NH82801DB S L8DE Intel, NH82801DB S L8DE Datasheet - Page 111

• Manufacturer Part Number: NH82801DB S L8DE
• Manufacturer: Intel

Specifications of NH82801DB S L8DE

• Lead Free Status / RoHS Status: Compliant

5.7.4.6
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Intel
®
82801DB ICH4 Datasheet
Edge and Level Triggered Mode
End of Interrupt Operations
Normal End of Interrupt
Automatic End of Interrupt Mode
Cascade Mode
The PIC in the ICH4 has one master 8259 and one slave 8259 cascaded onto the master through
IRQ2. This configuration can handle up to 15 separate priority levels. The master controls the
slaves through a three bit internal bus. In the ICH4, when the master drives 010b on this bus, the
slave controller takes responsibility for returning the interrupt vector. An EOI command must be
issued twice: once for the master and once for the slave.
In ISA systems this mode is programmed using bit 3 in ICW1, which sets level or edge for the
entire controller. In the ICH4, this bit is disabled and a new register for edge and level triggered
mode selection, per interrupt input, is included. This is the Edge/Level control Registers ELCR1
and ELCR2.
If an ELCR bit is 0, an interrupt request will be recognized by a low-to-high transition on the
corresponding IRQ input. The IRQ input can remain high without generating another interrupt. If
an ELCR bit is 1, an interrupt request will be recognized by a high level on the corresponding IRQ
input and there is no need for an edge detection. The interrupt request must be removed before the
EOI command is issued to prevent a second interrupt from occurring.
In both the edge and level triggered modes, the IRQ inputs must remain active until after the falling
edge of the first internal INTA#. If the IRQ input goes inactive before this time, a default IRQ7
vector will be returned.
An EOI can occur in one of two fashions: by a command word write issued to the PIC before
returning from a service routine, the EOI command; or automatically when AEOI bit in ICW4 is
set to 1.
In Normal EOI, software writes an EOI command before leaving the interrupt service routine to
mark the interrupt as completed. There are two forms of EOI commands: Specific and Non-
Specific. When a Non-Specific EOI command is issued, the PIC will clear the highest ISR bit of
those that are set to 1. Non-Specific EOI is the normal mode of operation of the PIC within the
ICH4, as the interrupt being serviced currently is the interrupt entered with the interrupt
acknowledge. When the PIC is operated in modes which preserve the fully nested structure,
software can determine which ISR bit to clear by issuing a Specific EOI. An ISR bit that is masked
will not be cleared by a Non-Specific EOI if the PIC is in the special mask mode. An EOI
command must be issued for both the master and slave controller.
In this mode, the PIC will automatically perform a Non-Specific EOI operation at the trailing edge
of the last interrupt acknowledge pulse. From a system standpoint, this mode should be used only
when a nested multi-level interrupt structure is not required within a single PIC. The AEOI mode
can only be used in the master controller and not the slave controller.
Functional Description
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