Fan Control Device with Hardware Monitoring and Acoustic Noise Reduction Features
Datasheet
6.1.4.6
Programming Options for Each Tachometer Input
The features defined in this section are programmable via the TACHx Option registers located at
offsets 90h-93h 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
the other edge options will require software to scale the values in the reading register to
correspond to the count for 1 RPM.
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.
PWM STRETCH TIME (MODE 2 ONLY)
The PWM max stretching time is programmable to 50msec, 100msec, 200msec, 400msec, 600msec,
800msec, and 950msec. This option is programmed via bits[7:5] in the TachX Option register. PWM
stretching can be disabled by selecting the bit combination 000 (default is disabled).
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.
6.1.4.7
Summary of Operation for Modes 1 & 2
The following summarizes the detection cases:
■
No edge occurs during the PWM ‘ON’ time or stretching 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 or
stretching time: indicate this condition as a slow fan.
−If the SLOW bit is ‘0’, 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 ‘1’, the tachometer reading register will be set to FFFFh. In this case, no
distinction is made between a slow or seized fan.
Note: The Slow Interrupt Enable feature (SLOW) is configured in the TACHx Options registers at
offsets 90h to 93h.
■
The programmed number of edges occurs:
-Mode 1: If the programmed number of edges occurs before the counter reaches FFFFh latch the
tachometer count
-Mode 2: If the programmed number of edges occurs during the PWM ‘ON’ time or stretching time:
latch the tachometer count and stop stretching PWM (see Note below).
Revision 0.4 (04-05-05)
SMSC EMC6D102
DATA4S0HEET
SMSC [ SMSC CORPORATION ]
