Fan Control Device with Hardware Monitoring and Acoustic Noise Reduction Features
Datasheet
The Fan Tachometer value may be updated every 300ms, 500ms, or 1000ms.
Internal PWM
PWM “ON”
Signal
Guard time A
Window for
Valid Tach
Pulses
2
3
4
5
6
F
1
Tach
Pulses
Tach
Pulses
B
C
D
E
A
Figure 6.5 PWM and Tachometer Concept
FAN TACHOMETER OPTIONS FOR MODE 2
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2, 3, 5 or 9 “edges” to calculate the fan speed (Figure 4)
Guard time A is programmable (8-63 clocks) to account for delays in the system (Figure 4)
PWM Stretching options: 50, 100, 200, 400, 600, 800msec, and 950msec.
The PWM frequencies for modes 1 & 2 are: 11.0 Hz, 14.6 Hz, 21.9 Hz, 29.3 Hz, 35.2 Hz, 44.0 Hz,
58.6 Hz, 87.7Hz
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Option to ignore first 3 tachometer edges after guard time
Option to force tach reading register to FFFEh to indicate a slow fan.
6.1.4.5
Fan Tachometer Reading Registers:
The Tachometer Reading registers are 16 bits, unsigned. When one byte of a 16-bit register is read, the
other byte latches the current value until it is read, in order to ensure a valid reading. The order 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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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).
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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.
SMSC EMC6D102
Revision 0.4 (04-05-05)
DATA3S9HEET
SMSC [ SMSC CORPORATION ]
