ADT7476ARQH ONSEMI [ON Semiconductor], ADT7476ARQH Datasheet - Page 29

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ADT7476ARQH

Manufacturer Part Number
ADT7476ARQH
Description
dBCOOL Remote Thermal Controller and Voltage Monitor
Manufacturer
ONSEMI [ON Semiconductor]
Datasheet
falling below a fan TACH limit by 1 sets the appropriate
status bit and can be used to generate an SMBALERT.
ADT7476 starts up, TACH measurements are locked. In
effect, an internal read of the low byte has been 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 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.
Register 3 (0x78) updates the fan TACH readings every
250 ms.
DC Bits
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. Once high frequency
mode is enabled in 4−wire fans, the dc bits do not need to be
set because this is automatically done internally.
Calculating Fan Speed
the ADT7476 programmed to measure two pulses per
revolution, fan speed is calculated by
where Fan TACH Reading is the 16−bit fan tachometer
reading.
Example:
Because the actual fan TACH period is being measured,
Measuring fan TACH has the following caveat: When the
Once the corresponding high byte has been read, TACH
The fan TACH limit registers are 16−bit values consisting
The fan TACH readings are normally updated once every
When set, the FAST bit (Bit 3) of Configuration
If any of the fans are not being driven by a PWM channel
Assuming a fan with two pulses per revolution, and with
Fan Speed (RPM) = (90,000 x 60)/Fan TACH Reading
TACH1 High Byte (0x29) = 0x17
TACH1 Low Byte (0x28) = 0xFF
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What is Fan 1 speed in RPM?
TACH Pulses per Revolution
four 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 (0x7B) for
each fan. Alternatively, this register can be used to determine
the number of pulses per revolution output by a given fan.
By plotting fan speed measurements at 100% speed with
different pulses per revolution settings, 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 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%. Fans have different
spin−up characteristics and take different times to overcome
inertia. The advantage of the ADT7476 is that it runs the fans
just fast enough to overcome inertia and is quieter on
spin−up than fans that are programmed to spin up for a given
time.
Fan Startup Timeout
up (because it is below running speed), the ADT7476
includes a fan startup timeout function. During this time, the
ADT7476 looks for two TACH pulses. If two TACH pulses
are not detected, an interrupt is generated.
(FSPDIS) of Configuration Register 1 (0x40).
Different fan models can output either one, two, three, or
The ADT7476 has a unique fan spin−up function. It spins
To prevent the generation of false interrupts as a fan spins
Fan startup timeout can be disabled by setting Bit 5
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
00 = 1 pulse per revolution.
01 = 2 pulses per revolution.
10 = 3 pulses per revolution.
11 = 4 pulses per revolution.

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