RPM-Based Fan Controller with HW Thermal Shutdown
Datasheet
5.3.4
FAN_MODE Pin
The FAN_MODE pin is used to determine the fan driver output levels at power-up before the EMC2102
has been programmed. After power-up, the fan driver will be set at the selected drive until the RPM
based Fan Control Algorithm is started or disabled.
The level on the pin determines the function as shown in Table 5.2.
Table 5.2 FAN_MODE Pin Functions
FAN_MODE
FUNCTION
Fan Driver set at 0% drive
0
open
1
Fan Driver set at 60% drive after Spin Up Routine
Fan Driver set at 75% drive after Spin Up Routine
5.3.5
32.768KHz Clock Source
The EMC2102 allows the user to choose between supplying an external 32.768KHz clock or use of
the internal 32.768KHz oscillator to measure the TACH signal. This clock source is used by the RPM
based Fan Control Algorithm to calculate the current fan speed. This fan controller accuracy is directly
proportional to the accuracy of the clock source.
To enable the external clock source, the CLK_SEL pin must be pulled to VDD_3V at power-up (see
Table 5.3). The CLK_SEL pin is must be in a known state at all times (either pulled high or pulled low)
and is latched upon power-up.
Table 5.3 CLK_SEL Pin Functions
CLK_SEL
FUNCTION
0
1
Internal oscillator used
External clock used
5.4
Watchdog Timer
The EMC2102 contains an internal Watchdog Timer. Once the device has powered up the watchdog
timer monitors the bus traffic for signs of activity. The Watchdog Timer starts when the VDD_5V supply
has reached its operating point. The Watchdog Timer only starts immediately after power-up and once
it has been triggered or deactivated will not restart.
If four (4) seconds elapse without the system host programming the device, then the following will
occur:
1. The WATCH status bit will be set.
2. The High Side Fan Driver will be set to full scale drive. It will remain at full scale drive until one of
the two conditions listed below are met.
If the Watchdog Timer is triggered, the following two operations will disable the timer and return the
device to normal operation.
1. Writing the RPM based Fan Control Algorithm TACH Target Register will disable the Watchdog
Timer regardless of the value. If a value is written that is greater than the Valid TACH Count
Register setting (other than FFh), the fan drive setting will be set based on the FAN_MODE pin
SMSC EMC2102
Revision 1.95 (10-19-06)
DATA2S5HEET
SMSC [ SMSC CORPORATION ]
