Rainbow Electronics MAX6651 - Manual (Page 23)

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
Register will cause the
ALERToutput to go inactive.
Read the Alarm Status Register.
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
with SMBus/I 2 C-Compatible Interface
______________________________________________________________________________________ 23
MAX6501
GPIO2
MAX6501
GPIO2
MAX6501
GPIO2
FAN
1
FAN
2
FAN
3
CLOCK OUT
CLOCK IN
CLOCK IN
Figure 14. Synchronizing Fans