EET 3350 Digital Systems Design

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EET 3350 Digital Systems Design
Introduction to HDLs Hardware Description
Languages

• Dan Solarek

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Hardware Description Languages

• Hardware Description Languages, or HDLs, are
  languages used to design hardware.
• Similar to procedural programming languages
  (e.g., C)
• Digital hardware only
• An HDL can also be used to describe the
  functionality of hardware as well as its
  implementation.
• Leads to simulation
• Allows functional verification

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Hardware Description Languages

• In electronics, a hardware description language
  or HDL is any language from a class of computer
  languages for formal description of digital
  electronic circuits.
• An HDL can describe a digital circuit's
• describe a digital circuits operation
• document its design
• test to verify its operation
• and simulate its functionality

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Hardware Description Languages

• Describe hardware modules at varying levels of
  abstraction (more later )
• Structural description
• Textual replacement for schematic
• Hierarchical composition of modules from
  primitives
• Behavioral/functional description
• Describe what module does, not how
• Synthesis step generates circuit for module
• Simulation semantics are included

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Levels of Design Abstraction

• Levels of abstraction for VLSI circuit design.
  Adapted from Michael D. Ciletti.

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Why HDLs ?

• The complexity of logic circuits has increased
  dramatically in the past few decades
• Other forms of EDAs are no longer effective
• HDLs offer a consistent and efficient method for
  both design and synthesis
• HDLs are relatively easy to learn

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Why HDLs ?
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Why HDLs ?
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Why HDLs ?
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Hardware Description Languages

• The principal feature of a hardware description
  language is that it contains the capability to
  describe the function (behavior) of hardware
  independent of implementation.
• The great advance with modern HDLs was the
  recognition that a single language could be used
  to describe the function of the design and also
  to describe the implementation.

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History of HDLs

• The first HDL was ISP, invented by C. Gordon
  Bell and Alan Newell at Carnegie Mellon
  University (CMU) and described in their book
  Computer Structures published in 1972.

Gordon Bell
Alan Newell
Instruction-set Processor language
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History of HDLs

• ISP was the first HDL to use the term
  register-transfer level (RTL).
• This term came from the use of ISP in describing
  the behavior of the PDP-8 computer as a set of
  registers and logical functions describing the
  transfer of data from source register to
  destination register.

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History of HDLs

• Subsequent HDLs included
• VHSIC HDL (VHDL) which was begun in 1979
• Verilog
• ABEL
• UDLI which was developed by NTT
• HiLo, which was the predecessor to Verilog
• ISP', which was a successor to ISP (implemented
  by the N-dot simulator).
• and more

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HDLs

• ISP (circa 1971) - research project at CMU
• Written by Bell and Newell
• Simulation, but no synthesis
• ABEL (circa 1983) - developed by Data-I/O
• Targeted to programmable logic devices
• Not good for much more than state machines
• Verilog (circa 1985) - developed by Gateway
  (absorbed by Cadence)
• Similar to Pascal and C
• Delays is only interaction with simulator
• Fairly efficient and easy to write
• IEEE standard

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HDLs

• VHDL (circa 1987) - DoD sponsored standard
• Similar to Ada (emphasis on re-use and
  maintainability)
• Simulation semantics visible
• Very general but can get verbose
• IEEE standard
• Updated in 1993

The current trend is to move away from
proprietary HDLs
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HDLs

• JHDL (1997, Just-Another Hardware Description
  Language)
• developed at BYU
• low level hardware description language
• focused primarily on building circuits via an
  Object Oriented approach
• bundles collections of gates into Java objects
• Implemented as a toolset and class library on top
  of the Java programming language
• Its primary use is for the design of
  field-programmable gate arrays (FPGAs)
• Increasing use as a replacement for VHDL for
  FPGAs
• VHDL/Verilog still popular for other uses
• Particular attention was paid to supporting the
  Xilinx series of chips

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HDLs vs. Programming Languages

• An HDL is a standard text-based expression of the
  temporal behavior (timing) and/or (spatial)
  circuit structure of an electronic system.
• In contrast to a software programming language,
  an HDL's syntax and semantics include explicit
  notations for expressing time and concurrency
  which are the primary attributes of hardware.

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HDLs vs. Programming Languages

• An HDL is analogous to a software programming
  language, but with subtle differences.
• Both types of language are processed by a
  compiler.
• An HDL compiler often works in several stages,
  first producing a logic description file in a
  proprietary format, then converting that to a
  logic description file in the industry-standard
  EDIF format, then converting that to a
  JEDEC-format file.
• The JEDEC file contains instructions to a PLD
  programmer for building logic.

Joint Electron Device Engineering Council
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HDLs vs. Programming Languages

• Program Structure
• Instantiation of multiple components of the same
  type
• Specify interconnections between modules via
  schematic
• Hierarchy of modules
• Assignment
• Continuous assignment (logic always produces an
  output signal)
• Propagation delay (signals take time)
• Timing of signals is important (when does a
  specific signal or set of signals have an effect
  on the circuit)

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HDLs vs. Programming Languages

• Data structures
• Size explicitly spelled out - no dynamic
  structures
• No pointers
• Parallelism
• Hardware is naturally parallel (must support
  multiple threads)
• Assignments can occur in parallel (not just
  sequentially)

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HDLS and Combinational Logic

• Modules specification of inputs, outputs,
  bidirectional, and internal signals
• Continuous assignment a gate's output is a
  function of its inputs at all times (doesn't need
  to wait to be "called")
• Propagation delay concept of time and delay in
  input affecting gate output
• Composition connecting modules together with
  wires
• Hierarchy modules encapsulate functional blocks
• Specification of don't care conditions
  (accomplished by setting output to x)

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HDLs and Sequential Logic

• Flip-Flops
• Representation of clocks - timing of state
  changes
• Asynchronous vs. synchronous
• FSMs
• Structural view (FFs separate from combinational
  logic)
• Behavioral view (synthesis of sequencers)
• Data-paths ALUs registers
• Use of arithmetic/logical operators
• Control of storage elements
• Parallelism
• Multiple state machines running in parallel
• Sequential don't cares