<s>
The	O
System	B-Algorithm
Management	I-Algorithm
Bus	I-Algorithm
(	O
abbreviated	O
to	O
SMBus	B-Algorithm
or	O
SMB	O
)	O
is	O
a	O
single-ended	O
simple	O
two-wire	O
bus	B-General_Concept
for	O
the	O
purpose	O
of	O
lightweight	O
communication	O
.	O
</s>
<s>
Most	O
commonly	O
it	O
is	O
found	O
in	O
computer	B-Device
motherboards	I-Device
for	O
communication	O
with	O
the	O
power	O
source	O
for	O
ON/OFF	O
instructions	O
.	O
</s>
<s>
It	O
is	O
derived	O
from	O
I²C	O
for	O
communication	O
with	O
low-bandwidth	O
devices	O
on	O
a	O
motherboard	B-Device
,	O
especially	O
power	O
related	O
chips	O
such	O
as	O
a	O
laptop	O
's	O
rechargeable	O
battery	O
subsystem	O
(	O
see	O
Smart	B-Algorithm
Battery	I-Algorithm
System	I-Algorithm
)	O
.	O
</s>
<s>
PCI	O
add-in	O
cards	O
may	O
connect	O
to	O
an	O
SMBus	B-Algorithm
segment	O
.	O
</s>
<s>
The	O
SMBus	B-Algorithm
is	O
generally	O
not	O
user	O
configurable	O
or	O
accessible	O
.	O
</s>
<s>
Although	O
SMBus	B-Algorithm
devices	O
usually	O
ca	O
n't	O
identify	O
their	O
functionality	O
,	O
a	O
new	O
PMBus	B-Algorithm
coalition	O
has	O
extended	O
SMBus	B-Algorithm
to	O
include	O
conventions	O
allowing	O
that	O
.	O
</s>
<s>
The	O
SMBus	B-Algorithm
was	O
defined	O
by	O
Intel	O
and	O
Duracell	O
in	O
1994	O
.	O
</s>
<s>
It	O
carries	O
clock	O
,	O
data	O
,	O
and	O
instructions	O
and	O
is	O
based	O
on	O
Philips	O
 '	O
I²C	O
serial	O
bus	B-General_Concept
protocol	O
.	O
</s>
<s>
(	O
PMBus	B-Algorithm
extends	O
this	O
to	O
400kHz	O
.	O
)	O
</s>
<s>
Its	O
voltage	O
levels	O
and	O
timings	O
are	O
more	O
strictly	O
defined	O
than	O
those	O
of	O
I²C	O
,	O
but	O
devices	O
belonging	O
to	O
the	O
two	O
systems	O
are	O
often	O
successfully	O
mixed	O
on	O
the	O
same	O
bus	B-General_Concept
.	O
</s>
<s>
SMBus	B-Algorithm
is	O
used	O
as	O
an	O
interconnect	B-General_Concept
in	O
several	O
platform	O
management	O
standards	O
including	O
:	O
ASF	O
,	O
DASH	O
,	O
IPMI	O
.	O
</s>
<s>
SMBus	B-Algorithm
is	O
used	O
to	O
access	O
DRAM	O
configuration	O
information	O
as	O
part	O
of	O
serial	B-General_Concept
presence	I-General_Concept
detect	I-General_Concept
.	O
</s>
<s>
SMBus	B-Algorithm
has	O
grown	O
into	O
a	O
wide	O
variety	O
of	O
system	O
enumeration	O
use	O
cases	O
other	O
than	O
power	O
management	O
.	O
</s>
<s>
While	O
SMBus	B-Algorithm
is	O
derived	O
from	O
I²C	O
,	O
there	O
are	O
several	O
major	O
differences	O
between	O
the	O
specifications	O
of	O
the	O
two	O
busses	O
in	O
the	O
areas	O
of	O
electricals	O
,	O
timing	O
,	O
protocols	O
and	O
operating	O
modes	O
.	O
</s>
<s>
Instead	O
of	O
relating	O
the	O
bus	B-General_Concept
input	O
levels	O
to	O
VDD	O
,	O
SMBus	B-Algorithm
defines	O
them	O
to	O
be	O
fixed	O
at	O
0.8	O
and	O
2.1V	O
.	O
</s>
<s>
SMBus	B-Algorithm
2.0	O
supports	O
VDD	O
ranging	O
from	O
3	O
to	O
5V	O
.	O
</s>
<s>
SMBus	B-Algorithm
3.0	O
supports	O
VDD	O
ranging	O
from	O
1.8	O
to	O
5V	O
and	O
VIH	O
=	O
1.35V	O
.	O
</s>
<s>
SMBus	B-Algorithm
2.0	O
defines	O
a	O
‘	O
High	O
Power’	O
class	O
that	O
includes	O
a	O
4	O
mA	O
sink	O
current	O
that	O
cannot	O
be	O
driven	O
by	O
I²C	O
chips	O
unless	O
the	O
pull-up	O
resistor	O
is	O
sized	O
to	O
I²C-bus	O
levels	O
.	O
</s>
<s>
NXP	O
devices	O
have	O
