way rotation. In this mode, a device receives the lowest

priority after being serviced. In the worst case, a device requesting an interrupt has to wait until

each of seven other devices are serviced at most once.

There are two ways to accomplish automatic rotation using OCW2; the Rotation on Non-Specific

EOI Command (R=1, SL=0, EOI=1) and the Rotate in Automatic EOI Mode which is set by (R=1,

SL=0, EOI=0).

Specific Rotation Mode (Specific Priority)

Software can change interrupt priorities by programming the bottom priority. For example, if IRQ5

is programmed as the bottom priority device, then IRQ6 is the highest priority device. The Set

Priority Command is issued in OCW2 to accomplish this, where: R=1, SL=1, and LO-L2 is the

binary priority level code of the bottom priority device.

In this mode, internal status is updated by software control during OCW2. However, it is

independent of the EOI command. Priority changes can be executed during an EOI command by

using the Rotate on Specific EOI Command in OCW2 (R=1, SL=1, EOI=1 and LO-L2=IRQ level

to receive bottom priority.

Poll Mode

Poll Mode can be used to conserve space in the interrupt vector table. Multiple interrupts that can

be serviced by one interrupt service routine do not need separate vectors if the service routine uses

the poll command. Polled Mode can also be used to expand the number of interrupts. The polling

interrupt service routine can call the appropriate service routine, instead of providing the interrupt

vectors in the vector table. In this mode, the INTR output is not used and the microprocessor

internal Interrupt Enable flip-flop is reset, disabling its interrupt input. Service to devices is

achieved by software using a Poll Command.

The Poll command is issued by setting P=1 in OCW3. The PIC treats its next I/O read as an

interrupt acknowledge, sets the appropriate ISR bit if there is a request, and reads the priority level.

Interrupts are frozen from the OCW3 write to the I/O read. The byte returned during the I/O read

contains a ‘1’ in bit 7 if there is an interrupt, and the binary code of the highest priority level in bits

2:0.

INTEL82801 Datasheet(PDF) 77 Page - Intel Corporation

INTEL82801 Datasheet(HTML) 77 Page - Intel Corporation

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