Hardware Description Languages 

The Hardware description languages ​​are used to model the behavior of the hardware components of a system. These languages ​​are mainly used to simulate or synthesize the resulting models of the system. The simulation allows to validate the behavior design  before implementation.

The HDLs most widely used for modeling embedded systems are VHDL, Verilog and SystemC.

VHDL and VHDL-AMS - Elements of Language

VHDL-AMS is a language for describing digital, analog and mixed-signal systems. It is an extension of the digital hardware description language VHDL. VHDL-AMS supports modeling continuous time systems described with sets of simultaneous equations. These equations are differential and algebraic equations with sets of unknowns that are continuous, analytic functions of time. We can model the function of analog systems at different levels of abstraction in ways similar to those just discussed for digital systems. In VHDL-AMS terminology, each module of a system is called a design entity, and the inputs and outputs are ports. In VHDL-AMS, a description of the internal implementation of an entity is called an architecture body of the entity.

Verilog and Verilog-AMS - Elements of Language

The basic structure for a design in Verilog is the module (Module). Within a module defines both interface elements and internal elements. The interface definition includes the list of ports, the statement of the types and direction of these. Where a port is defined as the points of connection to the module. Also included the definition of the parameters, which can be modified when the module is instantiated within other modules. Within the definition of the inner part is defined the behavior that will have the component and the interaction of other components.