a	O
higher	O
power	O
set	O
of	O
electrical	O
characteristics	O
than	O
SMBus	B-Algorithm
1.0	O
.	O
</s>
<s>
SMBus	B-Algorithm
‘	O
high	O
power’	O
devices	O
and	O
I²C-bus	O
devices	O
will	O
work	O
together	O
if	O
the	O
pull-up	O
resistor	O
is	O
sized	O
for	O
3mA	O
.	O
</s>
<s>
The	O
SMBus	B-Algorithm
clock	O
is	O
defined	O
from	O
10	O
–	O
100kHz	O
while	O
I²C	O
can	O
be	O
0	O
–	O
100kHz	O
,	O
0	O
–	O
400kHz	O
,	O
0	O
–	O
1MHz	O
and	O
0	O
–	O
3.4MHz	O
,	O
depending	O
on	O
the	O
mode	O
.	O
</s>
<s>
This	O
means	O
that	O
an	O
I²C	O
bus	B-General_Concept
running	O
at	O
less	O
than	O
10kHz	O
will	O
not	O
be	O
SMBus	B-Algorithm
compliant	O
since	O
the	O
SMBus	B-Algorithm
devices	O
may	O
time	O
out	O
.	O
</s>
<s>
Many	O
SMBus	B-Algorithm
devices	O
will	O
however	O
support	O
lower	O
frequencies	O
.	O
</s>
<s>
SMBus	B-Algorithm
3.0	O
adds	O
400kHz	O
and	O
1MHz	O
bus	B-General_Concept
speeds	O
.	O
</s>
<s>
SMBus	B-Algorithm
defines	O
a	O
clock	O
low	O
time-out	O
,	O
TIMEOUT	O
of	O
35ms	O
.	O
</s>
<s>
SMBus	B-Algorithm
specifies	O
TLOW:SEXT	O
as	O
the	O
cumulative	O
clock	O
low	O
extend	O
time	O
for	O
a	O
slave	O
device	O
.	O
</s>
<s>
SMBus	B-Algorithm
specifies	O
TLOW:MEXT	O
as	O
the	O
cumulative	O
clock	O
low	O
extend	O
time	O
for	O
a	O
master	O
device	O
.	O
</s>
<s>
SMBus	B-Algorithm
defines	O
both	O
rise	O
and	O
fall	O
time	O
of	O
bus	B-General_Concept
signals	O
.	O
</s>
<s>
The	O
SMBus	B-Algorithm
time-out	O
specifications	O
do	O
not	O
preclude	O
I²C	O
devices	O
co-operating	O
reliably	O
on	O
the	O
SMBus	B-Algorithm
.	O
</s>
<s>
It	O
is	O
the	O
responsibility	O
of	O
the	O
designer	O
to	O
ensure	O
that	O
I²C	O
devices	O
are	O
not	O
going	O
to	O
violate	O
these	O
bus	B-General_Concept
timing	O
parameters	O
.	O
</s>
<s>
There	O
are	O
the	O
following	O
differences	O
in	O
the	O
use	O
of	O
the	O
NACK	O
bus	B-General_Concept
signaling	O
:	O
</s>
<s>
SMBus	B-Algorithm
requires	O
devices	O
to	O
acknowledge	O
their	O
own	O
address	O
always	O
,	O
as	O
a	O
mechanism	O
to	O
detect	O
a	O
removable	O
device	O
's	O
presence	O
on	O
the	O
bus	B-General_Concept
(	O
battery	O
,	O
docking	O
station	O
,	O
etc	O
.	O
)	O
</s>
<s>
Other	O
than	O
to	O
indicate	O
a	O
slave	O
's	O
device-busy	O
condition	O
,	O
SMBus	B-Algorithm
also	O
uses	O
the	O
NACK	O
mechanism	O
to	O
indicate	O
the	O
reception	O
of	O
an	O
invalid	O
command	O
or	O
datum	O
.	O
</s>
<s>
Since	O
such	O
a	O
condition	O
may	O
occur	O
on	O
the	O
last	O
byte	O
of	O
the	O
transfer	O
,	O
it	O
is	O
required	O
that	O
SMBus	B-Algorithm
devices	O
have	O
the	O
ability	O
to	O
generate	O
the	O
not	O
acknowledge	O
after	O
the	O
transfer	O
of	O
each	O
byte	O
and	O
before	O
the	O
completion	O
of	O
the	O
transaction	O
.	O
</s>
<s>
This	O
is	O
important	O
because	O
SMBus	B-Algorithm
does	O
not	O
provide	O
any	O
other	O
resend	O
signaling	O
.	O
</s>
<s>
This	O
difference	O
in	O
the	O
use	O
of	O
the	O
NACK	O
signaling	O
has	O
implications	O
on	O
the	O
specific	O
implementation	O
of	O
the	O
SMBus	B-Algorithm
port	O
,	O
especially	O
in	O
devices	O
that	O
handle	O
critical	O
system	O
data	O
such	O
as	O
the	O
SMBus	B-Algorithm
host	O
and	O
the	O
SBS	O
components	O
.	O
</s>
<s>
Each	O
message	O
transaction	O
