DSD using VHDL

EECL 309B

Digital System Design using VHDL

Spring 2014 Semester

Books followed

Volnei A. Pedroni, Circuit Design with

VHDL.

Douglas L. Perry, VHDL Programming

by Example.

J Bhasker, A VHDL Primer.

Design Abstraction

How do we “describe” a system?

Design Abstraction

Example: Design a “system” which will complement input A

AF(x)

A and Y are single bit values

A Y

0 1

1 0

We can “describe” this design using a logical Truth Table

Y = A

Levels of Design Abstraction

Our goal in ECE is physical or hardware implementationsof the design.

DesignSpecs

a11

a22

3a3

4a4

b1

b2

b3

b4

5

6

7

8

Vcc1

0

GND

0

Hard-ware

DesignProcess

In ECE, we “design” at several levels of “abstraction”


System: Assembly Language Behavioral: VHDL Logical: Gates Electronic Circuit: Transistors Integrated Circuit: IC Layout Fabrication: IC Processing


System Level:

A ASMCode

8085 Assembly Language

NEG A


Y = A


Behavioral Level:

ANot A

VHDL

Y <= not A;


Y = A


Gate Level:

A

Inverter or NOT gate


Y = A


Circuit Level:

A

CMOS Technology


PFET

NFET

Y = A

Vdd


Digital IC Design:

A

CMOS Technology


VDD GND

Y = A


Fabrication Level:

NWELL

N+ N+P+P+

PSUB

Gajski’s Y-chartGajski’s Y-chart

Each axis represents type of description– Behavioral

Defines outputs as function of inputs

Algorithms but no implementation

– Structural Implements behavior by

connecting components with known behavior

– Physical Gives size/locations of

components and wires on chip/board

– Design process is illustrated by travel route

Behavior

Physical

Structural

Processors, memories

Registers, FUs, MUXs

Gates, flip-flops

Transistors

Sequential programs

Register transfers

Logic equations/FSM

Transfer functions

Cell Layout

Modules

Chips

Boards

VHDL Introduction

V- VHSIC Very High Speed Integrated Circuit

H- Hardware D- Description L- Language

VHDL for Simulation & Synthesis

Test Vector

Generator

A Series of RefinedModels

Executable Specification

TestVectors

Results,Errors

=

Final Chip

Model

Reasons for using VHDL

Shorter development times for electronic design Simpler maintenance Traditional way: schematic design

Origin of the VHDL

VHDL originated in the early 1980s The American Department of Defense initiated

the development of VHDL in the early 1980s because the US military needed a standardized

method of describing electronic systems VHDL was standardized in 1987 by the

IEEE It is now accepted as one of the most

important standard languages for specifying verifying designing of electronics

Standardization 1

IEEE standard specification language (IEEE 1076-1993) for describing digital hardware used by industry worldwide

VHDL enables hardware modeling from the gate level to the system level

All the major tool manufacturers now support the VHDL standard

VHDL is now a standardized language, with the advantage that it is easy to move VHDL code between different commercial platforms (tools)

=> VHDL code is interchangeable among the different tools

Standardization 2

VHDL is an acronym of VHSIC Hardware Description Language

VHSIC is an acronym of Very High Speed Integrated Circuits

All the major tool manufacturers now support the VHDL standard

VHDL is now a standardized language, with the advantage that it it easy to move VHDL code between different commercial platforms (tools)

=> VHDL code is interchangeable among the different tools

Standardization 3

It was the American Department of Defense which initiated the development of VHDL in the early 1980s because the US military needed a standardized method of describing electronic systems

VHDL was standardized in 1987 by the IEEE IEEE Std-1076-1987

ANSI Standard in 1988 Added Support for RTL Design

VITAL: VHDL Initiative Towards ASIC Library Revised version in 1993

IEEE Std-1076-1993

Standardization 4

1995: numeric_std/bit: IEEE-1076.3 VITAL: IEEE-1076.4

1999: IEEE-1076.1 (VHDL-AMS ) 2000:

IEEE-1076-2000 IEEE-1076.1-2000 (VITAL-2000, SDF 4.0)

Added mixed-signal support to VHDL in 2001 -> VHDL-AMS

IEEE Std-1076.1-2001 2002: IEEE-1076-2002

Tools

Good VHDL tools, and VHDL simulators in particular, have also been developed for PCs

Prices have fallen dramatically, enabling smaller companies to use VHDL, too

There are also PC synthesis tools, primarily for FPGAs and EPLDs

Usage

High-tech companies Texas Instruments, Intel use VHDL most European companies use VHDL

