ADT7476AARQZ 查看數據表（PDF） - ON Semiconductor

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ADT7476AARQZ

dBCOOL® Remote Thermal Controller and Voltage Monitor

ON Semiconductor 

ADT7476A

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

ADT7476A 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.

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

The fan TACH readings are normally updated once every

second.

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

Register 3 (0x78) updates the fan TACH readings every

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 ADT7476A 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 ADT7476A 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 ADT7476A 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 ADT7476A

includes a fan startup timeout function. During this time, the

ADT7476A 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).

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