MAX6651EEE Datasheet MAX6650EUB+T, MAX6651EEE+T, MAX6650EUB+ | P7-P9

Slave Address Command byte: Selects

which register you are

writing to.

Data byte: Data goes into

the register set by the

command byte (to set

thresholds, configuration

masks, and sampling rate).

Figure 2a. SMBus Protocol: Write Byte Format

Slave Address 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.

Figure 2b. SMBus Protocol: Read Byte Format

Data byte: Reads data

from the register com-

manded by the last

read-byte or write-byte

transmission; also

used for SMBus alert

response return address.

Figure 2c. SMBus Protocol: Receive Byte Format

S = Start condition Shaded = Slave transmission WR = Write = 0

P = Stop condition ACK = Acknowledged = 0 RD = Read =1

A = Not acknowledged = 1

Fan-Speed Regulators and Monitors

with SMBus/I

C-Compatible Interface

Maxim Integrated 7

MAX6650/MAX6651

COMMANDS

8 bits

PACKDATA

8 bits

ACKADDRESS

7 bits

ACK

COMMANDS

8 bits

ACKADDRESS

7 bits

ACK

S P

DATA

8 bits

ADDRESS

7 bits

ACK

S P

DATA

8 bits

ADDRESS

7 bits

ACK

Slave Address

Fan-Speed Regulators and Monitors

with SMBus/I

C-Compatible Interface

8 Maxim Integrated

MAX6650/MAX6651

SMBCLK

AB CD

FG H

SMBDATA

SU:STA

HD:STA

LOW

HIGH

SU:DAT

HD:DAT

SU:STO

BUF

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

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER

G = MSB OF DATA CLOCKED INTO SLAVE

H = LSB OF DATA CLOCKED INTO SLAVE

I = SLAVE PULLS SMBDATA LINE LOW

J = ACKNOWLEDGE CLOCKED INTO MASTER

K = ACKNOWLEDGE CLOCK PULSE

L = STOP CONDITION, DATA EXECUTED BY SLAVE

M = NEW START CONDITION

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

AB CD

FG H

SMBDATA

SU:STA

HD:STA

LOW

HIGH

SU:DAT

SU:STO

BUF

E = SLAVE PULLS SMBDATA LINE LOW

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER

G = MSB OF DATA CLOCKED INTO MASTER

H = LSB OF DATA CLOCKED INTO MASTER

I = ACKNOWLEDGE CLOCK PULSE

J = STOP CONDITION

K = NEW START CONDITION

Figure 3. SMBus Write Timing Diagram

Figure 4. SMBus Read Timing Diagram

Command-Byte Functions

The 8-bit Command-Byte Register (Table 2) is the mas-

ter index that points to the various other registers within

MAX6650/MAX6651. The register’s power-on reset

(POR) state is 0000 0000, so that a receive-byte trans-

mission (a protocol that lacks the command byte)

occurring immediately after POR returns the current

speed setting.

Fan-Speed Register

In closed-loop mode, the MAX6650/MAX6651 use the

Fan-Speed Register to set the period of the tachometer

signal that controls the fan speed. The Fan-Speed

MAX6650/MAX6651 regulate the fan speed by forcing

the tachometer period (t

TACH

) equal to the scaled reg-

ister value. One revolution of the fan generates two

tachometer pulses, so the required Fan-Speed Register

value (K

TACH

) may be calculated as:

TACH

= 1 / (2 x Fan Speed)

TACH

= [t

TACH

x K

SCALE

x (f

CLK

/ 128)] - 1

where the fan speed is in rotations per second (RPS),

TACH

is the period of the tachometer signal, f

CLK

is the

internal oscillator frequency (254kHz ±10%), and

SCALE

is the prescaler value (see

Configuration-Byte

). Since the fan speed is inversely proportional

to the tachometer period, the Fan-Speed Register value

TACH

) does not linearly control the fan speed (Table

3). Select the prescaler value so the fan’s full speed is

achieved with a register value of approximately 64

(0100 0000) to optimize speed range and resolution.

The MAX6651 may be controlled by an external oscilla-

Fan-Speed Regulators and Monitors

with SMBus/I

C-Compatible Interface

Maxim Integrated 9

MAX6650/MAX6651

tor that overrides the internal oscillator (see

General-

Purpose Input/Output

). When using an external oscillator

OSC

), calculate the Fan-Speed Register value with f

CLK

equal to f

OSC

. Codes above F8h (1111 1000) are

allowed, but will not significantly decrease the frequency.

Configuration-Byte Register

The Configuration-Byte Register (Table 4) adjusts the

prescaler, changes the tachometer threshold voltage,

and sets the mode of operation. The three least-signifi-

cant bits configure the prescaler division used to scale

the tachometer period. Select the prescaler value so the

fan’s full speed is achieved with a register value of

approximately 64 (0100 0000) to optimize speed range

and resolution (see the

Fan Speed Register

section). The

fourth bit selects the fan operating voltage.

The fifth and sixth bits configure the operating mode.

The MAX6650/MAX6651 have four modes of operation:

full-on, full-off (shutdown), closed-loop, and open-loop.

In closed-loop operation, the external microcontroller

(µC) sets the desired speed by writing an 8-bit word to

the Fan-Speed Register (see the

Fan-Speed Register

section). The MAX6650/MAX6651 monitor the fan’s

tachometer output and automatically adjust the voltage

Table 2. Command-Byte Assignments

COUNT

SPEED 0000 0000

xCONFIG 0000 0010

READ

GPIO DEF 0000 0100

0001 0110 x

WRITE

x 02h Tachometer count time

FFh

0Ah

00h

POR (DEFAULT)

STATE

COMMAND

GPIO definition

Configuration

Fan speed

FUNCTION

DAC 0000 0110 x x 00h DAC

ALARM ENABLE 0000 1000 x x 00h Alarm enable

ALARM 0000 1010 x — 00h Alarm status

TACH0 0000 1100 x — 00h Tachometer 0 count

TACH1 0000 1110 x — 00h Tachometer 1 count

TACH2 0001 0000 x — 00h Tachometer 2 count

TACH3 0001 0010 x — 00h Tachometer 3 count

GPIO STAT 0001 0100 x — 1Fh GPIO status

Table 3. Fan Speed

0000 0000 1.0

*0000 0001 1.0

0000 0010 1.5

SCALE

(ms)

TACH

SCALE

330

500

TACH

* *

FAN SPEED (RPS)

—

500

480

—

240

— — — — — —

—

0001 1110 16 3.9 * 32 128

0001 1111 16 4.0 1.0 31 124

0010 0000 17 4.2

20,000

1.0 30 120

30,000

— — — — — —

30,000

0100 0000 33 8.2 2.1 15.3 61.1

— — — — — —

—

1111 1000 125 31 7.8 4 15.9

1900

1800

—

910

—

240

SCALE

—

7700

7400

7200

—

3700

—

960

FAN SPEED (RPM)

—

30,000

29,000

—

15,000

—

3830

4 16 11 4 16 1 4 16

The minimum allowed tachometer period is 1ms.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P25

MAX6651EEE

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