332:437 Lecture 7
Verilog Hardware Description Language
Basics



Motivation for Hardware Description Language
Tool flow
FPGA Design Methodology and Synthesis
 Logic Synthesis Problems

Summary
Material from The Verilog Hardware Description Language,
by Thomas and Moorby, Kluwer Academic Publishers,
VHDL for Programmable Logic, by Kevin Skahill, Addison Wesley
Longman.
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Motivation
 Now possible to automatically design hardware using a
computer program (Synopsys design_analyzer)
 Translate high-level hardware description language
(Verilog or VHDL) automatically into logic gates
 Massive saving of time for hardware design
 Widely used at all electronics companies
 Problem: Resulting hardware design is not always good
 Designer must check the design to determine its quality
If unacceptable, redesign manually using K-maps and
lower-level hardware synthesis tools
Example: NJ company went out of business because
they used a bad design created with VHDL
Too many logic gates, too slow, too expensive
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Synopsys System at Rutgers
 Industrial Cost: 50 licenses X $1 million / license = $50 million
 University Cost: 3 X $5000 = $15000
 Tools used in DSD:
 design_analyzer: Translates Verilog into logic gates
 vcs – Verilog behavioral and logic simulator
Good check to make certain that design_analyzer
correctly created the logic
 Many other tools are in the tool suite
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Verilog Language
 Concurrent hardware description language

Expresses parallelism in the hardware
 DO NOT code Verilog like a C or FORTRAN program

Serializes the hardware operations

Leads to a BIG increase in the amount of the hardware
design_analyzer adds interlock logic gates to make
sure that the hardware runs serially -- unnecessary
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Choice of Hardware Description
Language
 VHDL
 Used in all Dept. of Defense (DoD) military system designs
 Used throughout Europe, Japan, and IBM
 Has problems with type conversions between Boolean and
arithmetic
 Verilog
 Preferred in the commercial electronics industry
 Best for converting data types between bit vector and
arithmetic notations
 Best for configuring large designs produced by large
design teams
 Best for describing low-level logic (more concise)
 Reality: Probably need to know both languages
 Impossible to say which is better – matter of taste
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Shortcomings of Verilog or VHDL
1. You lose some control of defining the gate-level circuit
implementation
 You don’t have time to do that, anyway
2. Logic synthesized by the Verilog compiler is sometimes
inefficient
 A real problem – Must learn to “explain” the design to the
compiler in the “right” way to get maximum hardware
parallelism, which leads to the best design
3. Quality of synthesis varies from tool to tool:
 Use Synopsys for high quality – used at Intel, IBM, Agere,
everywhere – knowing this helps you get a job
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Activities of Design at Levels of Design
Synopsys
Design Analyzer
ARCHITECTURE & BEHAVIORAL
REGISTER TRANSFER – Verilog/VHDL
Cadence
Verilog-XL
LOGIC
Synopsys
Behavioral
Simulation
Synopsys
Logic Sim.
SWITCH LEVEL
SPICE and
CIRCUIT (TRANSISTORS)
Cadence
Spectre
Cadence
LAYOUT & TEST PATTERNS
LayoutPlus
FABRICATION LINE
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Design Activities
 Implemented by complex Computer-Aided Design programs

You must know how to parameterize these correctly to get
correct results
 Estimation – Estimate likely design parameters
 Synthesis – Translate design into lower level of representation
 Simulation – Mimic design behavior at level of representation
to see if it is correct
 Analysis – Analyze design parameters at a level
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Fitting and Routing
 Fitting – Fit logic produced by synthesis, place it onto a
particular programmable logic device, transforming the logic
as needed
 Place & Route – Place logic in a particular combinational
Logic Block on an FPGA, such that the wiring delay between
the block and others is acceptable
 Must place critical circuit portions together to minimize
wiring delays
 Propagation delay of signals depends significantly on
routing delay
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Design Verification
 Can simulate the placed & routed device with fairly realistic
logic gate delays
 Use Synopsys PrimeTime static timing analyzer
 Simulation always essential to verify correct design behavior
 Can avoid making application-specific integrated circuits
(ASICs), burning field-programmable gate arrays (FPGAs),
or making full-custom chips that do not work
 Must simulate at both behavioral and logic levels
 Behavioral simulation finds logic errors
 Logic simulation verifies that Synopsys designed logic
correctly
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FPGA Modern Design Methodology
always
mumble
mumble
blah
blah
gates, gates, gates, …
Synthesis
Synthesizable Verilog
Place
and
Route
LE 1
LE 2
Logic Elements in FPGA Chip
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What’s a Logic Element (LE)?
A mux selects which
element of memory
to send to output
Arbitrary programmable
Boolean function of K inputs




K=4 in our particular example.
Usually see K=3, 4, 5 in real
FPGAs
It has a memory — you
download to the memory to
program the device
You also program
connections between these
Logical Elements
Synthesis tool partitions logic
into groups of 5-input
functions
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0000:
0
0001:
0
0010:
0
0011:
1
0100:
0
0101:
0
0110:
0
0111:
1
1000:
0
1001:
0
1010:
0
1011:
1
1100:
1
1101:
1
1110:
1
1111:
1
Really just a
1-bit memory
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mux
F
BD
AC
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What Do We Mean by “Synthesis”?
 Logic synthesis

A program that “designs” logic from abstract descriptions of the
logic
 takes constraints (e.g. size, speed)
 uses a library (e.g. 3-input gates)
 How?

You write an “abstract” Verilog description of the logic

The synthesis tool provides alternative implementations
constraints
Verilog blah
blah blah
synthesis
or …
library
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An Example
 What’s cool?

You type the left, synthesis gives you the gates

It used a different library than you did. (2-input gates only)

One description suffices for a variety of alternate
implementations!

... but this assumes you know a gate level implementation —
that’s not an “abstract” Verilog description.
module gate (f, a, b, c);
output f;
input
a, b, c;
and
A (a1, a, b, c),
B (a2, a, ~b, ~c),
C (a3, ~a, o1);
or
D (o1, b, c),
E (f, a1, a2, a3);
endmodule
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a
b
f
c
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What Do We Want Here…?
 Goal

To specify a combination ckt, inputs->outputs…

… in a form of Verilog that synthesis tools will correctly read

… and then use to make the right logic
 And...

We know the function we want, and can specify in C-like form...

… but we don’t now the exact gates (nor logic elements); we want the
tool to figure this out.
A
B
Combinational
Logic
F
C
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Synthesis and Libraries
 Synopsys can synthesize hardware for the
components in a wide variety of libraries, as well as
for complex programmable logic devices (CPLDs)
and field-programmable gate arrays (FPGAs)
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Problems with Automatic Logic
Synthesis
 Verilog synthesis may frequently interpret code differently
from Verilog simulation
 Unneeded Logic May Not Be Detected

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Both circuits are equivalent
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Ease of Synthesis Depends on
Description Form
Easier to
synthesize
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Summary
Motivation for Hardware Description Language
FPGA Design Methodology and Synthesis
Logic Synthesis Problems
Tool flow
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The Verilog Hardware Description Language - IUMA