Fan Control Device with High Frequency PWM Support and Hardware Monitoring Features
Datasheet
7.1.4.6
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.
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.
7.1.4.7
Summary of Operation for Modes 1 & 2
The following summarizes the detection cases:
■
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 set to 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 set to one, 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 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: latch the
tachometer count. (see Note below).
Notes:
■
Whenever the programmed number of edges is detected, the edge detection ends and the state
machine is reset. The tachometer reading register is updated with the tachometer count value at
this time. See Note 7.1 below for the one exception to this behavior.
SMSC EMC6D103
Revision 0.4 (04-04-05)
DATA4S5HEET
SMSC [ SMSC CORPORATION ]
