ADT7475 Analog Devices, Inc., ADT7475 Datasheet - Page 29

• Manufacturer Part Number: ADT7475
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

Reading Fan Speed from the ADT7475
The measurement of fan speeds involves a 2-register read for
each measurement. The low byte should be read first. This
causes the high byte to be frozen until both high and low byte
registers have been read, preventing erroneous TACH readings.
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 essentially
measuring the fan TACH period, the higher the count value, the
slower the fan is actually running. A 16-bit fan tachometer read-
ing of 0xFFFF indicates that the fan either 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.
Register 0x54, TACH1 Minimum Low Byte = 0xFF default
Register 0x55, TACH1 Minimum High Byte = 0xFF default
Register 0x56, TACH2 Minimum Low Byte = 0xFF default
Register 0x57, TACH2 Minimum High Byte = 0xFF default
Register 0x58, TACH3 Minimum Low Byte = 0xFF default
Register 0x59, TACH3 Minimum High Byte = 0xFF default
Register 0x5A, TACH4 Minimum Low Byte = 0xFF default
Register 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 (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 the following:
where Fan TACH Reading is the 16-bit fan tachometer reading.
Example
TACH1 High Byte (Register 0x29) = 0x17
TACH1 Low Byte (Register 0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TACH Reading = 0x17FF = 6143 (decimal)
RPM = (f × 60)/Fan 1 TACH Reading
RPM = (90,000 × 60)/6143
Fan Speed = 879 RPM
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 Tach Pulses per
Revolution register (Register 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 measurements at 100% speed with different pulses per
revolution setting, the smoothest graph with the lowest ripple
determines the correct pulses per revolution value.
Tach Pulses per Revolution Register
Bits [1:0] Fan 1 default = 2 pulses per revolution.
Bits [3:2] Fan 2 default = 2 pulses per revolution.
Bits [5:4] Fan 3 default = 2 pulses per revolution.
Bits [7:6] Fan 4 default = 2 pulses per revolution.
00 = 1 pulse per revolution
01 = 2 pulses per revolution
10 = 3 pulses per revolution
11 = 4 pulses per revolution
Fan Speed (RPM) = (90,000 × 60)/Fan TACH Reading
ADT7475