Verilog HDL is used to model digital circuits at different levels of abstraction. There are several modeling styles in Verilog including structural, dataflow, behavioral, and mixed modeling. Structural modeling connects instances of modules through nets, dataflow uses continuous assignments, and behavioral describes algorithms without hardware details. Modules have interfaces with input, output, and inout ports that connect to external signals. Simulation verifies designs operate as required and uses the `timescale directive to specify time units for delays.
3. Modules
A Verilog HDL module consist of two major parts:
The interface
The internal (body)
Block diagram of a full adder is shown below
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4. The Interface: Port Declaration
The interface signals can be grouped into one of the
following three types: input, output or inout
A complete interface can be divided into three parts: port
list, port declaration and data type declaration
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5. Port Declaration
The declaration of a port and its associated data type can
be combined into a single line as below:
Port list, port declaration and their associated data types
can be put into a single list. This is called port list
declaration or ANSI C style
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6. Port Connections
Two methods are used to connect ports to external
signals:
Named association: ports to be connected to external signals
are specified by listing their names. The order of the ports is
not important
Positional association: the signals to be connected must have
the same order as in the port list. All unconnected ports are
left blank
The two methods cannot be mixed
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8. Modeling the Internal of a Module
The internal or body can be modeled using one of the
following styles:
Structural Style
Gate level
Switch level
Dataflow Style
Behavioral Style
Mixed Style
RTL = Behavioral + Dataflow Styles
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9. Structural Modeling
Structural modeling of a design is by connecting required
instantiations of built-in primitives, user defined primitives
or other modules through nets
Structural style is one way to model a complex digital
system in a hierarchical manner
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11. Dataflow Modeling
The module is described by specifying the data flow
between registers and how the data is processed
This is done through a set of continuous assignment
statements
A continuous assignment starts with the keyword assign
and has a syntax as below:
assign [delay] l_value = expression;
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14. Behavioral Modeling
The module is described in terms of the desired design
algorithm without concerning the hardware
implementation details
The module uses procedural constructs: initial and always
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15. Mixed modeling
The design is described in terms of the mixing of the
previous styles
It is most commonly used in modeling large designs
Register-Transfer level (RTL) is usually used in industry to
mean the combination of behavioral and dataflow
constructs
Below is a full adder constructed with basic logic gates:
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17. Simulation
For a design to be useful, it must be verified to make sure
that it can operate according to the requirement
Two basic simulation structures are available in Verilog:
The first is to take the UUT as an instantiated module in the
stimulus module
The second considers both stimulus block and UUT as the
separate instantiated module at the top-level module
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18. Related Compiler Directives:
`timescale Directive
In simulations, we need to specify the physical unit of
measure
This is accomplished using:
`timescale time_unit/time_precision
time unit : This is the time to be used as one unit for all
the delays used in the design.
time precision : This represents the minimum delay which
needs to be considered during simulation or it decides
that how many decimal point would be used with the time
unit.
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