Specifications

Interrupt Controller (V40)
Level- or Edge-Triggered
The two primary methods of sensing interrupt requests are to sense
the logical state (level) or the transition between logical states (edge)
of the interrupt request inputs. The ICU is programmed for level- or
edge-triggered interrupt sensing with the IIW1 register.
Level-Triggered Sensing
If programmed for level-triggered mode, the ICU recognizes a logical
1 as an interrupt request. There is a level of inversion between the
ICU and the interrupt request block. This means that all STD bus and
frontplane (J3) interrupt requests are active low.
There is a minimum and maximum pulse width for the interrupt
request input to guarantee proper operation. The minimum pulse
width requirement states that the request must remain active until the
falling edge of the second interrupt acknowledge pulse. The
maximum amount of time the request remains active is determined by
when the "finish interrupt" command is written. If the request remains
active after the "finish interrupt" command is issued, a second
interrupt is generated on the same request.
Edge-Triggered Sensing
Edge-triggering is the second mode of interrupt sensing. The ICU
recognizes a transition from a logical 0 to a logical 1 as an interrupt
request. There is a minimum pulse width for the interrupt request
input to guarantee proper operation. As in the level-triggered mode,
the request must remain active until the falling edge of the second
interrupt acknowledge pulse. Unlike the level-triggered mode, there is
no maximum pulse width. This means that once the request
transitions to the active state and is serviced, there are no further
interrupts generated until the request returns inactive and then
transitions back into the active state.
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