MAX6651EEE datasheet(23/25 Pages) MAXIM

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Synchronizing Fans

GPIO2 can be configured to allow multiple MAX6651s

to synchronize the speeds of the fans they are driving

(Figure 14). Synchronization is accomplished by having

one of the MAX6651s (or an external clock) serve as

the clock master by configuring one of the GPIO2s in

the system as a clock output. The remaining GPIO2s in

the system need to be configured as clock inputs:

• Electrically connect all MAX6651 GPIO2s together.

• Configure one of the MAX6651’s GPIO2s to be a

clock output, using the GPIO Definition Register (set

bits 5 and 4 to 01).

• Configure the rest of the GPIO2s as clock inputs, using

the GPIO Definition Register (set bits 5 and 4 to 00).

• Configure all MAX6651s in closed-loop mode.

• Configure all prescaler values to be equal.

• Write identical values to all speed registers.

Setting Up Alarms

The MAX6650/MAX6651 can be configured to generate

an ALERT output on GPIO0 whenever certain events,

such as control loop out of regulation, tachometer over-

flow, or GPI01/GPI02 being driven low, occur. This is

designed to enhance the “set and forget” functionality

of the fan control system.

Configure GPIO0 to be an ALERT output (see above).

Minimum/Maximum Output Level Alarm

The minimum/maximum output level alarms are

designed to warn the system when the MAX6650/

MAX6651 are unable to maintain speed regulation in

closed-loop mode. The MAX6650/MAX6651 maintain

speed regulation by adjusting the voltage across the

fan. If the desired speed can’t be attained, one of these

alarms will be generated. Possible causes for failure to

attain the desired speed include system programming

problems, incipient fan failure, and a programmed

speed that the fan cannot support.

The minimum output alarm occurs when the DAC out-

put is 00h. A DAC value of 00h means that the

MAX6650/MAX6651 have applied the largest available

voltage across the fan. This typically means the fan is

unable to spin as fast as the desired speed.

The maximum output alarm occurs when the DAC value

is FFh. A DAC value of FFh means the MAX6650/MAX6651

have tried to reduce the voltage across the fan to 0.

Although this would seem to indicate the fan is spinning

faster than the desired speed, this should rarely hap-

pen. If this alarm occurs, it probably indicates some

type of system error.

Enable the minimum/maximum output level alarm by

setting bits 0 and 1 of the alarm enable register to 11.

Tachometer Overflow Alarm

If any tachometer counter overflows (reaches a count of

255), this alarm will be set.

Enable the overflow output level alarm by setting bit 2

of the alarm enable register bit to 1.

GPIO1/2 Pulled Low

Enabling this alarm causes the ALERT output to go low

whenever GPIO1 or GPIO2 is pulled low. This will occur

independent of the configuration of GPIO1 or GPIO2.

Enable the GPIO1/GPIO2 output level alarms by setting

bits 3 and/or 4 of the alarm enable register bit to 1.

Clearing the ALERT

Once an ALERT is generated, determine which alarm

caused the ALERT pin to go low. Do this by reading the

Alarm Status Register. An ALERT output will stay active

(low) even if the condition that caused the alert is

removed. Reading the Alarm Status Register clears the

ALERT, if the condition that caused the alert is gone. If

the condition has not gone away, the ALERT will stay

active. Disabling the alarm with the Alarm Enable

Read the Alarm Status Register.

Fan-Speed Regulators and Monitors

with SMBus/I2C-Compatible Interface

Maxim Integrated

MAX6650/MAX6651

MAX6501

GPIO2

MAX6501

GPIO2

MAX6501

GPIO2

FAN

CLOCK OUT

CLOCK IN

Figure 14. Synchronizing Fans

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