Universities VHDL groups to support new users

IEEE

IEEE is the Institute of Electrical and Electronics Engineers

The reference manual is called IEEE VHDL Language Reference Manual Draft Standard version 1076/B

It was ratified in December 1987 as IEEE 1076-1987

Important: the VHDL is standardized for system specification but not for design

Technology independence

The design of VHDL components can be technology-independent or more-or-less technology independent for a technical family

The components can be stored in a library for reuse in several different designs

VHDL models of commercial IC standard components can now be bought, which is a great advantage when it comes to verifying entire circuit boards

Analog world

VHDL has not yet been standardized for analog electronics

Standardization is in progress on VHDL with an analog extension (AHDL) to allow analog systems to be described as well

This new standard will be based wholly on the VHDL standard and will have a number of additions for describing analog functions

VHDL-Related Newsgroups

comp.arch.fpga comp.lang.vhdl comp.cad.synthesis

Other HDL languages

There are several other language extensions built to either aid in RTL construction or assist in modeling: Verilog ParaCore - http://www.dilloneng.com/paracore.shtml RubyHDL -

http://www.aracnet.com/~ptkwt/ruby_stuff/RHDL/index.shtml MyHDL - http://jandecaluwe.com/Tools/MyHDL/Overview.shtml JHDL - http://www.jhdl.org/ Lava - http://www.xilinx.com/labs/lava/ HDLmaker - http://www.polybus.com/hdlmaker/users_guide/ SystemC AHDL – http://www.altera.com

It is good for Altera-made chips only, which limits its usefulness

But it is easy to pick up and use successfully The main purpose of a language -- programming, hdl, or otherwise

-- is to ease the expression of design

Fundamental sections of a basic VHDL code.

A VHDL code for the full adderFull-adder Schematic

Simulation results from the VHDL design

Example code - VHDL

VHDL Constructs

Entity definition Architecture definition Configuration Process Subprogram Package

A VHDL design can be broken into multiple files.

Each file contains entity and architecture

definitions, or packages.

Overall structure of a VHDL file

32

What are they?

VHDL 1. ver.3a 33

Entity declaration

Architecture body

A VHDL fileA VHDL file

Library declaration, e.g. IEEE library

EntityArchitectureBody: definesthe processing

ArchitectureBody: definesthe processing

LIBRARY

A LIBRARY is a collection of commonly used pieces of code. Placing such pieces inside a library allows them to be reused or shared by other designs.

The typical structure of a library is shown aside.

Library Declarations

To declare a LIBRARY (that is, to make it

visible to the design) two lines of code are

needed, one containing the name of the library,

and the other a use clause. The syntax is as follows LIBRARY library_name ; USE library_name.package_name.package_parts

;

Contd..

At least three packages, from three different libraries, are usually needed in a design :

ieee.std_logic_1164(from the ieee library),

standard (from the std library) , and work (work library)

Library Declarations

LIBRARY ieee; -- A semi-colon (;) indicates the end of a statement or a declaration

USE ieee.std_logic_1164.all ;

LIBRARY std ; -- a double dash (--) indicates a comment.

USE std . Standard . all ;

LIBRARY work ;

USE work. all;

Contd..

The std_logic_1164 package of the ieee library specifies a multi-level logic system; std is a resource library (data types, text i/o, etc.) for the VHDL design environment; and the work library is where we save our design (the .vhd file, plus all files created by the compiler, simulator, etc.).

STD_LOGIC type

Value Meaning

‘U’ Uninitialized

‘X’ Forcing (Strong driven) Unknown

‘‘0’0’ Forcing (Strong driven) 0Forcing (Strong driven) 0

‘‘1’1’ Forcing (Strong driven) 1Forcing (Strong driven) 1

‘‘Z’Z’ High ImpedanceHigh Impedance

‘W’ Weak (Weakly driven) Unknown

‘L’Weak (Weakly driven) 0.Models a pull down.

‘H’Weak (Weakly driven) 1. Models a pull up.

‘-’ Don't Care

ENTITY

Entity describes the design interface. The interconnections of the design unit with the

external world are enumerated. The properties of these interconnections are

defined.entity declaration:

entity <entity_name> is

port ( <port_name> : <signal mode> <type>;

….

);

end <entity_name>;

Signals in Entity

There are four modes for the ports in VHDL

in, out, inout, buffer These modes describe the different kinds of

interconnections that the port can have with the external circuitry.

Sample program:

ENTITY andgate isport ( c : out bit;

a : in bit;

b : in bit

);

end andgate;

Contd..

