SCH3112I-NE SMSC [SMSC Corporation], SCH3112I-NE Datasheet - Page 234

• Manufacturer Part Number: SCH3112I-NE
• Description: LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
• Manufacturer: SMSC [SMSC Corporation]

Rev 0.2 (09-28-04)
23.14.2.3
23.14.2.4Mode 2 – Monitor Tach input When PWM is ‘ON’
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Mode 1 – Always Monitoring
Mode 1 is the simple case. In this mode, the Fan is always powered when it is ‘ON’ and the fan
tachometer output ALWAYS has a valid output. This mode is typically used if a linear DC Voltage
control circuit drives the fan.
pulses between the programmed number of edges (default = 5 edges). The fan tachometer reading
registers are continuously updated.
The counter is used to determine the period of the Fan Tachometer input pulse. The counter starts
counting on the first edge and continues counting until it detects the last edge or until it reaches FFFFh.
If the programmed number of edges is detected on or before the counter reaches FFFFh, the reading
register is updated with that count value. If the counter reaches FFFFh and no edges were detected
a stalled fan event has occurred and the Tach Reading register will be set to FFFFh. If one or more
edges are detected, but less than the programmed number of edges, a slow fan event has occurred
and the Tach Reading register will be set to either FFFEh or FFFFh depending on the state of the Slow
Tach bits located in the TACHx Options registers at offsets 90h - 93h. Software can easily compute
the RPM value using the tachometer reading value if it knows the number of edges per revolution.
Notes:
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In this mode, the PWM is used to pulse the Fan motor of a 3-wire fan. 3-wire fans use the same
power supply to drive the fan motor and to drive the tachometer output logic. When the PWM is ‘ON’
the fan generates valid tach pulses. When the PWM is not driving the Fan, the tachometer signal is
not generated and the tach signal becomes indeterminate or tristate. Therefore, Mode 2 only makes
tachometer measurements when the associated PWM is driving high during an update cycle. As a
result, the Fan tachometer measurement is “synchronized” to the PWM output, such that it only looks
for tach pulses when the PWM is ‘ON’.
Note: Any fan tachometer input may be associated with any PWM output (see
During an update cycle, if an insufficient number of tachometer pulses are detected during this time
period, the following applies: If at least one edge but less than the programmed number of edges is
detected, the fan is considered slow. If no edge is detected, the fan is considered stopped.
Notes:
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Slow interrupt disabled (Don't force FFFEh)
Tach interrupt enabled via enable bit
Tach Limit = FFFFh
Tach readings updated once a second
If the PWM output associated with a tach input is configured for the high frequency option then the
tach input must be configured for Mode 1.
Some enhanced features added to support Mode 2, are available to Mode 1 also. They are:
programmable number of tach edges and force tach reading register to FFFEh to indicate a SLOW
fan.
Five edges or two tach pulses are generated per revolution.
If a tach input is left unconnected it must be configured for Mode 1.
The interrupt status bits are set, if enabled, to indicate that a slow or stopped fan event has
occurred when the tach reading registers are greater than the tach limit registers.
At some duty cycles, the programmed number of edges will appear during some PWM High times,
but not all. If opportunistic mode is enabled, the tach logic will latch the count value any time it
detects the programmed number of edges and reset the update counter. (See Bit[5] of PME_STS1.)
Tachometers to PWMs on page
In this mode, the fan tachometer simply counts the number of 90kHz
DATASHEET
LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
223.)
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SMSC SCH311X
Linking Fan
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