EVAL-ADT7462EBZ ON Semiconductor, EVAL-ADT7462EBZ Datasheet - Page 30

• Manufacturer Part Number: EVAL-ADT7462EBZ
• Description: BOARD EVALUATION FOR ADT7462
• Manufacturer: ON Semiconductor
• Type: Temperature Sensorr

Specifications of EVAL-ADT7462EBZ

• Contents: Evaluation Board
• For Use With/related Products: ADT7462
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

read on both TACH inputs synchronized to the particular
PWM output. If one fan is connected to one PWM output,
the PWM output is pulse stretched until the counter has
timed out on the disconnected TACH input. In this case, the
pulse is stretching longer than necessary in an effort to sense
a disconnected fan. The speed of the connected fan may be
increased and an audible change in fan speed may be
observed. There are two options to prevent the PWM output
from being stretched longer than necessary in this case.
•
•
Bit 0 controls pulse stretching on TACH1 and TACH5.
Bit 1 controls pulse stretching on TACH2 and TACH6.
Bit 2 controls pulse stretching on TACH3 and TACH7.
Bit 3 controls pulse stretching on TACH4 and TACH8.
Note that the TACH assignments in this register differ from
the TACHs synchronized to each PWM output. Therefore,
if the intention is to drive and sense four fans, connecting the
TACHs together as described in Option 1 allows pulse
stretching on all channels.
set the FAST bit (Bit 0) of Configuration Register 2 (0x02).
When the FAST bit is set, fan TACH readings are updated
every 250 ms.
Fan Speed Measurement Registers
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 (because 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 either that the fan has stalled or is running
very slowly (<100 rpm).
an ALERT is generated if the reading falls below a fan
TACH limit. This ALERT sets the appropriate status bit and
can be used to generate an SMBALERT. The TACH limit is
an 8−bit value that is compared with the TACH high byte of
the TACH reading.
The PWM output is pulse stretched until a valid TACH is
To enable fan speed measurements four times a second,
Fan speed measurement involves a 2−register read for
The actual fan TACH period is being measured; therefore,
Connect the two synchronized TACH inputs together;
for example, if PWM1 is driving a single fan being
sensed on TACH1 only, connect TACH1 and TACH2
together.
Turn off pulse stretching on the unused TACH input;
that is, if PWM1 is driving a single fan being sensed on
TACH1 only, turn off pulse stretching on TACH 2 in
Register 0x08. In this register:
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30
Calculating Fan Speed
pulses per revolution being measured), fan speed is
calculated by:
Fan Speed(RPM) + (freq
Example:
of 2/3 of a revolution. Therefore, the result of Equation 5
should be divided by 1.5. Similarly, if the fan used is an
8−pole fan, then the result should be divided by 2.
Fan Spin−Up
the fan at 100% PWM duty cycle until two TACH pulses are
detected on the TACH input. Once two TACH pulses have
been detected, the PWM duty cycle goes to the expected
running value, for example, 33%. The advantage of this
process is that fans have different spin−up characteristics
and require different times to overcome inertia. The
ADT7462 runs the fans just fast enough to overcome inertia
and the fans are quieter on spin−up than fans programmed
to spin up for a given spin−up time.
Fan 1 Tachometer Reading + 0
RPM + (freq
RPM + (90000
Fan Speed + 879 RPM
Table 21. Tachometer Value and Limit Registers
TACH1
TACH2
TACH3
TACH4
TACH5
TACH6
TACH7
TACH8
TACH
Assuming a fan with two pulses per revolution (and two
What is the speed of Fan 1 in rpm?
If the fan is a 6−pole fan, the count value is representative
The ADT7462 has a unique fan spin−up function. It spins
where:
TACH1 high byte (Register 0x99) = 0x17
TACH1 low byte (Register 0x98) = 0xFF
Fan Tachometer Reading is the 16−bit fan
tachometer reading. freq is the oscillator
frequency, 90 kHz.
Value Register
Low Byte
0x9C
0x98
0x9A
0x9E
0xA2
0xA4
0xA6
0xA8
60) Fan 1 Tachometer Reading
60) 6143
Value Register
60) Fan Tachometer Reading
High Byte
0x9D
0x99
0x9B
0x9F
0xA3
0xA5
0xA7
0xA9
17FF + 6143 Decimal
8−Bit Limit
Register
0x7C
0x7D
0x7A
0x7B
0x7E
0x7F
0x78
0x79
(eq. 1)
(eq. 2)
(eq. 3)
(eq. 4)
(eq. 5)