1. 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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Bushnell: Digital Systems Design Lecture 7

2. Bushnell: Digital Systems Design Lecture 7
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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Bushnell: Digital Systems Design Lecture 7

3. 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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Bushnell: Digital Systems Design Lecture 7

4. Bushnell: Digital Systems Design Lecture 7
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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Bushnell: Digital Systems Design Lecture 7

5. 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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Bushnell: Digital Systems Design Lecture 7

6. Shortcomings of Verilog or VHDL
You lose some control of defining the gate-level circuit implementation
You don’t have time to do that, anyway
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
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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Bushnell: Digital Systems Design Lecture 7

7. Activities of Design at Levels of Design
ARCHITECTURE & BEHAVIORAL
REGISTER TRANSFER – Verilog/VHDL
LOGIC
SWITCH LEVEL
CIRCUIT (TRANSISTORS)
LAYOUT & TEST PATTERNS
FABRICATION LINE
Synopsys
Design Analyzer
Synopsys
Behavioral
Simulation
Cadence
Verilog-XL
Synopsys
Logic Sim.
SPICE and
Cadence
Spectre
Cadence
LayoutPlus
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Bushnell: Digital Systems Design Lecture 7

8. Bushnell: Digital Systems Design Lecture 7
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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Bushnell: Digital Systems Design Lecture 7

9. Bushnell: Digital Systems Design Lecture 7
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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Bushnell: Digital Systems Design Lecture 7

10. Bushnell: Digital Systems Design Lecture 7
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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Bushnell: Digital Systems Design Lecture 7

11. FPGA Modern Design Methodology
always
mumble
blah
Synthesis
gates, gates, gates, …
Technology Mapping
Synthesizable Verilog
Place and Route
LE 1
LE 2
Logic Elements in FPGA Chip
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Bushnell: Digital Systems Design Lecture 7

12. 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
Really just a 1-bit memory
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Bushnell: Digital Systems Design Lecture 7

13. 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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Bushnell: Digital Systems Design Lecture 7

14. Bushnell: Digital Systems Design Lecture 7
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 a b c f
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Bushnell: Digital Systems Design Lecture 7

15. Bushnell: Digital Systems Design Lecture 7
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.
Combinational
Logic A B C F
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Bushnell: Digital Systems Design Lecture 7

16. 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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Bushnell: Digital Systems Design Lecture 7

17. Problems with Automatic Logic Synthesis
Verilog synthesis may frequently interpret code differently from Verilog simulation
Unneeded Logic May Not Be Detected
Both circuits are equivalent
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Bushnell: Digital Systems Design Lecture 7

18. Ease of Synthesis Depends on Description Form
Easier to
synthesize
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Bushnell: Digital Systems Design Lecture 7

19. Bushnell: Digital Systems Design Lecture 7
Summary
Motivation for Hardware Description Language
FPGA Design Methodology and Synthesis
Logic Synthesis Problems
Tool flow
2/22/2019
Bushnell: Digital Systems Design Lecture 7