Temperature Monitors and

PWM Fan Controllers

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7

Figure 1. SMBus Protocols

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 which

register you are writing to

Data Byte: data goes into the register

set by the co mma nd byte ( to se t

thresholds, configuration masks, and

sampling rate)

Slave Address: equiva-

lent to chip-select line

Command Byte: selects

which register you are

reading from

Slave Address: repeated

due to change in data-

flow direction

Data Byte: reads from

the register set by the

command byte

Data Byte: writes data to the

register commanded by the

last read byte or write byte

transmission

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

P = Stop condition

Shaded = Slave transmission

/ / / = Not acknowledged

ACK

7 bits

ADDRESS

ACK

8 bits

DATA

ACK

P

8 bits

S

COMMAND

WR

ACK

7 bits

ADDRESS

ACK

S

ACK

8 bits

DATA

7 bits

ADDRESS

RD

8 bits

P

S

COMMAND

/ / /

WR

ACK

7 bits

ADDRESS

8 bits

COMMAND

ACK

P

S

WR

ACK

7 bits

ADDRESS

RD

8 bits

DATA

P

S

/ / /

Figure 2. SMBus Write Timing Diagram

SMBCLK

AB

C

D

E

FG

H

I

J

K

SMBDATA

A = START CONDITION

B = MSB OF ADDRESS CLOCKED INTO SLAVE

C = LSB OF ADDRESS CLOCKED INTO SLAVE

D = R/W BIT CLOCKED INTO SLAVE

E = SLAVE PULLS SMBDATA LINE LOW

L

M

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, DATA EXECUTED BY SLAVE

M = NEW START CONDITION