ADT7468 Analog Devices, Inc., ADT7468 Datasheet - Page 32

• Manufacturer Part Number: ADT7468
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

ADT7468
The fan tachometer reading registers report back the number of
11.11 μs period clocks (90 kHz oscillator) gated to the fan speed
counter, from the rising edge of the first fan TACH pulse to the
rising edge of the third fan TACH pulse (assuming two pulses
per revolution are being counted). Because the device is essen-
tially measuring the fan TACH period, the higher the count
value, the slower the fan is actually running. A 16-bit fan tach-
ometer reading of 0xFFFF indicates either that the fan has
stalled or is running very slowly (<100 RPM).
Because the actual fan TACH period is being measured, falling
below a fan TACH limit by 1 sets the appropriate status bit and
can be used to generate an SMBALERT .
Fan TACH Limit Registers
The fan TACH limit registers are 16-bit values consisting of two
bytes.
Reg. 0x54, TACH1 minimum low byte = 0xFF default
Reg. 0x55, TACH1 minimum high byte = 0xFF default
Reg. 0x56, TACH2 minimum low byte = 0xFF default
Reg. 0x57, TACH2 minimum high byte = 0xFF default
Reg. 0x58, TACH3 minimum low byte = 0xFF default
Reg. 0x59, TACH3 minimum high byte = 0xFF default
Reg. 0x5A, TACH4 minimum low byte = 0xFF default
Reg. 0x5B, TACH4 minimum high byte = 0xFF default
Fan Speed Measurement Rate
The fan TACH readings are normally updated once every
second.
The fast bit (Bit 3) of Configuration Register 3 (Reg. 0x78),
when set, updates the fan TACH readings every 250 ms.
If any of the fans are not being driven by a PWM channel but
are powered directly from 5 V or 12 V, their associated dc bit in
Configuration Register 3 should be set. This allows TACH
readings to be taken on a continuous basis for fans connected
directly to a dc source. For optimal results, the associated dc bit
should always be set when using 4-wire fans.
High limit: >comparison performed
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Calculating Fan Speed
Assuming a fan with a two pulses per revolution (and two
pulses per revolution being measured), fan speed is calculated
by
where the Fan TACH reading is the 16-bit fan tachometer
reading.
Example:
Fan Pulses per Revolution
Different fan models can output either 1, 2, 3, or 4 TACH pulses
per revolution. Once the number of fan TACH pulses has been
determined, it can be programmed into the fan pulses per
revolution register (Reg. 0x7B) for each fan. Alternatively, this
register can be used to determine the number or pulses per
revolution output by a given fan. By plotting fan speed measure-
ments at 100% speed with different pulses per revolution
setting, the smoothest graph with the lowest ripple determines
the correct pulses per revolution value.
Fan Pulses per Revolution Register
<1:0> Fan 1 default = 2 pulses per revolution
<3:2> Fan 2 default = 2 pulses per revolution
<5:4> Fan 3 default = 2 pulses per revolution
<7:6> Fan 4 default = 2 pulses per revolution
Fan Speed (RPM) = (90,000 × 60)/Fan TACH Reading
TACH1 high byte (Reg. 0x29) = 0x17
TACH1 low byte (Reg. 0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TACH Reading = 0x17FF = 6143 (decimal)
RPM = (f × 60)/Fan 1 TACH Reading
RPM = (90000 × 60)/6143
Fan Speed = 879 RPM
00 = 1 pulse per revolution
01 = 2 pulses per revolution
10 = 3 pulses per revolution
11 = 4 pulses per revolution