to ensure that the limit is high enough not to cause sporadic alerts. The fan tachometer will not cause a bit to be set in Interrupt

Status Register 2 if the current value in Current PWM Duty registers is 00h or if the fan 1 disabled via the Fan Configuration

Register. Interrupts will never be generated for a fan if its minimum is set to FF FFh.

Given the insignificance of Bit 0 and Bit 1, these bits could be programmed to remember which fan is which, as follows.

Fan

Bit 1

Bit 0

CPU

0

0

Memory

0

1

Chassis Front

1

0

Chassis Rear

1

1

Setting the Ready/Lock/Start/Override register Lock bit has no effect these registers.

4.14 Registers 5C-5Eh: Fan Configuration

Register

Address

Read/

Write

Register

Name

Bit 7

(MSB)

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

(LSB)

Default

Value

Lock?

5Ch

R/W

Fan1 Configuration

ZON2

ZON1

ZON0

INV

RES

SPIN2

SPIN1

SPIN0

62h

U

5Dh

R/W

Fan2 Configuration

ZON2

ZON1

ZON0

INV

RES

SPIN2

SPIN1

SPIN0

62h

U

5Eh

R/W

Fan3 Configuration

ZON2

ZON1

ZON0

INV

RES

SPIN2

SPIN1

SPIN0

62h

U

This register becomes Read Only when the Ready/Lock/Start/Override register Lock bit is set. Any further attempts to write to this

register shall have no effect. After power up the default value is used whenever the Ready/Lock/Start/Override register Start bit

is cleared even though modifications to this register are possible.

Bits [7:5] Zone/Mode

Bits [7:5] of the Fan Configuration registers associate each fan with a temperature sensor. When in Auto Fan Mode the fan will

be assigned to a zone, and its PWM duty cycle will be adjusted according to the temperature of that zone. If ‘Hottest’ option is

selected (101 or 110), the fan will be controlled by the hottest of zones 2 and 3, or of zones 1, 2, and 3. To determine the ‘Hottest’

zone the PWM level for each zone is calculated then the the highest PWM value is selected. When in manual control mode, the

Current PWM duty registers (30h-32h) become Read/Write. It is then possible to control the PWM outputs with software by writing

to these registers. When the fan is disabled (100) the corresponding PWM output should be driven low (or high, if inverted).

Zone 1: External Diode 1 (processor)

Zone 2: Internal Sensor

Zone 3: External Diode 2

TABLE 4. Fan Zone Setting

ZON[2:0]

Fan Configuration

000

Fan on zone 1 auto

001

Fan on zone 2 auto

010

Fan on zone 3 auto

011

Fan always on full

100

Fan disabled

101

Fan controlled by hottest of zones 2, 3

110

Fan controlled by hottest of zones 1, 2, 3

111

Fan manually controlled (Test Mode)

Bit [4] PWM Invert

Bit [4] inverts the PWM output. If set to 0, 100% duty cycle will yield an output that is always high. If set to 1, 100% duty cycle

will yield an output that is always low.

Bit [3] Reserved

Bits [2:0] Spin Up

Bits [2:0] specify the ‘spin up’ time for the fan. When a fan is being started from a stationary state, the PWM output is held at 100%

duty cycle for the time specified in the table below before scaling to a lower speed.

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