ADT7462ACPZ-R7 datasheet(30/81 Pages) ONSEMI

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ADT7462

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The PWM output is pulse stretched until a valid TACH is

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.

• 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

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.

To enable fan speed measurements four times a second,

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

Fan speed measurement 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 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).

The actual fan TACH period is being measured; therefore,

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.

Table 21. Tachometer Value and Limit Registers

TACH

Low Byte

Value Register

High Byte

Value Register

8−Bit Limit

TACH1

0x98

0x99

0x78

TACH2

0x9A

0x9B

0x79

TACH3

0x9C

0x9D

0x7A

TACH4

0x9E

0x9F

0x7B

TACH5

0xA2

0xA3

0x7C

TACH6

0xA4

0xA5

0x7D

TACH7

0xA6

0xA7

0x7E

TACH8

0xA8

0xA9

0x7F

Calculating Fan Speed

Assuming a fan with two pulses per revolution (and two

pulses per revolution being measured), fan speed is

calculated by:

Fan Speed(RPM) + (freq

60) Fan Tachometer Reading

(eq. 1)

where:

Fan Tachometer Reading is the 16−bit fan

tachometer reading. freq is the oscillator

frequency, 90 kHz.

Example:

TACH1 high byte (Register 0x99) = 0x17

TACH1 low byte (Register 0x98) = 0xFF

What is the speed of Fan 1 in rpm?

Fan 1 Tachometer Reading + 0

17FF + 6143 Decimal

(eq. 2)

RPM + (freq

60) Fan 1 Tachometer Reading

(eq. 3)

Fan Speed + 879 RPM

(eq. 4)

RPM + (90000

60) 6143

(eq. 5)

If the fan is a 6−pole fan, the count value is representative

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 ADT7462 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%. 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.

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