Fan Control Device with Hardware Monitoring and Acoustic Noise Reduction Features
Datasheet
Table 6.5 Minimum RPM Detectable– With PWM Stretching
MINIMUM RPM AT STRETCHED PULSE WIDTH
NUMBER OF EDGES
FOR DETECTION
50MSEC 100MSEC 200MSEC 400MSEC 600MSEC 800MSEC 950MSEC
9
5
3
2
2400
1200
600
1200
600
300
150
600
300
150
75
300
150
75
200
100
50
150
75
126
63
38
32
300
38
25
19
16
Note: Minimum RPM values shown use minimum guard time (88.88usec).
6.1.4.9
Detection of a Stalled Fan
There is a fan failure bit (TACHx) in the interrupt status register used to indicate that a slow or stalled
fan event has occurred. If the tach reading value exceeds the value programmed in the tach limit
register the interrupt status bit is set. See Interrupt Status register 2 at offset 42h.
Notes:
■
■
The reading register will be forced to FFFFh if a stalled event occurs (i.e., stalled event =no edges
detected.)
The reading register will be forced to either FFFFh or FFFEh if a slow fan event occurs. (i.e., slow
event: 0 < #edges < programmed #edges). If the control bit, SLOW, located in the TACHx Options
registers at offsets 90h - 93h, is ‘0’ then FFFEh will be forced into the corresponding Tach Reading
Register to indicate that the fan is spinning slowly.
■
The fan tachometer reading register stays at FFFFh in the event of a stalled fan. If the fan begins
to spin again, the tachometer logic will reset and latch the next valid reading into the tachometer
reading register.
6.1.4.10
Fan Interrupt Status Bits
The status bits for the fan events are in Interrupt Status Register 2 (42h). These bits are set when the
reading register is above the tachometer minimum and the Interrupt Enable 2 (Fan Tachs) register bits
are configured to enable Fan Tach events. No interrupt status bits are set for fan events (even if the
fan is stalled) if the associated tachometer minimum is set to FFFFh (registers 54h-5Bh).
Note: The Interrupt Enable 2 (Fan Tachs) register at offset 80h defaults to enabled for the individual
tachometer status events bits. The group Fan Tach INT# bit defaults to disabled. This bit needs
to be set if Fan Tach interrupts are to be generated on the external INT# pin.
See Figure 5.1 Interrupt Controlon page 22.
6.1.5
Linking Fan Tachometers to PWMs
The TACH/PWM Association Register at offset 81h is used to associate a Tachometer input with a
PWM output. This association has three purposes:
1. The auto fan control logic supports a feature called SpinUp Reduction. If SpinUp Reduction is
enabled (SUREN bit), the auto fan control logic will stop driving the PWM output high if the
associated TACH input is operating within normal parameters. (Note: SUREN bit is located in the
Configuration Register at offset 7Fh)
2. To measure the tachometer input in Mode 2, the tachometer logic must know when the associated
PWM is ‘ON’.
SMSC EMC6D102
Revision 0.4 (04-05-05)
DATA4S3HEET
SMSC [ SMSC CORPORATION ]
