SMBus Fan Control with 1°C Accurate Temperature Monitoring
Datasheet
command can be issued that will refresh the temperature data. The limits are only checked when
the temperature data is updated.
3. Mixed - the temperature monitoring block is disabled, but the fan driver block is active. The device
will not update temperature data automatically and the fan driver output will not be updated
automatically based on temperature. A one-shot command can be issued that will refresh the
temperature data and update the fan driver based on the values in the Fan Control Look-Up Table
(if the Fan Control Look-Up Table is enabled).
5.4
ALERT / TACH Output
The ALERT / TACH pin (Pin 6) is an open drain output and requires a pull-up resistor to VDD when
configured as an ALERT output.
APPLICATION NOTE: When configured as a TACH input, the ALERT / TACH pin will not function as an ALERT
output. Error conditions will not trigger an interrupt (though will be updated in the Status
Registers as normal) and the MASK bits will do nothing. Likewise, the device will not respond
to the ARA command.
For the EMC2101 - R, the value of this pull-up resistor determines the initial FAN output mode of
operation as well as whether the device auto loads from an EEPROM or via an SMBus host per
Table 5.1.
After power-up, the EMC2101-R requires 10ms to initialize and determine the operating mode.
When configured as an interrupt, the ALERT / TACH pin is maskable for each alert condition. If the
ALERT / TACH pin is masked, then it will not respond to the corresponding condition (though the Alert
Status Register will update normally). This pin has multiple functions described below and is controlled
by ALERT_COMP bit (bit 0) in the Averaging Filter Register (BFh) (see Section 6.23).
Table 5.1 ALERT/ TACH Pull-up Resistors - SMBus / FAN MODE for EMC2101-R
POLARITY BIT
ALERT / TACH
PULL-UP RESISTOR
SETTING (SEE
Section 6.16)
SMBUS MODE
FAN MODE
5.6k Ohm ±5%
Host Load via SMBus FAN output initialize to 100%
Duty Cycle
1
0
1
0
10k Ohm ±5%
18k Ohm ±5%
33k Ohm ±5%
Host Load via SMBus FAN output initialize to 0% Duty
Cycle
Auto Load via
EEPROM
FAN output initialize to 100%
Duty Cycle
Auto Load via
EEPROM
FAN output initialize to 0% Duty
Cycle
5.4.1
ALERT / TACH as a Temperature Comparator
When the ALERT / TACH pin is used as a temperature comparator, the ALERT / TACH output is
asserted when an out of limit measurement (> high limit, < low limit, or > TCRIT limit) is detected on
any diode (low limits only apply to the external diode channel) or when the external diode connections
are open. When the condition is no longer true, the ALERT / TACH output will de-assert. Reading from
the Status Register will cause the ALERT / TACH pin to be released however it will not prevent it from
being re-asserted based on the temperature comparisons.
Setting the MASK bit will not affect the ALERT / TACH pin when it is configured as a temperature
comparator, however the individual channel mask bits will block the ALERT / TACH pin from being
asserted.
Revision 2.53 (03-13-07)
SMSC EMC2101
DATA2S0HEET
SMSC [ SMSC CORPORATION ]
