Datasheet

ACPI Devices
Intel
®
Atom™ Processor E6xx Series Datasheet
221
Interrupt priorities can be changed in the rotating priority mode.
11.3.6.2 Special Fully Nested Mode
This mode will be 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 will be 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 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.
• 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.
11.3.6.3 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 will have 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).
11.3.6.4 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 will be 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.
11.3.6.5 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 8259 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 will contain a ‘1’ in bit 7 if there is an interrupt, and
the binary code of the highest priority level in bits 2:0.