SMBus-Compatible Temperature Monitor with
Automatic PWM Fan-Speed Controller
8  _______________________________________________________________________________________
Write Byte Format
Read Byte Format
Send Byte Format
Receive Byte Format
Slave address: equiva-
lent to chip-select line of
a 3-wire interface
Command byte: selects to
which register you are writing
Data byte: data goes into the register
set by the command byte (to set
thresholds, configuration masks, and
sampling rate)
Slave address: equivalent
to chip-select line
Command byte: selects
from which register you
are reading
Slave address: repeated
due to change in data-
flow direction
Data byte: reads from
the register set by the
command byte
Command byte: sends com-
mand with no data, usually
used for one-shot command
Data byte: reads data from
the register commanded
by the last read byte or
write byte transmission;
also used for SMBus alert
response return address
S = Start condition
Shaded = Slave transmission
P = Stop condition
/// = Not acknowledged
Figure 1. SMBus Protocols
S
ADDRESS
RD
ACK
DATA
///
P
7 bits
8 bits
WR
S
ACK
COMMAND
ACK
P
8 bits
ADDRESS
7 bits
P
1
ACK
DATA
8 bits
ACK
COMMAND
8 bits
ACK
WR
ADDRESS
7 bits
S
S
ADDRESS
WR
ACK
COMMAND
ACK
S
ADDRESS
7 bits
8 bits
7 bits
RD
ACK
DATA
8 bits
///
P
SMBCLK
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
A
B
C
D
E    F
G
H
I
J
SMBDATA
t
SU:STA
t
HD:STA
t
LOW
t
HIGH
t
SU:DAT
t
SU:STO
t
BUF
L
M
K
E = SLAVE PULLS SMBDATA LINE LOW
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
I = MASTER PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION
Figure 2. SMBus Write Timing Diagram
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