on	O
SMBus	B-Algorithm
follows	O
the	O
format	O
of	O
one	O
of	O
the	O
defined	O
SMBus	B-Algorithm
protocols	O
.	O
</s>
<s>
The	O
SMBus	B-Algorithm
protocols	O
are	O
a	O
subset	O
of	O
the	O
data	O
transfer	O
formats	O
defined	O
in	O
the	O
I²C	O
specifications	O
.	O
</s>
<s>
I²C	O
devices	O
that	O
can	O
be	O
accessed	O
through	O
one	O
of	O
the	O
SMBus	B-Algorithm
protocols	O
are	O
compatible	O
with	O
the	O
SMBus	B-Algorithm
specifications	O
.	O
</s>
<s>
I²C	O
devices	O
that	O
do	O
not	O
adhere	O
to	O
these	O
protocols	O
cannot	O
be	O
accessed	O
by	O
standard	O
methods	O
as	O
defined	O
in	O
the	O
SMBus	B-Algorithm
and	O
Advanced	B-Device
Configuration	I-Device
and	I-Device
Power	I-Device
Interface	I-Device
(	O
ACPI	B-Device
)	O
specifications	O
.	O
</s>
<s>
The	O
SMBus	B-Algorithm
uses	O
I²C	O
hardware	O
and	O
I²C	O
hardware	O
addressing	O
,	O
but	O
adds	O
second-level	O
software	O
for	O
building	O
special	O
systems	O
.	O
</s>
<s>
SMBus	B-Algorithm
has	O
a	O
time-out	O
feature	O
which	O
resets	O
devices	O
if	O
a	O
communication	O
takes	O
too	O
long	O
.	O
</s>
<s>
This	O
explains	O
the	O
minimum	O
clock	O
frequency	O
of	O
10kHz	O
to	O
prevent	O
locking	O
up	O
the	O
bus	B-General_Concept
.	O
</s>
<s>
I²C	O
can	O
be	O
a	O
‘	O
DC’	O
bus	B-General_Concept
,	O
meaning	O
that	O
a	O
slave	O
device	O
stretches	O
the	O
master	O
clock	O
when	O
performing	O
some	O
routine	O
while	O
the	O
master	O
is	O
accessing	O
it	O
.	O
</s>
<s>
There	O
is	O
no	O
limit	O
in	O
the	O
I²C-bus	O
protocol	O
as	O
to	O
how	O
long	O
this	O
delay	O
can	O
be	O
,	O
whereas	O
for	O
an	O
SMBus	B-Algorithm
system	O
,	O
it	O
would	O
be	O
limited	O
to	O
35	O
ms.	O
</s>
<s>
The	O
SMBus	B-Algorithm
protocol	O
just	O
assumes	O
that	O
if	O
something	O
takes	O
too	O
long	O
,	O
then	O
it	O
means	O
that	O
there	O
is	O
a	O
problem	O
on	O
the	O
bus	B-General_Concept
and	O
that	O
all	O
devices	O
must	O
reset	O
in	O
order	O
to	O
clear	O
this	O
mode	O
.	O
</s>
<s>
SMBus	B-Algorithm
1.1	O
and	O
later	O
define	O
optional	O
Packet	O
Error	O
Checking	O
(	O
PEC	O
)	O
.	O
</s>
<s>
The	O
byte	O
is	O
calculated	O
as	O
CRC-8	O
checksum	B-Algorithm
,	O
calculated	O
over	O
the	O
entire	O
message	O
including	O
the	O
address	O
and	O
read/write	O
bit	O
.	O
</s>
<s>
The	O
polynomial	O
used	O
is	O
x8+x2+x+1	O
(	O
the	O
CRC-8-ATM	O
HEC	B-Protocol
algorithm	O
,	O
initialized	O
to	O
zero	O
)	O
.	O
</s>
<s>
The	O
SMBus	B-Algorithm
has	O
an	O
extra	O
optional	O
shared	O
interrupt	B-Application
signal	O
called	O
SMBALERT#	O
,	O
which	O
can	O
be	O
used	O
by	O
slaves	O
to	O
tell	O
the	O
host	O
to	O
ask	O
its	O
slaves	O
about	O
events	O
of	O
interest	O
.	O
</s>
<s>
SMBus	B-Algorithm
also	O
defines	O
a	O
less	O
common	O
"	O
Host	O
Notify	O
Protocol	O
"	O
,	O
providing	O
similar	O
notifications	O
but	O
passing	O
more	O
data	O
and	O
building	O
on	O
the	O
I²C	O
multi-master	O
mode	O
.	O
</s>
<s>
SMBus	B-Algorithm
devices	O
are	O
supported	O
by	O
FreeBSD	B-Operating_System
,	O
OpenBSD	B-Operating_System
,	O
NetBSD	B-Device
,	O
DragonFly	B-Application
BSD	I-Application
,	O
Linux	B-Application
,	O
Windows	B-Device
98	I-Device
and	O
newer	O
and	O
Windows	B-Operating_System
CE	I-Operating_System
.	O
</s>
