VHDL
Structured Logic Design
School of Electrical Engineering
University of Belgrade
Department of Computer Engineering
Ivan Dugic idugic@verat.net
Veljko Milutinovic vm@etf.bg.ac.yu
Table of contents
 HDL Introduction
 Structured Design Concepts
 Basic Features of VHDL
 Design Process Highlights
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HDL Introduction
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HDL Introduction
Modern chip design aspects

Modern chips became too complex

The number of transistors in a modern chip is over a 100 M

Transistor count per chip and chip speed rise up to 50% per year

Estimated time needed for manual implementation
(100 M transistor, 10 sec/transistor) – 135.5 years!!!
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HDL Introduction
Modern ASIC design approach

ASIC – Application Specific Integrated Circuit

Modeling system should be designed and described
in the highest abstraction level possible

Simulation and testing at high abstraction level

Conversion of the modeled system
into the low abstraction level model (gate, circuit, silicon level)
using sophisticated synthesis tools

Key point – CAD (Computer Aided Design)
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HDL Introduction
Modern ASIC design approach
Design
RTL Model
Description
Simulation & Testing
HIGH ABSTRACTION LEVEL
Conversion
LOW ABSTRACTION LEVEL
Gate Level Model

HDLs (Hardware Description Languages)
are used for system description at the high abstraction level
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HDL Introduction
VHDL

VHDL - VHSIC Hardware Description Language

VHSIC - Very High Speed Integrated Circuit

Development of VHDL began in 1983, sponsored by
Department of defense, further developed by the IEEE
and released as IEEE Standard 1076 in 1987

Today it is De facto industry standard for hardware description
languages
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Structural Design Concepts
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Structural Design Concepts
The abstraction hierarchy

The abstraction hierarchy can be expressed in two domains:
structural domain, behavioral domain

Structural domain – component model is described
in terms of an interconnection of more primitive components

Behavioral domain – component model is described
by defining its input/output response

VHDL is used for both structural and behavioral description

Six abstraction hierarchy levels of detail commonly used in design:
silicon, circuit, gate, register, chip and system
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Structural Design Concepts
Design process

The design cycle consists of a series of transformations,
synthesis steps:
(1) Transformation from English to an algorithmic representation,
natural language synthesis
(2) Translation from an algorithmic representation
to a data flow representation,
algorithmic synthesis
(3) Translation from data flow representation
to a structural logic gate representation,
logic synthesis
(4) Translation from logic gate to layout and circuit representation,
layout synthesis
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Structural Design Concepts
Design process

The design cycle steps can be carried out automatically
in all stages except the first that is currently an active area of research

VHDL tools are used for algorithmic synthesis
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Structural Design Concepts
Design tools

Editors – textual (circuit level – SPICE gate, register, chip – VHDL)
or graphic (used at all levels)

Simulators – stochastic (system level)
or deterministic (all levels above the silicon level)

Checkers and Analyzers – employed at all levels, used for example
(1) to insure that the circuit layout
can be fabricated reliably (rule checkers),
(2) to check for the longest path through a logic circuit or system
(timing analyzers)

Synthesizers and Optimizers – improving a form
of the design representation
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Basic Features of VHDL
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Basic Features of VHDL
Design entities

In VHDL a logic circuit is represented as a design entity

A design entity consists of two different VHDL types of description:
(1) Interface description (reserved word is entity)
(2) One or more architectural bodies (reserved word is architecture)
entity D_FF
D
Q
defining D FF interface (ports)
D FF
CLK
R
architecture of D_FF
specifying the behavior of the entity
Designed digital device
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VHDL representation
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Basic Features of VHDL
Entity

The entity part provides system’s interface specification
as seen from the outside and is generally comprised of:
(1) Parameters (such as bus width or max clock frequency)
(2) Connections (system input and output ports)
entity DesignEntityName is
-- parameters
…
-- connections
port (ports);
end entity DesignEntityName;
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Basic Features of VHDL
Architectural bodies

