ADT7476ARQZ-R7 datasheet(29/67 Pages) ONSEMI

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ADT7476

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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.

Measuring fan TACH has the following caveat: When the

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.

Once the corresponding high byte has been read, TACH

measurements are unlocked and interrupts are processed as

normal.

Fan TACH Limit Registers

The fan TACH limit registers are 16−bit values consisting

of two bytes.

Fan Speed Measurement Rate

The fan TACH readings are normally updated once every

second.

When set, the FAST bit (Bit 3) of Configuration

250 ms.

DC Bits

If any of the fans are not being driven by a PWM channel

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

Assuming a fan with two pulses per revolution, and with

the ADT7476 programmed to measure two pulses per

revolution, fan speed is calculated by

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 (0x29) = 0x17

TACH1 Low Byte (0x28) = 0xFF

What is Fan 1 speed in RPM?

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

TACH Pulses per Revolution

Different fan models can output either one, two, three, or

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.

00 = 1 pulse per revolution.

01 = 2 pulses per revolution.

10 = 3 pulses per revolution.

11 = 4 pulses per revolution.

Fan Spin−Up

The ADT7476 has a unique fan spin−up function. It spins

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

To prevent the generation of false interrupts as a fan spins

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.

Fan startup timeout can be disabled by setting Bit 5

(FSPDIS) of Configuration Register 1 (0x40).

ADT7476ARQZ-R7 Datasheet(PDF) 29 Page - ON Semiconductor

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