<s>
In	O
a	O
synchronous	B-Application
system	I-Application
,	O
operations	O
(	O
instructions	B-Language
,	O
calculations	B-General_Concept
,	O
logic	O
,	O
etc	O
.	O
)	O
</s>
<s>
An	O
asynchronous	B-Application
system	I-Application
,	O
in	O
contrast	O
,	O
has	O
no	O
global	O
clock	O
.	O
</s>
<s>
Asynchronous	B-Application
systems	I-Application
do	O
not	O
depend	O
on	O
strict	O
arrival	O
times	O
of	O
signals	O
or	O
messages	O
for	O
reliable	O
operation	O
.	O
</s>
<s>
Coordination	O
is	O
achieved	O
using	O
event-driven	B-Application
architecture	I-Application
triggered	O
by	O
network	B-Protocol
packet	I-Protocol
arrival	O
,	O
changes	O
(	O
transitions	O
)	O
of	O
signals	O
,	O
handshake	B-Protocol
protocols	O
,	O
and	O
other	O
methods	O
.	O
</s>
<s>
Asynchronous	B-Application
systems	I-Application
–	O
much	O
like	O
object-oriented	B-Language
software	O
–	O
are	O
typically	O
constructed	O
out	O
of	O
modular	B-Architecture
'	O
hardware	O
objects	O
 '	O
,	O
each	O
with	O
well-defined	O
communication	O
interfaces	B-Application
.	O
</s>
<s>
These	O
modules	B-Architecture
may	O
operate	O
at	O
variable	O
speeds	O
,	O
whether	O
due	O
to	O
data-dependent	O
processing	O
,	O
dynamic	O
voltage	O
scaling	O
,	O
or	O
process	B-Algorithm
variation	I-Algorithm
.	O
</s>
<s>
The	O
modules	B-Architecture
can	O
then	O
be	O
combined	O
to	O
form	O
a	O
correct	O
working	O
system	O
,	O
without	O
reference	O
to	O
a	O
global	O
clock	O
signal	O
.	O
</s>
<s>
Furthermore	O
,	O
several	O
asynchronous	O
styles	O
have	O
been	O
shown	O
to	O
accommodate	O
clocked	O
interfaces	B-Application
,	O
and	O
thereby	O
support	O
mixed-timing	O
design	O
.	O
</s>
<s>
Hence	O
,	O
asynchronous	B-Application
systems	I-Application
match	O
well	O
the	O
need	O
for	O
correct-by-construction	O
methodologies	O
in	O
assembling	O
large-scale	O
heterogeneous	O
and	O
scalable	O
systems	O
.	O
</s>
<s>
Slightly	O
less	O
robust	O
,	O
but	O
much	O
more	O
useful	O
,	O
are	O
quasi-delay-insensitive	O
circuits	O
(	O
also	O
known	O
as	O
speed-independent	O
circuits	O
)	O
,	O
such	O
as	O
delay-insensitive	B-General_Concept
minterm	O
synthesis	O
,	O
which	O
operate	O
correctly	O
regardless	O
of	O
gate	O
delays	O
;	O
however	O
,	O
wires	O
at	O
each	O
fanout	O
point	O
must	O
be	O
tuned	O
for	O
roughly	O
equal	O
delays	O
.	O
</s>
<s>
with	O
setup/hold	O
requirements	O
on	O
when	O
new	O
inputs	O
can	O
be	O
received	O
)	O
,	O
and	O
bundled	O
datapaths	B-General_Concept
using	O
matched	O
delays	O
(	O
see	O
below	O
)	O
.	O
</s>
<s>
At	O
the	O
extreme	O
,	O
high-performance	O
"	O
timed	O
circuits	O
"	O
have	O
been	O
proposed	O
,	O
which	O
use	O
tight	O
two-side	O
timing	O
constraints	O
,	O
where	O
the	O
clock	O
can	O
still	O
be	O
avoided	O
but	O
careful	O
physical	O
delay	O
tuning	O
is	O
required	O
,	O
such	O
as	O
for	O
some	O
high-speed	O
pipeline	B-General_Concept
applications	O
.	O
</s>
<s>
In	O
a	O
'	O
4-phase	O
handshaking	B-Protocol
protocol	O
 '	O
(	O
or	O
return-to-zero	O
)	O
,	O
the	O
request	O
is	O
asserted	O
by	O
the	O
sender	O
component	O
,	O
and	O
the	O
receiver	O
responds	O
by	O
asserting	O
the	O
acknowledge	O
;	O
then	O
both	O
signals	O
are	O
de-asserted	O
in	O
turn	O
.	O
</s>
<s>
In	O
a	O
'	O
2-phase	O
handshaking	B-Protocol
protocol	O
 '	O
(	O
or	O
transition-signalling	O
)	O
,	O
the	O
requester	O
simply	O
toggles	O
the	O
value	O
on	O
the	O
request	O
wire	O
(	O
once	O
)	O
,	O
and	O
the	O
receiver	O
responds	O
by	O
toggling	O
the	O
value	O
on	O
the	O
acknowledge	O
wire	O
.	O
</s>
<s>
Asynchronous	O
datapaths	B-General_Concept
are	O
typically	O
encoded	O
using	O
several	O
schemes	O
.	O
</s>
<s>
synchronous-style	O
)	O
function	O
block	O
can	O
be	O
used	O
,	O
with	O
an	O
accompanying	O
worst-case	B-General_Concept
matched	O
delay	O
.	O
</s>
<s>
While	O
this	O
scheme	O
has	O
timing	O
constraints	O
,	O
they	O
are	O
simple	O
,	O
localized	O
(	O
unlike	O
in	O
synchronous	B-Application
systems	I-Application
)	O
,	O
and	O
one-sided	O
,	O
hence	O
are	O
usually	O
easy	O
to	O
validate	O
.	O
</s>