The mode of the signal can be IN, OUT, INOUT, or BUFFER .

IN and OUT are truly unidirectional pins, while INOUT is bidirectional.

BUFFER, on the other hand, is employed when the output signal must be used (read) internally.

The name of the entity can be basically any name, except VHDL reserved words

ARCHITECTURE

Architecture defines the functionality of the entity.

It forms the body of the VHDL code. An architecture belongs to a specific

entity. Various constructs are used in the

description of the architecture.

architecture declaration:architecture <architecture_name> of

<entity_name> is

<declarations>

begin

<vhdl statements>

end <architecture_name> ;

Contd-

An architecture has two parts : a declarative part (optional), where signals and constants (among others) are declared, and the code part (from BEGIN down).

architecture declaration:

architecture <architecture_name> of <entity_name> is

<declarations>

begin

<vhdl statements>

end <architecture_name> ;

EXAMPLE OF A VHDL ARCHITECTURE

ENTITY andgate is

port (c : out bit;

a : in bit;

b : in bit

);

end andgate;

ARCHITECTURE arc_andgate of andgate is

begin

c <= a and b;

end arc_andgate;

Architecture Types

The digital system can be represented in different levels of abstractions such as a behavioral model or a structural model or a data flow. These levels of abstraction help the designer to develop any complex digital system efficiently.

1. Data Flow2. Behavioral3. Structural

Data Flow Modelling

A Dataflow model specifies the functionality of the entity without explicitly specifying its structure.

This functionality shows the flow of information through the entity, which is expressed primarily using concurrent signal assignment statements and block statements.

The primary mechanism for modeling the dataflow behavior of an entity is using the concurrent signal assignment statement.

mostly used for design of combinational logics

Behavioral

Behavioral level describes the system, the way it behaves and describes the relationship between the input and output signals.

The description can be a Register Transfer Level (RTL) or Algorithmic (set of instruction) or simple Boolean equations.

Algorithmic level is mostly used for design of sequential logics.

Structural Modelling

The Structural level describes the digital system as gates or as component blocks interconnected to perform the desired operations.

Structural level is primarily the graphical representation of the digital system and so it is closer to the actual physical representation of the system.

Complete AND GATE Example

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_unsigned.all;

Entity and_example is

port(a,b: in std_logic;

ya,yb,yc: out std_logic);

End entity and_example;

Architecture test of and_example is

begin

-- dataflow model (ya)

ya <= a and b;

-- structural model (yb)

and2:a_7408 port map(a,b,yb);

-- behavioral model (yc)

process(a,b)

begin

yc <= ‘0’;

if((a=‘1’) and (b = ‘1’)) then

yc <= ‘1’;

else yc <= ‘0’;

end if;

end process;

End architecture test;

AND GATE Example (cont)

When synthesized, we obtain the following logic circuit

Ya

Yb

Yc

A

B

Synthesis tool creates three ANDgates.

Block Diagram

An example of a comparator

1 entity eqcomp4 is 2 port (a, b: in std_logic_vector(3

downto 0 ); 3 equals: out std_logic); 4 end eqcomp4; 5 6 architecture dataflow1 of eqcomp4 is 7 begin 8 equals <= '1' when (a = b) else

'0’; 9-- “comment” equals is active high 10 end dataflow1;

Entitydeclaration

Architecturebody



Entity declaration: defines IOsEntity declaration: defines IOs

How to read it?

1 entity eqcomp4 is 2 port (a, b: in std_logic_vector(3

downto 0 ); 3 equals: out std_logic); 4 end eqcomp4; 5 6 architecture dataflow1 of eqcomp4

is 7 begin 8 equals <= '1' when (a = b) else

'0’; 9-- “comment” equals is active high 10 end dataflow1;

•A bus, use downto to define it.

•E.g. in std_logic_vector(3 downto 0);

Entity enclosed by the entity name – eqcomp4 (entered by the user)

•Port defines the I/O pins.

Entitydeclaration

Architecturebody

Exercise

In the eqcomp4 VHDL code: How many Input /

Output pins? Answer: _______

What are their names and their types? Answer: ___________ ___________________

What is the difference between std_logic and std_logic_vector? Answer: __________ __________________

1 entity eqcomp4 is2 port (a, b: in std_logic_vector(3 downto

0 );3 equals: out std_logic);4 end eqcomp4;56 architecture dataflow1 of eqcomp4 is7 begin8 equals <= '1' when (a = b) else '0’;9-- “comment” equals is active high10 end dataflow1;

Documents

DSD using VHDL