SCH3112I-NE SMSC [SMSC Corporation], SCH3112I-NE Datasheet - Page 236

• 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)
23.14.2.6
23.14.2.7
is LSB first, MSB second. The value FFFFh indicates that the fan is not spinning, or the tachometer
input is not connected to a valid signal (this could be triggered by a counter overflow). These registers
are read only – a write to these registers has no effect.
Notes:
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Programming Options for Each Tachometer Input
The features defined in this section are programmable via the TACHx Option registers located at
offsets 90h-92h and the PWMx Option registers located at offsets 94h-96h.
TACH READING UPDATE TIME
In Mode 1, the Fan Tachometer Reading registers are continuously updated. In Mode 2, the fan
tachometer registers are updated every 300ms, 500msec, or 1000msec. This option is programmed
via bits[1:0] in the PWMx Option register. The PWM associated with a particular TACH(s) determines
the TACH update time.
PROGRAMMED NUMBER OF TACH EDGES
In modes 1 & 2, the number of edges is programmable for 2, 3, 5 or 9 edges (i.e., ½ tachometer pulse,
1 tachometer pulse, 2 tachometer pulses, 4 tachometer pulses). This option is programmed via bits[2:1]
in the TachX Option register.
Note: The “5 edges” case corresponds to two tachometer pulses, or 1 RPM for most fans. Using
GUARD TIME (MODE 2 ONLY)
The guard time is programmable from 8 to 63 clocks (90kHz). This option is programmed via bits[4:3]
in the TachX Option register.
IGNORE FIRST 3 TACHOMETER EDGES (MODE 2 ONLY)
Option to ignore first 3 tachometer edges after guard time. This option is programmed for each
tachometer via bits[2:0] in the TACHx Option register. Default is do not ignore first 3 tachometer edges
after guard time.
Summary of Operation for Modes 1 & 2
The following summarizes the detection cases:
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The Fan Tachometer Reading registers always return an accurate fan tachometer measurement,
even when a fan is disabled or non-functional.
FFFFh indicates that the fan is not spinning, or the tachometer input is not connected to a valid
signal (This could be triggered by a counter overflow).
The Tachometer registers are read only – a write to these registers has no effect.
Mode 1 should be enabled and the tachometer limit register should be set to FFFFh if a tachometer
input is left unconnected.
No edge occurs during the PWM ‘ON’ time: indicate this condition as a stalled fan
− The tachometer reading register contains FFFFh.
One edge (or less than programmed number of edges) occurs during the PWM ‘ON’ time:
indicate this condition as a slow fan.
− If the SLOW bit is enabled, the tachometer reading register will be set to FFFEh to indicate that
this is a slow fan instead of a seized fan. Note: This operation also pertains to the case where the
tachometer counter reaches FFFFh before the programmed number of edges occurs.
− If the SLOW bit is disabled, the tachometer reading register will be set to FFFFh. In this case,
no distinction is made between a slow or seized fan.
the other edge options will require software to scale the values in the reading register to
correspond to the count for 1 RPM.
DATASHEET
LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
220
SMSC SCH311X
Datasheet