EVAL-ADT7475EB ON Semiconductor, EVAL-ADT7475EB Datasheet - Page 24

• Manufacturer Part Number: EVAL-ADT7475EB
• Description: BOARD EVALUATION FOR ADT7475
• Manufacturer: ON Semiconductor
• Series: dBCool®r
• Type: Temperature Sensorr

Specifications of EVAL-ADT7475EB

• Contents: Evaluation Board
• For Use With/related Products: ADT7475
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

made for each TACH input. The net result of this is that all
TACH readings are locked until the high byte is read from the
corresponding TACH registers. All TACH related interrupts
are also ignored until the appropriate high byte is read.
measurements are unlocked and interrupts are processed as
normal.
Fan Speed Measurement Registers
of a 2−byte read from the ADT7475.
Register 0x28, TACH1 Low Byte = 0x00 default
Register 0x29, TACH1 High Byte = 0x00 default
Register 0x2A, TACH2 Low Byte = 0x00 default
Register 0x2B, TACH2 High Byte = 0x00 default
Register 0x2C, TACH3 Low Byte = 0x00 default
Register 0x2D, TACH3 High Byte = 0x00 default
Register 0x2E, TACH4 Low Byte = 0x00 default
Register 0x2F, TACH4 High Byte = 0x00 default
Reading Fan Speed from the ADT7475
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 ms 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 reading of
0xFFFF indicates that the fan either has stalled or is running
very slowly (<100 RPM).
falling below a fan TACH limit by 1 sets the appropriate
status bit and can be used to generate an SMBALERT.
CLOCK
Once the corresponding high byte has been read, TACH
The fan tachometer readings are 16−bit values consisting
The measurement of fan speeds involves a 2−register read
Because the actual fan TACH period is being measured,
TACH
PWM
High Limit > Comparison Performed
1
Figure 39. Fan Speed Measurement
2
3
4
http://onsemi.com
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Fan TACH Limit Registers
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
second. The FAST bit (Bit 3) of Configuration Register 3
(0x78), when set, updates the fan TACH readings
every 250 ms.
but are powered directly from 5.0 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.
Calculating Fan Speed
pulses per revolution being measured), fan speed is
calculated by the following:
Fan Speed (RPM) = (90,000 x 60)/Fan TACH Reading
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 Pulses per Revolution
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.
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.
The fan TACH limit registers are 16−bit values consisting
The fan TACH readings are normally updated once every
If any of the fans are not being driven by a PWM channel
Assuming a fan with a two pulses per revolution (and two
Different fan models can output either 1, 2, 3, or 4 TACH
Alternatively, this register can be used to determine the
Fan 1 TACH Reading = 0x17FF = 6143 (decimal)
RPM = (f x 60)/Fan 1 TACH Reading
RPM = (90,000 x 60)/6143
Fan Speed = 879 RPM