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ADM1027

Fan Speed Measurement Registers

The fan tachometer readings are 16-bit values consisting of a

2-byte read from the ADM1027.

Reg. 0x28 TACH1 Low Byte = 0x00 default

Reg. 0x29 TACH1 High Byte = 0x00 default

Reg. 0x2A TACH2 Low Byte = 0x00 default

Reg. 0x2B TACH2 High Byte = 0x00 default

Reg. 0x2C TACH3 Low Byte = 0x00 default

Reg. 0x2D TACH3 High Byte = 0x00 default

Reg. 0x2E TACH4 Low Byte = 0x00 default

Reg. 0x2F TACH4 High Byte = 0x00 default

Reading Fan Speed From the ADM1027

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 have been read from. This prevents erroneous TACH

readings.

The fan tachometer reading registers report back the number of

11.11

s 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 that the fan either has

stalled or is running very slowly (< 100 RPM).

HIGH LIMIT: > COMPARISON PERFORMED

Since actual fan TACH period is being measured, exceeding a

fan TACH limit by 1 will set the appropriate status bit and can

be used to generate an

SMBALERT.

Fan Tach Limit Registers

The fan TACH limit registers are 16-bit values consisting of

two bytes.

Reg. 0x54 TACH1 Minimum Low Byte = 0xFF default

Reg. 0x55 TACH1 Minimum High Byte = 0xFF default

Reg. 0x56 TACH2 Minimum Low Byte = 0xFF default

Reg. 0x57 TACH2 Minimum High Byte = 0xFF default

Reg. 0x58 TACH3 Minimum Low Byte = 0xFF default

Reg. 0x59 TACH3 Minimum High Byte = 0xFF default

Reg. 0x5A TACH4 Minimum Low Byte = 0xFF default

Reg. 0x5B TACH4 Minimum High Byte = 0xFF default

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 powered directly from 5 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.

Calculating Fan Speed

Assuming a fan with 2 pulses/revolution (and 2 pulses/rev being

measured), fan speed is calculated by:

Fan Speed RPM

Fan Tach Reading

() =¥

()

90 000

where Fan Tach Reading = 16-bit fan tachometer reading.

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 = 6143 decimal

RPM = (f

¥ 60)/fan 1 TACH reading

RPM = (90000

¥ 60)/6143

Fan Speed = 879 RPM

FAN PULSES PER REVOLUTION

Different fan models can output either 1, 2, 3, or 4 TACH

pulses per revolution. Once the number of fan TACH pulses

has been determined, it can be programmed into the fan pulses

per revolution register (Reg. 0x7B) for each fan. Alternatively,

this register can be used to determine the number or pulses/

revolution output by a given fan. By plotting fan speed measure-

ments at 100% speed with different pulses/rev setting, the

smoothest graph with the lowest ripple determines the correct

pulses/rev value.

Fan Pulses Per Revolution Register

<1:0> FAN1 default = 2 pulses/rev

<3:2> FAN2 default = 2 pulses/rev

<5:4> FAN3 default = 2 pulses/rev

<7:6> FAN4 default = 2 pulses/rev

00 = 1 pulse/rev

01 = 2 pulses/rev

10 = 3 pulses/rev

11 = 4 pulses/rev

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