ADT7460 Datasheet, PDF (26/46 Pages) Analog Devices – dB COOL Remote Thermal Controller and Fan Controller

ADT7460
12V
V_CC
<1kΩ
R1*
TACH
OUTPUT
TACH
R2*
ADT7460
FAN SPEED
COUNTER
*SEE TEXT
Figure 50. Fan with Strong TACH Pullup to > V_CC or
Totem−Pole Output, Attenuated with R1/R2
Fan Speed Measurement
The fan counter does not count the fan TACH output
pulses directly because the fan speed may be less than
1000 RPM. It would take several seconds to accumulate a
reasonably large and accurate count. Instead, the period of
the fan revolution is measured by gating an on−chip 90 kHz
oscillator into the input of a 16−bit counter for N periods of
the fan TACH output (Figure 51). The accumulated count is
actually proportional to the fan tachometer period and
inversely proportional to the fan speed.
CLOCK
PWM
TACH 1
2
3
4
Figure 51. Fan Speed Measurement
N, the number of pulses counted, is determined by the
settings of Register 0x7B (fan pulses per revolution
register). This register contains two bits for each fan,
allowing one, two (default), three, or four TACH pulses to
be counted.
The fan tachometer readings are 16−bit values consisting
of a 2−byte read from the ADT7460.
Table 27. Fan Speed Measurement Registers
Register
Description
Default
0x28
0x29
0x2A
0x2B
0x2C
0x2D
0x2E
0x2F
TACH1 low byte
TACH1 high byte
TACH2 low byte
TACH2 high byte
TACH3 low byte
TACH3 high byte
TACH4 Low byte
TACH4 high byte
0x00
0x00
0x00
0x00
0x00
0x00
0x00
0x00
Reading Fan Speed from the ADT7460
If fan speeds are being measured, this 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 are read from. This prevents
erroneous TACH readings.
The fan tachometer reading registers report 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).
Since 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 that it is
running very slowly (<100 RPM).
High Limit: > Comparison Performed
Since the actual fan TACH period is being measured,
exceeding a fan TACH limit by 1 sets the appropriate status bit
and can be used to generate an SMBALERT. The fan TACH
limit registers are 16−bit values consisting of two bytes.
Table 28. Fan TACH Limit Registers
Register
Description
Default
0x54
0x55
0x56
0x57
0x58
0x59
0x5A
0x5B
TACH1 minimum low byte
TACH1 minimum high byte
TACH2 minimum low byte
TACH2 minimum high byte
TACH3 minimum low byte
TACH3 minimum high byte
TACH4 minimum low byte
TACH4 minimum high byte
0xFF
0xFF
0xFF
0xFF
0xFF
0xFF
0xFF
0xFF
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 instead powered directly from 5.0 V or 12 V, its
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.
Calculating Fan Speed
Assuming a fan with a two pulses/revolution (and two
pulses/ revolution being measured), fan speed is calculated
by:
Fan Speed (RPM) = 90,000 × 60/Fan TACH Reading
where:
Fan TACH Reading = 16−Bit Fan Tachometer Reading
For example:
TACH1 High Byte (Reg. 0x29) = 0x17
TACH1 Low Byte (Reg. 0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TACH Reading = 0x17FF = 6143d
RPM = (f × 60)/Fan 1 TACH Reading
RPM = (90000 × 60)/6143
Fan Speed = 879 RPM
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