
Advuttd
DigitaJ D esign with
the
Verilog
HDL
HDLs
and
synlhesis lools focus
an
engin
ee
r's allention on functionality rather
than
on
individual transistors or gates; they synthesize a
ci
rcuit that will r
ea
li
ze the de-
sired functionalit
y,
and
satisfy a rea
andlor
performance constraints. Moreover, alterna-
tive architectures can
be
generated
fr
om a single
HDL
model and evaluated quickly to
perform design tra
de
offs. Functional models
ar
e also referred to as behavioral model
s.
HDLs
serve as a platform for
seve
ral
too
l
s:
design e ntry, design ver
ifi
cation. test
generation, fault analysis
and
simulation,
ti
ming analysis and/or
ve
rification, synthesi
s,
and
automatic generation
of
sc
hematics. This
br
eadth
of
use improves the efficiency of
the design now by eliminating translations
of
de
sign descriptions as t
he
design moves
through the tool chain.
Tw
o languages enjoy wides
pr
ead industry s
up
port:Veril
og
U I
(lJ
and
VHDL
[2
J.
Both languages are I
EEE
(In
stitute
of
Electrical
and
Electronics Engincers) stan-
dards;
both
are supported by synthesis tools for ASICs
(app
li
cation-speci
fic
integrated
circuits) a
nd
FPGAs
(field-pr
og
rammabl
e
gale
arrays). Languages
fo
r
ana
log
ci
rcuit
dc
sign, such as Spice (3). play an impo
rt
ant role in verifying critical timing paths
of
a
ci
rcuit,
but
these languages impose a prohibitive co
mput
ational
burd
en
on
large de-
si
gn
s,
cannot support abstract styles
of
design,
and
become impractical when used on a
large scal
c.
Hybrid languages (e.g
..
Verilog-
A)
(4)
are used
in
designing mixed-signal
circuits, which have both di
gi
tal and analog circuitry. System-level design languages,
such as SystemC
[5J
and SuperlogTM 16], are now emerging to support a higher level
of
de
sign abstrac
ti
on than
ca
n
be
s
up
po
rted by Veril
og
or
VHDL.
1
.1
Design Methodology-An Introduction
ASICs
and
FPG
As are design
ed
systematically to maximize
the
likelih
ood
that a de-
sign will
be
correct and will be fabricated
wi
thout fatal naw
s.
Designers follow a "de-
sign now" like that shown in Figure 1-1, which specifies a se
qu
ence of maj
or
steps
that
will be taken to design.
ve
rif
y,
synth
es
ize.
and
test a digital circuit.
ASIC
design nows
in
vo
lve several activitie
s,
from specificati
on
and
design entry.
to
place-
and
-route and
timing closure
of
the circuit in s
il
icon. Tuning
cl
osure is attain
ed
when all of the signal
paths in the design satisfy the timing constraints imposed by the interface circuitry, the
circuit's sequential elements,
and
the system
cl
ock. Although the desi
gn
fl
ow
appears
10
be
linear, in practice
it
is not. Various steps might
be
revisited as design errors are
discovered, requir
eme
nts change,
or
per
f
orma
nce a
nd
design constraints are violatcd.
For example, if a circuit
fa
il
s to m
eet
timing constraints, a new place
ment
and
routing
step
will have to be taken,
perhaps
including redesign
of
critical paths.
Design nows f
or
standard-cell-based ASJCs
ar
e
more
complcx than those
fo
r
FP
GAs because the architecture of
an
ASIC is not
fix
e
d.
Consequcntly, the perfor-
mance that can
be
realized from a d
es
ign de
pend
s
on
the physical placement
and
r
ou
t-
ing
of
the cells on the die, as we
ll
as
the underlying device properties. Inte rconnect
delays play a significant role in delcrmining performance in submicron designs below
0.18
pm
, in which p
re
layout
estimates
of
path
delays do
not
gu
arantee
timing clos
ure
of
the routed design.
Th
e following sections will clarify the design flow described in Fig
ure
1-1.