Always block

Definition

An always block is a fundamental construct in hardware description languages, such as Verilog and SystemVerilog, used to describe the behavior of digital circuits. It allows designers to define how a circuit responds to changes in input signals and can be triggered by specific events like clock edges or changes in signal values. Always blocks are essential for creating combinational and sequential logic in designs.

5 Must Know Facts For Your Next Test

1. Always blocks can be defined in both combinational and sequential contexts, making them versatile for different design needs.
2. In Verilog, the syntax for an always block starts with the keyword 'always' followed by a sensitivity list or an event control statement.
3. In SystemVerilog, always blocks can be enhanced with new features like 'always_ff' for sequential logic and 'always_comb' for combinational logic.
4. An always block can contain multiple statements and can include conditional statements to implement complex logic.
5. When using an always block, it’s crucial to understand the order of execution, especially in sequential designs where clock edges determine when the block executes.

Review Questions

• How does the always block function differently in combinational versus sequential logic designs?
  • In combinational logic designs, an always block executes whenever any signal in its sensitivity list changes, immediately reflecting input changes in output. In contrast, for sequential logic designs, an always block often responds to clock edges, meaning it will only execute its contents at specific times dictated by the clock signal. This distinction is crucial as it affects how memory elements are modeled in digital designs.
• Discuss the role of the sensitivity list in defining the behavior of an always block in Verilog.
  • The sensitivity list defines which signals will trigger the execution of the always block when they change. If a signal in the sensitivity list changes state, it causes the block to execute and update outputs accordingly. This ensures that the circuit accurately represents the intended behavior by only reacting to relevant signal changes. Inappropriate use of sensitivity lists can lead to simulation mismatches and unintended circuit behavior.
• Evaluate how SystemVerilog enhances the traditional always block concept compared to Verilog.
  • SystemVerilog introduces more specific constructs like 'always_comb', 'always_ff', and 'always_latch', which clarify the intended behavior of each block. These enhancements help avoid common pitfalls associated with traditional always blocks by enforcing rules that ensure proper synthesis of combinational and sequential logic. The clear distinctions made by these constructs help designers avoid unintended latches or incorrect timing behaviors, significantly improving design reliability and readability.

Related terms

Combinational Logic: A type of logic circuit whose output is determined solely by its current inputs, without any memory of past inputs.

Sequential Logic: A type of logic circuit whose output depends on both current inputs and past inputs, allowing for memory storage and state retention.

Sensitivity List: A list of signals that an always block is sensitive to; when any signal in this list changes, the block is triggered to execute.