Intel C2518 FH8065501516710 Data Sheet

Intel

®

 Atom™ Processor C2000 Product Family for Microserver

Datasheet, Vol. 2 of 3

September 2014

554

Order Number: 330061-002US

In this mode, interrupt requests are ordered in priority from 0 through 7, with 0 being 

the highest. When an interrupt is acknowledged, the highest priority request is 

determined and its vector placed on the bus. Additionally, the ISR for the interrupt is 

set. This ISR bit remains set until: the processor issues an EOI command immediately 

before returning from the service routine or if in AEOI mode, on the trailing edge of the 

second INTA#. While the ISR bit is set, all further interrupts of the same or lower 

priority are inhibited, while higher levels generate another interrupt. Interrupt priorities 

are changed in the rotating priority 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 priority logic, and further interrupt requests from higher 

priority interrupts within the slave are recognized by the master and initiate the 

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, the 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 is also sent to the master.

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

automatic rotation mode provides for a sequential eight-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 the seven other devices are serviced at 

most once.

Two ways to accomplish automatic rotation using OCW2.REOI are: the rotation on the 

Non-specific EOI command (OCW2.REOI=101b) and the rotation in the automatic EOI 

mode which is set by (OCW2.REOI=100b).

The software changes the 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 

OCW2.REOI=11xb, and OCW2.ILS is the binary priority level code of the bottom 

priority device.

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

However, it is independent of the EOI command. Priority changes are executed during 

an EOI command by using the Rotate-on-Specific EOI command in OCW2 

(OCW2.REOI=111b) and OCW2.ILS=IRQ level to receive bottom priority.