Architectural bodies are specifying the behavior of the entity
architecture ArchitectureName of DesignEntityName is
-- signal declarations
begin
-- concurrent statements
end architecture ArchitectureName;

There are two types of architectural bodies: algorithmic, structural

Algorithmic - at the beginning of the design process,
designers usually would like to check the accuracy of the algorithm
without specifying the detailed implementation

Structural - the logic design stage, detailed implementation,
entity as a set of interrelated components
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Basic Features of VHDL
Processes

Process is another major modeling element in VHDL:
ProcessLabel: ProcessName (sensitivity_list_of_signals) is
begin
-- sequential statements;
end process;

Processes are used inside architectural bodies,
specifying entity behavioral in algorithmic way

Whenever a signal in sensitivity list changes,
the process is activated

Process execution is similar to program execution,
barring one important difference:
a process generally repeats indefinitely
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Basic Features of VHDL
Sequential and parallel processing

The statements within process are performed sequentially

The statements within architectural body are performed concurrent

Sequential and concurrent combination is called VHDL duality and
it presents powerful mechanism
for description of complex systems
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Basic Features of VHDL
Variables and signals

VHDL variable concept in many ways correspondents
to a variable inherited from traditional sequential programming

Signals are the basic vehicle for information transmission
in electronic systems

Signals model real devices’ wires and buses

Variable assignment is different from signal assignment

Main difference between variables and signals
is that signal changes are visible only after process termination
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Design Process Highlights
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Design Process Highlights
MAC (Multiply Accumulator) unit

Incoming part is based on MAC unit design and synthesis
as part of Computer VLSI Systems,
subject lectured by Dr. Veljko Milutinovic

Basic specification elements of MAC unit:
(1) purpose – hardware support for numerous succeeding
multiplication
(2) Wishbone compatible
(3) structural elements: FIFO, sequential multiplier, accumulator

MAC units are used as special CPU resource
for digital signal processing
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Design Process Highlights
MAC (Multiply Accumulator) unit

MAC unit conceptual scheme:
MAC unit
FIFO
Sequential Multiplier
Accumulator
Wishbone Interface
data flow
control data flow
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Design Process Highlights
MAC (Multiply Accumulator) unit

MAC unit detailed scheme – synthesis outcome:
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Design Process Highlights
MAC: Lessons Learned

Testing is extremely important aspect of device design

In the component design process it is essential
to test all structure components of top-level entity separately,
and after that top-level entity itself

While projecting MAC unit so called Regression Testing is used

Regression Testing includes
testing both structural and behavioral architecture
of every entity simultaneously
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Design Process Highlights
MAC: Lessons Learned

An example of regressive testing:
entity TestBench is
end entity TestBench;
architecture Regression of TestBench is
-- signal declaration
begin
BehModel: entity myModel (beh)
port map (…);
StructModel:entity myModel (struct)
port map (…);
stimulus: process is
begin
-- stimulation
end process stimulus;
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Design Process Highlights
MAC: Lessons Learned
verify: process (…) is
begin
assert behOutSignal_i= structOutSignal_i and
behOutSignal_j = structOutSignal_j
report “Implementation Error!”
severity error;
end process verify;
end architecture Regression;
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Design Process Highlights
MAC: Lessons Learned

Special problem in hardware component design:
accommodation of VHDL source code with tool for synthesis

It is possible that VHDL code can be compiled regularly
but synthesis tool registers errors

The solution of the problem:
VHDL coding concerning synthesis tool requirements

It is necessary that VHDL code describes designed device
as close as possible to the particular hardware elements
which synthesis tool recognizes and synthesizes easily
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References

James R. Armstrong, F. Gail Gray,
Structured Logic Design with VHDL

Peter J. Ashenden,
The Designer’s Guide to VHDL

Milutinovic Veljko,
Surviving the Design of a 200 MHz RISC Microprocessor:
Lessons Learned
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Authors
 Ivan Dugic, idugic@verat.net
 Dr. Veljko Milutinovic, vm@galeb.etf.bg.ac.yu
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