Structured Logic design With VHDL

Report
HDL
Logic Chip Design
ดร.ธีรยศ เวียงทอง
ภาควิชาวิศวกรรมอิเล็กทรอนิกส์
ห้อง A502
Course Description
2/29
HDL Introduction
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!!!
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)
HDL Introduction
Modern ASIC design approach
Design
RTL Model
Description
Simulation & Testing
HIGH ABSTRACTION LEVEL
Conversion
LOW ABSTRACTION LEVEL
Gate Level Model

Transistor Level
Model
HDLs (Hardware Description Languages)
are used for system description at the high abstraction level
Verilog vs VHDL
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
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
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
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
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
Basic Features of VHDL
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
VHDL representation
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;
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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