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What does signal assertion mean?

Published in Digital Logic 3 mins read

Signal assertion means that a signal is in its active state, performing the function it's designed to do. Essentially, it signifies that the signal is currently "doing what its name says."

Understanding Signal Assertion

When we talk about signal assertion in digital logic and electronics, we're referring to the signal being in the state that triggers or activates a specific function or behavior. It's crucial to understand that "active" doesn't inherently mean "high" (logic 1). Signals can be either:

  • Active High: The signal is asserted (active) when it's at a high voltage level (usually represented as 1).
  • Active Low: The signal is asserted (active) when it's at a low voltage level (usually represented as 0).

Examples of Signal Assertion

To illustrate, consider these common signals:

  • RESET (Active High): If the RESET signal is active high, it means the system resets when the RESET line is HIGH. When RESET is asserted, the system is being reset.

  • ENABLE (Active High): If the ENABLE signal is active high, the connected component is enabled only when the ENABLE signal is HIGH. When ENABLE is asserted, the component is active.

  • INTERRUPT (Active Low): If the INTERRUPT signal is active low, it means the interrupt is triggered when the INTERRUPT line is LOW. When INTERRUPT is asserted, the processor is being interrupted.

  • CHIP_SELECT (Active Low): Microprocessors frequently use active low signals to select peripherals; a component might be selected and enabled when CHIP_SELECT is LOW. When CHIP_SELECT is asserted, the corresponding chip is selected.

Importance of Knowing Assertion Level

It is vitally important to know whether a signal is active high or active low to properly interface with the circuit. Misunderstanding the assertion level can lead to unexpected and incorrect behavior. The signal name often, but not always, gives a clue, usually when an active low signal is indicated by a bar over its name in diagrams, or by a suffix such as "_L" or "_N" in textual descriptions.

In summary

Signal assertion simply means a signal is active; it is at the voltage level (high or low) that causes it to perform its designated function. Whether that's a high voltage or a low voltage depends on whether the signal is active high or active low, respectively.

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