Fan Control Device with Hardware Monitoring and Acoustic Noise Reduction Features
Datasheet
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.
6.1.4.4 Mode 2 –Monitor Tach input When PWM is ‘ON’
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 Linking Fan
Tachometers to PWMs on page 43.)
During an update cycle, if an insufficient number of tachometer pulses are detected during this time
period, the PWM output is “stretched” (held high) until either the programmed number of edges is
detected or the the maximum stretch time expires, whichever comes first. (See Summary of Operation
for Modes 1 & 2 on page 40)
PWM STRETCHING
PWM stretching will be done by the hardware each update of the tachometer registers (every 300ms,
500ms, or 1000ms). The PWM will be stretched until the programmed number of edges is detected or
until the programmed max PWM stretching timeout occurs. 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.
Note: 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.
The PWM maximum stretching times (50, 100, 200, 400, 600, 800, 950 msec) represent the maximum
time that the PWM will be ‘on’. So if the hardware needs to stretch the PWM ‘on’ time then it will be
stretched beyond the PWM duty cycle ‘on’ time up to the programmed PWM stretch time.
If PWM stretching is disabled, the tachometer will only be monitored during the PWM duty cycle ‘ON’
time. The hardware will never stretch the PWM ‘ON’ time. The hardware treats the disabled case as
Stretch Time equals PWM ‘ON’ time. Interrupts are reported in the same manner as if stretching is
enabled.
Note: When stretching is enabled, the hardware will continue the edge detection until the required
number of edges occur or the stretching time period expires.
ASSUMPTIONS (REFER TO FIGURE 4 - PWM AND TACHOMETER CONCEPT):
The Tachometer pulse generates 5 transitions per fan revolution (i.e., two fan tachometer periods per
revolution, edges 2→6). One half of a revolution (one tachometer period) is equivalent to three edges
(2→4 or 3→5). One quarter of a revolution (one-half tachometer period) is equivalent to two edges.
To obtain the fan speed, count the number of 90Khz pulses that occurs between 2 edges i.e., 2→3,
between 3 edges i.e., 2→4, or between 5 edges, i.e. 2→6 (the case of 9 edges is not shown). The
time from 1-2 occurs through the guard time and is not to be used. For the discussion below, an edge
is a high-to-low or low-to-high transition (edges are numbered – refer to Figure 4 - PWM and
Tachometer Concept
The Tachometer circuit begins monitoring the tach when the associated PWM output transitions high
and the guard time has expired. Each tach circuit will continue monitoring until either the stretch time
ends or the programmed number of edges has been detected, whichever comes first. The associated
PWM output will continue to stretch until all tachs that are associated with it have taken a
measurement.
Revision 0.4 (04-05-05)
SMSC EMC6D102
DATA3S8HEET
SMSC [ SMSC CORPORATION ]
