Mastering the Always Block in Verilog - Part 12 of our Verilog Series

2024-03-13 | By DWARAKAN RAMANATHAN

What is an always block?

In Verilog, an "always" block is a fundamental construct used to describe sequential logic behavior within digital designs. It plays a pivotal role in defining how a circuit responds to changes in its inputs and the progression of time. The "always" block is primarily used for modeling the behavior of registers, flip-flops, and other sequential elements.

Here are the key characteristics and functions of an "always" block:

Sensitive to Events: An "always" block is sensitive to specific events or conditions. It can be triggered by changes in input signals, clock edges, or a combination of conditions. These events are specified in the sensitivity list of the "always" block.
Edge Sensitivity: In many cases, "always" blocks are used in conjunction with clock signals to model clocked sequential logic. Common edge types include "posedge" (positive clock edge) and "negedge" (negative clock edge) sensitivity.
Combinational and Sequential Logic: Inside an "always" block, you can describe both combinational logic (combinations of inputs that produce outputs immediately) and sequential logic (logic that relies on memory elements like flip-flops).
Behavioral Descriptions: "Always" blocks are often used to provide a behavioral description of a circuit's operation, making it easier to design, simulate, and understand the intended functionality.
Sequential Logic Modeling: The "always" block is employed to model sequential logic, which is logic that depends on previous states and events. It's commonly used to describe how flip-flops and registers in a digital circuit behave over time.

Here's a simplified example of an "always" block describing the behavior of a D flip-flop:

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always @(posedge clock, posedge reset) begin  
if (reset) q <= 1'b0; // Reset the flip-flop to 0 when 'reset' is active.  
else q <= d; // Otherwise, store the value of 'd' on the rising edge of the ‎clock.  
end

In this example:

The "always" block is sensitive to both the rising edge of the clock signal (posedge clock) and the rising edge of the reset signal (posedge reset).

When the reset signal is active (reset is 1), the flip-flop is cleared to 0.
Otherwise, on each rising edge of the clock, the value of the input d is stored in the flip-flop's output q.

The "always" block allows designers to specify how sequential elements behave in response to specific events or conditions, making it a crucial construct for describing digital hardware behavior in Verilog.

What is sensitivity list?

In Verilog, a sensitivity list is a crucial component of an "always" block or an "initial" block. It defines the list of signals or events to which the block is sensitive. The sensitivity list determines when the block will execute its code based on changes or events related to the specified signals.

Here are the key points to understand about sensitivity lists:

Trigger for Execution: The sensitivity list specifies the conditions or events that will trigger the execution of the code within the associated "always" or "initial" block. When any of the signals or events in the list experience a change that matches the specified condition, the block is executed.
Syntax: The sensitivity list is placed within parentheses immediately following the "always" or "initial" keyword. It lists the signals, edges, or other conditions that should trigger the block's execution.
Signal Types: The sensitivity list can include different types of signals or events, including:

Signal Names: These are typically specified as input or output signals from the module.
Edges: Such as "posedge" (positive clock edge) or "negedge" (negative clock edge) to trigger on clock transitions.
Level-Sensitive Conditions: Such as "a or b," indicating that the block should execute when either signal 'a' or 'b' changes.
Combinations: You can combine signals and edges in the sensitivity list to create complex triggering conditions.

Suppose you have a simple Verilog module that represents a D flip-flop with an asynchronous reset:

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module d_flip_flop (  
input wire d, // Data input  
input wire clk, // Clock input  
input wire reset, // Asynchronous reset input 
output wire q // Data output );  
always @(posedge clk or posedge reset) begin  
‎ if (reset) begin // When 'reset' is active (1), asynchronously reset the ‎flip-flop.  
‎  q <= 1'b0;  
‎ end  
‎ else begin // On the rising edge of the clock, store the value of 'd'. 
‎  q <= d;  
‎ end  
end  
endmodule

In this example:

The "always" block has a sensitivity list (posedge clk or posedge reset), indicating that it will execute on the positive edge of the 'clk' signal or the positive edge of the 'reset' signal.
When the 'reset' signal experiences a positive edge (transition from 0 to 1), the flip-flop is asynchronously reset to 0.
On the rising edge of the 'clk' signal (transition from 0 to 1), the value of the 'd' input is stored in the flip-flop's output 'q'.

This Verilog module models a D flip-flop with both clocked and asynchronous reset behavior, and it demonstrates how the sensitivity list determines when the "always" block executes based on changes in the specified signals or events.

Conclusion:

In conclusion, the "always" block is a foundational construct in Verilog that plays a central role in modeling the behavior of digital hardware, particularly sequential logic. Here's a summary of key points regarding the "always" block:

Sequential Logic Modeling: "always" blocks are used to model sequential logic, which encompasses components like flip-flops, registers, and state machines. These components depend on past states and events.
Event-Driven Execution: "always" blocks execute in response to specific events or conditions defined in their sensitivity lists. These events include signal changes, clock edges, or logical conditions.
Edge Sensitivity: Many "always" blocks are edge-sensitive and are used in clocked logic to respond to rising ("posedge") or falling ("negedge") clock edges, ensuring synchronous operation.
Combining Combinational and Sequential Logic: Within "always" blocks, you can describe both combinational logic (combinations of inputs producing immediate outputs) and sequential logic (logic with memory elements).
Behavioral Descriptions: "always" blocks are often used to provide high-level, behavioral descriptions of circuits, focusing on what the circuit should do without specifying implementation details.
Simulation and Verification: These blocks are crucial for simulating and verifying digital designs, allowing designers to predict how the hardware behaves under different conditions.
Clarity and Modularity: "always" blocks enhance code clarity and modularity by encapsulating specific functionality. This makes it easier to design, simulate, and maintain complex digital systems.