MAX6651EEE datasheet(10/25 Pages) MAXIM

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across the fan until the desired speed is reached. Open-

loop operation allows the µC to regulate fan speed direct-

ly. The µC reads the fan speed from the Tach-

ometer-Count Register. Based on the tachometer

count, the µC decides if the fan speed requires adjust-

ment, and changes the voltage across the fan by writ-

ing an 8-bit word to the DAC Register. Full-on mode

applies the maximum voltage across the fan, forcing it

to spin at full speed. Configuring GPIO1 (see the

General-Purpose Input/Output section) as an active-low

input provides additional hardware control that fully

turns on the fan and overrides all software commands.

General-Purpose Input/Output

The GPIO pins connect to the drain of the internal N-

channel MOSFET and pullup resistor (Figure 5). When

the N-channel MOSFET is off (Table 5), the pullup resis-

tor provides a logic-level high output. However, with the

MOSFET off, the GPIO may serve as an input pin and

its state is read from the GPIO Status Register (Table

6). The MAX6650/MAX6651 power up with the MOSFET

off, so input signals may be safely connected to the

GPIO pins. When using the GPIO pin as a general-pur-

pose output, change the output by writing to the GPIO

Definition Register.

GPIO0 may be configured as an ALERT output that will

go low whenever a fault-condition is detected (see the

Alarm-Enable and Status Registers section). GPIO1

may be configured as a FULL ON input to allow hard-

ware control to fully turn on the fan in case of software

or µC failure. GPIO2 (MAX6651 only) may be config-

ured as an internal clock output or as an external clock

input to allow synchronization of multiple devices.

Alarm-Enable and Status Registers

The alarms are enabled only when the appropriate bits of

the Alarm-Enable Register are set (Table 7). The maxi-

mum and minimum output level alarms function only

when the device is configured to operate in the closed-

loop mode (see the

Configuration-Byte Register section).

The Alarm Status Register allows the system to deter-

mine which alarm caused the alert output (Table 8).

The set-alarm and alert outputs clear after reading the

Fan-Speed Regulators and Monitors

with SMBus/I2C-Compatible Interface

Maxim Integrated

MAX6650/MAX6651

Table 4. Configuration Byte Register

Figure 5. General-Purpose Input/Output Structure

MAX6650

MAX6651

100k

GPIO

STATUS

VCC

3.0V TO 5.5V

VCC

CBYPASS

GPIO_

GND

GPIO

DEFINITION

BIT

NAME

POR (DEFAULT)

STATE

FUNCTION

5 to 4

7 (MSB) to 6

—

Always 0

Operating Mode:

00 = Software full-on (default)

01 = Software off (shutdown)

10 = Closed-loop operation

11 = Open-loop operation

MODE

5/12V

Fan/Tachometer Voltage:

0 = 5V

1 = 12V (default)

2 to 0 (LSB)

SCALE

010

Prescaler Division:

000 = Divide by 1

001 = Divide by 2

010 = Divide by 4 (default)

011 = Divide by 8

100 = Divide by 16

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