Environmental Monitoring and Control Device
Datasheet
Spin Up
MAX
PWM
PWM may be at
MIN or OFF when
below MIN temp
Duty
Cycle
MIN
MIN
MAX
Hysteresis
Temperature
FIGURE 7.2 - AUTOMATIC FAN CONTROL
7.4.3 SPIN UP
When a fan is being started from a stationary state, the part will cause the fan to “spin up” by going to
100% duty cycle for a programmable amount of time to overcome the inertia of the fan. Following this spin
up time, the fan will go to a duty cycle computed by the auto fan algorithm.
During spin-up, the PWM duty cycle is reported as 0%.
To limit the spin-up time and thereby reduce fan noise, the part uses feedback from the tachometers to
determine when each fan has started spinning properly. The following tachometer feedback is included
into the auto fan algorithm during spin-up. This feature defaults to enabled; it can be disabled by clearing
bit 4 of the Configuration register (7Fh). If disabled, the all fans go to 100% duty cycle for the duration of
their associated spin up time. Note that the Tachometer x minimum registers must be programmed to a
value less than FFFFh in order for the spin up reduction to work properly.
1. The PWM goes to 100% duty cycle until the tachometer reading register is below the minimum limit,
or the spin-up time expires, whichever comes first. This causes spin-up to continue until the
tachometer enters the valid count range, unless the spin up time expires. If the spin up expires before
the tachometer enters the valid range, an interrupt status bit will be set once spin-up expires. Note
that more than one tachometer may be associated with a PWM, in which case all tachometers
associated with a PWM must be in the valid range for spin-up to end.
2. The tachometer reading register always gives the actual reading of the tachometer input.
3. No interrupt bits are set during spin-up.
SMSC EMC6D100/EMC6D101
Page 34
Rev. 09-09-04
DATASHEET
SMSC [ SMSC CORPORATION ]
