MAX31785 MAXIM [Maxim Integrated Products], MAX31785 Datasheet - Page 45

• Manufacturer Part Number: MAX31785
• Description: 6-Channel Intelligent Fan Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

The MFR_READ_FAN_PWM command returns the latest real-time value for the fan PWM in % duty cycle.
MFR_READ_FAN_PWM is updated whenever the PWM duty cycle is updated. This is determined by the RATE bits in
MFR_FAN_CONFIG. The 2 data bytes are in DIRECT format.
The MFR_FAN_FAULT_LIMIT command sets the value of the fan speed (in RPM) or percentage of target fan speed that
causes a fan fault. Fans operating below these limits for over 10s continuous trip the fault. When the fan is operating in
RPM mode, the 10s checking period starts after the device has completed a PWM ramp of the fan speed.
The slowest tachometer signal the device can measure for a fan is 60 RPM. In dual tachometer applications, the slow-
est RPM is 360. Tachometer signals slower than these minimums are reported as 0 RPM. Fault and warning limits
should be set to RPM values greater than these minimum RPM limits.
The 2 data bytes are in DIRECT format. Set to 0000h to disable the limit checking. Set to 0001h to only alarm when the
tachometer input is locked for greater than 10s (this mode should be used when the fan only has a locked rotor output). In
response to violation of the MFR_FAN_FAULT_LIMIT or if a stalled/locked rotor is detected, the device does the following:
1) Sets the NONE OF THE ABOVE bit in STATUS_BYTE.
2) Sets the NONE OF THE ABOVE and FANS bits in STATUS_WORD.
3) Sets the FAN_1_FAULT bit in STATUS_FANS_1_2.
4) Responds as specified in the MFR_FAULT_RESPONSE.
5) Notifies the host using ALERT assertion (if enabled in MFR_MODE).
USER NOTE: For proper fan operation, MFR_FAN_FAULT_LIMIT must be configured.
The MFR_FAN_WARN_LIMIT command sets the value of the fan speed (in RPM) or percentage of target fan speed that
causes a fan speed warning. Fans operating below these limits for over 10s continuous trip the warning. When the fan is
operating in RPM mode, the 10s checking period starts after the device has completed a PWM ramp of the fan speed.
Normally, the MFR_FAN_WARN_LIMIT is set higher than the MFR_FAN_FAULT_LIMIT. The 2 data bytes are in DIRECT
format. Set to 0000h to disable the limit checking. Set to 0001h to generate a warning when using a locked rotor fan.
In response to violation of the MFR_FAN_WARN_LIMIT, the device does the following:
1) Sets the NONE OF THE ABOVE bit in STATUS_BYTE.
2) Sets the NONE OF THE ABOVE and FANS bits in STATUS_WORD.
3) Sets the FAN_1_WARN bit in STATUS_FANS_1_2.
4) Notifies the host using ALERT assertion (if enabled in MFR_MODE).
Table 29. Monitored Fan Fault and Warning Parameters
FAN CONTROL MODE
Automatic PWM
Automatic RPM
Manual PWM
Manual RPM
6-Channel Intelligent Fan Controller
Fan speed (in RPM)
Percentage of programmed
target fan speed
Fan speed (in RPM)
Percentage of LUT target fan
speed
LIMIT PARAMETER
> (limit x 110%)
> (limit + 5%)
> (limit x 110%)
> (limit + 5%)
CLEARING THE FAULT/
HYSTERESIS FOR
WARNING
MFR_FAN_FAULT_LIMIT (F5h)
MFR_FAN_WARN_LIMIT (F6h)
MFR_READ_FAN_PWM (F3h)
Checked once a second
Checked once a second
Checked once a second
Checked once a second
COMPARISON INTERVAL
45