In microprocessor systems, what is meant by a level triggered interrupt input?

Introduction / Context:
Microprocessors support different types of hardware interrupt triggering modes, mainly level triggered and edge triggered. The way an interrupt input is sampled and recognized affects how external devices should generate interrupt signals. Designers must know whether an interrupt line expects a level or an edge in order to wire and program hardware correctly. This question focuses on the definition of a level triggered interrupt input.

Given Data / Assumptions:

• The processor has external interrupt pins that can be configured as level triggered or edge triggered.
• Devices assert their interrupt request signals when they need service.
• The processor acknowledges and clears these requests through hardware and software mechanisms.

Concept / Approach:
A level triggered interrupt is one where the processor recognizes a request whenever the interrupt input is held at a defined active level, such as logic low or logic high, depending on the design. The request remains pending as long as the signal stays active. The processor typically samples this input at specific times during instruction execution. Once the interrupt has been acknowledged, the device is expected to deassert the line. This contrasts with edge triggered interrupts, which respond to transitions from inactive to active and may latch the request even if the level changes back quickly.

Step-by-Step Solution:
Step 1: Identify that the question is specifically about level triggering, not edge triggering.Step 2: Recall that in level triggered mode, the interrupt input is sampled and considered active as long as it remains at the active level.Step 3: Understand that the device must keep the line asserted until it is sure that the processor has recognized and begun servicing the interrupt.Step 4: Select the option that clearly describes an interrupt being recognized while the signal remains at an active level.

Verification / Alternative check:
In practical systems, level triggered interrupts help ensure that an interrupt is not missed if the processor is temporarily busy. As long as the device keeps the line active, the processor will eventually detect it when interrupts are enabled. Documentation for many processors specifies which interrupt inputs are level sensitive and how they should be cleared in software by writing to specific acknowledgement registers. This behavior matches the description in the correct option.

Why Other Options Are Wrong:
Option B: Describes edge triggered behavior, where the processor responds only to a transition, not a sustained level.Option C: Talks about priority levels, which are different from triggering modes.Option D: Refers to scheduling of software routines and does not define how hardware interrupt inputs are triggered.

Common Pitfalls:
A common confusion is mixing up level and edge triggering. Students sometimes think that any interrupt that can be masked or has a priority is level triggered, which is not correct. Another pitfall is forgetting that level triggered inputs may cause repeated interrupts if the device does not clear the request properly after service. Good design must ensure that the hardware clears or toggles the interrupt line at the right time.

Final Answer:
The correct answer is An interrupt input that is recognized as long as the signal remains at an active logic level until the processor acknowledges it.