Title: CSET 4650 Field Programmable Logic Devices
1CSET 4650 Field Programmable Logic Devices
Introduction to HDLs Hardware Description
Languages
2Hardware 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
3Hardware 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 digital circuit's
- operation
- its design
- and tests to verify its operation
- by means of simulation
4Hardware 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
5Levels of Design Abstraction
- Levels of abstraction for VLSI circuit design.
Adapted from Michael D. Ciletti.
6Why 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
7Why HDLs ?
8Why HDLs ?
9Why HDLs ?
10Hardware 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.
11History 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
12History 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.
13History 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
14HDLs
- 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
15HDLs
- 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 and towards the two leading
standards, VHDL and Verilog HDL.
16HDLs 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.
17HDLs 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
18HDLs 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)
19HDLs 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)
20HDLS 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)
21HDLs 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