Environmental Monitoring and Control Device
Datasheet
temperature is around the minimum temperature limit. This value is programmed in Registers 6Dh-
6Eh: Zone Hysteresis registers.
3. The speed of the fan is controlled by the duty cycle set for that device. The duty cycle for the
minimum fan speed must be programmed in Registers 64h-66h: Fanx PWM Minimum. This value
corresponds to the speed of the fan when the temperature reading is equal to the minimum
temperature LIMIT setting. As the actual temperature increases/decreases and is above the Zone
LIMIT temperature and below the Absolute Temperature Limit, the PWM will be determined by a linear
function based on the Auto Fan Speed Range bits in Registers 5Fh-61h.
4. Set the absolute temperature for each zone in Registers 6Ah-6Ch: Absolute Limit (Auto Fan Mode
only). If the actual temperature is equal to or exceeds the absolute temperature in one or more zones,
all Fans will be set to Full on, regardless of which zone they are operating in (except those that are
disabled). Note: fans can be disabled via the Fan Configuration registers and the absolute
temperature safety feature can be disabled by writing 80h into the Absolute Temp Limit registers.
5. To set the mode to operate in auto mode, set Bits[7:5] Zone/Mode, located in Registers 5Ch-5Eh: Fan
Configuration: Bits[7:5]=’000’ for Fan on Zone 1; Bits[7:5]=’001’ for Fan on Zone 2; Bits[7:5]=’010’ for
Fan on Zone 3. If the “Hottest” option is chosen (101 or 110), then the fan is controlled by the zone
that results in the highest PWM duty cycle value.
Notes:
Software can be alerted of an out-of-limit condition by the INT# pin if a status bit is set and enabled
(EMC6D100 only).
Software can monitor the operation of the Fans through the Fan Tachometer Reading registers and by the
Fan x Current PWM duty registers. It can also monitor current temperature readings through the
Temperature Limit Registers if hardware monitoring is enabled.
Fan control in auto mode is implemented without any input from external processor and without any
consideration of the fan tachometer register values except during spin up. See description below.
In auto “Zone” mode, the speed is adjusted automatically as shown in the figure below. Fans are assigned
to a zone. It is possible to have more than one fan in a thermal zone or one fan monitoring two or three
fans.
FIGURE 7.2 shows the control flow for the auto fan algorithm. The part allows a minimum temperature to
be set, below which the fan will not run or will run at minimum speed. A hysteresis value is included to
prevent the fan continuously switching on and off if the temperature is close to the minimum. A
temperature range is specified over which the part will automatically adjust the fan speed. When the
temperature exceeds the minimum, the fan will “spin up” by going on full for a programmable amount of
time. Following this spin up time, the fan will go to a duty cycle computed by the auto fan algorithm. As the
temperature rises, the duty cycle will increase until the fan is running at full-speed when the temperature
reaches the minimum plus the range value. The effect of this is a temperature feedback loop, which will
cause the temperature to reach equilibrium between the minimum temperature and the minimum
temperature plus the range. Provided that the fan has adequate cooling capacity for all environmental and
power dissipation conditions, this system will maintain the temperature within acceptable limits, while
allowing the fan to run slower (and quieter) when less cooling is required.
SMSC EMC6D100/EMC6D101
Page 33
Rev. 09-09-04
DATASHEET
SMSC [ SMSC CORPORATION ]
