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FAN FAILURE DETECTION
DuringSpin-UpProcess
NormalOperationofFanFailure
DetectionandSpin-Up
BitFANS=0
ClearSpin-Up
TimeCounter
TACHData >LowLimit
MeasureRPM
Yes
Yes
No
No
No
No
Yes
Yes
NoSpin-UpifDisabled
BitFANS=1
Spin-UpTimeCount+1
Spin-UpProcess
Spin-UpTimeCount 5³
Fanfailure,
pin
goeslow.
FAN-FAULT
Spin-UpDisabled?
MeasuretheRPMcontinuouslyonceevery0.25s(TACH-FAST =1)or1sbit (TACH-FAST=0),
evenafterafanfailure.However,therearenoFANSdetectionsduringspin-up.
The pinisnegatedifthefanreturnstoanormalRPMrange.
FAN-FAULT
AMC6821
SBAS386C – MAY 2006 – REVISED JULY 2007
When the TACH data are larger than the TACH low limit, the fan runs at a speed below the predefined minimum
RPM. When this condition occurs, a spin-up process is applied to start the fan again when spin-up is enabled.
Bits [STIME2:STIME0] of the Fan Characteristics Register define this time period. Figure 19 shows the function
of the fan failure detection. Refer to the Fan Spin-Up section.
The fan speed is measured immediately after spin-up; the TACH-FAST bit of Configuration Register 4
determines the monitoring rate. If the fan does not return to a normal range after five consecutive spin-ups, a
FAN-FAILURE occurs; the FAN-FAULT pin goes low when it is enabled (the FAN-FAULT-EN bit of Configuration
Register 1 is set), and the spin-up process continues. If the fan returns to a normal speed range before the fifth
spin-up, the FAN-FAULT pin does not go low even though the FANS bit is still set to '1'. No FANS (fan-slow)
detections are performed during spin-up. After the FAN-FAULT pin goes low, spin-up is performed indefinitely
until the RPM reading returns to within normal range or the spin-up is disabled.
Figure 19. Fan Failure Detection and Spin-Up
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