Why interrupts are active low?

What interrupts are active low in digital circuits?

In digital circuits, an interrupt is a signal that causes the processor to temporarily stop its current task and execute a predefined subroutine. Interrupts can be triggered by various sources, such as timers, external devices, or software instructions.

One way to classify interrupts is by their polarity, which indicates whether the interrupt signal is active high or active low. Active high means that the interrupt signal is asserted when it is at a logic high level, while active low means that the interrupt signal is asserted when it is at a logic low level.

Active low interrupts are commonly used in digital circuits for several reasons. First, active low interrupts can simplify the circuit design, as they can be implemented using open-drain or open-collector outputs, which do not require pull-up resistors. Second, active low interrupts can reduce power consumption, as they only draw current when the interrupt signal is asserted. Third, active low interrupts can improve noise immunity, as they are less susceptible to interference from voltage spikes or glitches.

If you consider the transistor level of a module, active low means the capacitor in the output terminal gets charged or discharged based on low to high and high to low transition respectively.

When it goes from high to low it depends on the pull down resistor that pulls it down and it is relatively easy for the output capacitance to discharge rather than charging. hence people prefer using active low signals.

Interrupts are often designed to be active low for practical and historical reasons. An active-low signal means that the interrupt is triggered or recognized when the signal is at a logic low voltage level. There are a few reasons for this convention:

Compatibility with older technology: In early digital systems and integrated circuits, active-low signals were more common. As technology evolved, this convention persisted to maintain compatibility with existing designs and systems.

Reliability: Active-low signals can be more reliable in certain situations. When an interrupt line is actively pulled low, it is less susceptible to noise and interference. This characteristic can be crucial in systems where reliability and stability are top priorities.

Design simplicity: In many cases, designing circuits with active-low signals can simplify the overall design. It may lead to fewer inverters and simpler logic gates in the interrupt handling circuitry.

Historical practices: The use of active-low signals in digital electronics goes back to earlier design practices, and once a convention becomes established, there is often resistance to change for the sake of backward compatibility and familiarity.

Some examples of active low interrupts in digital circuits are:

1. The INT0 and INT1 pins of the 8051 microcontroller, which can be configured as external interrupts that are triggered by a falling edge or a low level on the pins.
2. The INT pin of the 74LS138 decoder, which generates an active low interrupt when one of its outputs is selected by the address inputs.
3. The IRQ pin of the 8259 programmable interrupt controller, which outputs an active low interrupt request to the processor when one of its inputs receives an interrupt signal from a peripheral device.

It's important to note that while active-low interrupts are common, active-high interrupts also exist, and there's no inherent advantage of one over the other in all situations. Design choices, historical practices, and the specific requirements of a system influence whether an active-low or active-high approach is chosen for interrupts or other signals.