Lesson 09: Behavioral Modeling

Behavioral modeling in Verilog is used to describe a digital circuit using logic expression and C-like constructs. Behavioral modeling allows for the description of circuits based on their function rather than their hardware components. Usually, it describes how the outputs are computed as functions of the inputs.

In Behavioral modeling, the data assignments are not carried out continuously instead they happened on specific events specified in the sensitivity list. This type of modeling scheme is implemented using procedural blocks. All procedures in Verilog are specified within one of the following four blocks:

  • always block
  • initial block
  • task
  • function

Tasks and functions are procedures that are enabled from one or more places in other procedures. Tasks and functions are covered in "EE2440-FPGA Lab 13: Memory Blocks".

The initial and always statements are enabled at the beginning of the simulation. The initial statement executes only once and its activity dies when the statement has finished. The always statement executes repeatedly. Its activity dies only when the simulation is terminated. They can be compared to the setup() and loop() functions in the Arduino C respectively. There is no limit to the number of initial and always blocks that can be defined in a module.

Also, the output variables must be declared as reg because they need to keep hold of the previous value until a new assignment occurs after any change in the specified sensitivity list.









  1. Use behavioral modeling to describe the circuit in the following figure.
    Q1 Circuit