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In several books for VHDL I have come across the term "behavioural
description" of a system (circuit etc etc).Is there any other
description that VHDL can express?Is so what are the differences?

On Tue, 27 Sep 2005 00:46:26 +0200, Nikos Mitas <>
wrote:

>+<In several books for VHDL I have come across the term "behavioural
>+<description" of a system (circuit etc etc).Is there any other
>+<description that VHDL can express?Is so what are the differences?
*****

To me, Behavioral Description is using programming language to
describe a black box operation of a circuit. You need not describe the
actual contents of what makes up the black box but only to describe
the input and the output functionality and timing.

One can also use programing language to describe the structure of a
block to the gate level and how to hook up the various gates of a
circuit. Using this method you in a sense use programming language to
dictate the bounds of the synthesizer. In a behavioral description you
use constraint files built into the tools to guide and give
intelligence to help the synthesizer develope a circuit and wiring.


An example is a full adder. You can describe the behavior as:

A <= B + C.

Or you can describe the XOR and AND gates plus how they are wired.
Both do functionaly the same.

james

In behavioural, you can (sometimes) omit clocks etc.

There are other descriptions:

structural - a (description of a) digital circuit connecting
components using (signal) wires
gate-level - structural, but using logic elements from a vendor
library as components
register-transfer-level: using processes mostly with clk/reset can
be synthesized, glitch-resistent
...

a gate-level VHDL description can be the result of a synthesis tool,
which is fed by a register-transfer-level description.

A behavioural description (aka behaviour model) can normally not be
synthesized to a gate-level "netlist".

In VHDL, at the lowest level (inside the components), there are always
processes (all concurrent statements can be replaced to equivalent to
process descriptions, the language reference manual describes how to do
this).

Hubble.