SMBus Digital Interface

From a software perspective, the MAX6639 appears as

a set of byte-wide registers. This device uses a stan-

access the internal registers.

The MAX6639 features an address select input (ADD)

that allows the MAX6639 to have three unique addresses

(see Table 1).

The MAX6639 employs four standard SMBus protocols:

write byte, read byte, send byte, and receive byte

(Figures 1, 2, and 3). The shorter receive byte protocol

allows quicker transfers, provided that the correct data

register was previously selected by a read byte instruc-

tion. Use caution with the shorter protocols in multimas-

ter systems, since a second master could overwrite the

command byte without informing the first master.

Table 4 details the register addresses and functions,

whether they can be read or written to, and the power-

on reset (POR) state. See Tables 5–9 for all other regis-

ter functions and the

Register Descriptions section.

Temperature Reading

Temperature data can be read from registers 00h and

01h. The temperature data format for these registers is

8 bits, with the LSB representing 1°C (Table 2) and the

MSB representing +128°C. The MSB is transmitted first.

Three additional temperature bits provide resolution

down to 0.125

°C and are in the channel 1 extended

temperature (05h) and channel 2 extended temperature

(06h) registers. All values below 0°C clip to 00h.

2-Channel Temperature Monitor with Dual,

Automatic, PWM Fan-Speed Controller

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7

SCL

A = START CONDITION

B = MSB OF ADDRESS CLOCKED INTO SLAVE

C = LSB OF ADDRESS CLOCKED INTO SLAVE

D = R/W BIT CLOCKED INTO SLAVE

AB

C

D

E

FG

HI

J

SDA

LM

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

SCL

AB

C

D

E

FG

H

I

J

K

SDA

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 MASTER

H = LSB OF DATA CLOCKED INTO MASTER

I = MASTER PULLS DATA LINE LOW

J = ACKNOWLEDGE CLOCKED INTO SLAVE

K = ACKNOWLEDGE CLOCK PULSE

L = STOP CONDITION

M = NEW START CONDITION

Figure 3. SMBus Read Timing Diagram

ADD INPUT STATE

BINARY

EQUIVALENT

5Eh

0101 111

Floating

5Ch

0101 110

GND

58h

0101 100