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FAN CONTROLLER REGISTERS

AMC6821

SBAS386C – MAY 2006 – REVISED JULY 2007

Status Register 2 (Address 0x03, Value After Power-On or Reset = 0x00)

BIT

NAME

R/W

DEFAULT

DESCRIPTION

Status of the THERM pin as an input. When this input is pulled low, THERM-IN = 1, and

THERM-IN

the fan is driven at full speed. This bit is cleared when reading this register and be

written to '1' if the pin persists "pulled-low".

Local temperature over the local THERM limit flag. L-THERM = 1 when the local

temperature is greater than the value of the Local-THERM-Limit register. Otherwise,

L-THERM = 0. When L-THERM is set to 1, the THERM pin goes low. It also generates a

L-THERM

THERM interrupt through the SMBALERT pin, if enabled (THERMOVIE = 1). This bit is

cleared on a read of Status Register 1. Once cleared, this bit is not reasserted until the

temperature falls 5

°C below the THERM limit, even if the THERM condition persists.

Refer to the THERM Pin and External Hardware Control section.

Active control temperature below the PSV (passive cooling) temperature flag. This bit is

set to '1' when the active control temperature is equal to or below the PSV temperature.

Otherwise, this bit is cleared ('0'). LPSV = 1 generates a PSV interrupt on SMBALERT, if

LPSV

enabled (PSVIE = 1). This bit is cleared when reading this register. If the active control

temperature remains equal to or below the PSV temperature, this bit reasserts on the

next monitoring cycle.

Local temperature over the local critical temperature flag. This bit is set ('1') when the

local temperature is equal to or above the local critical temperature. LTC = 0 if the local

critical temperature is below this value. LTC = 1 asserts the OVR pin low and generates

LTC

an LTC interrupt (non-maskable) though the SMBALERT pin. This bit is cleared when

reading this register. If the over-critical limit condition persists, this bit reasserts on the

next monitoring cycle.

Remote temperature over the remote critical temperature flag. This bit is set to '1' when

the remote temperature is equal to or above the remote critical temperature. RTC = 0 if

the remote critical temperature is below this value. RTC = 1 asserts the OVR pin low and

RTC

generates an RTC interrupt (non-maskable) though the SMBALERT pin. This bit is

cleared when reading this register. If the over-critical limit condition persists, this bit

reasserts on next monitoring cycle.

Reserved

Reserved. Reading returns '0'.

Reserved

Reserved. Reading returns '0'.

Reserved

Reserved. Reading returns '0'.

DCY (Duty Cycle) Register (Address 0x22, Value After Power-On or Reset = 0x55)

BIT

NAME

DEFAULT

DESCRIPTION

7 (MSB)

DCY7 (MSB)

DCY CODE

DUTY CYCLE

DCY6

0x00

DCY5

0x01

0.392%

DCY4

... ...

DCY3

0x40

25%

DCY2

... ...

DCY1

0x80

50%

DCY0

... ...

0xFF

100%

The DCY register stores the value of the PWM duty cycle, 0x00 corresponds to 0%, and 0xFF to 100%. 1LSB

corresponds to 0.392%. Power-on default = 0x55, 33.2%.

In a read operation, with the two following exceptions, the returned data are the actual duty cycle (DCY) value

driving the PWM-Out pin:

1. When TACH-MODE = 0 and the system is in software-RPM control mode, if the calculated duty cycle is less

than 30%, the returned value is the calculated value, not the actual PWM-OUT pin duty cycle which is forced

to 30%.

2. When TACH-MODE = 0 and the system is in software DCY-control mode or Auto Temperature-Fan mode, if

the calculated duty cycle is less than 7%, the returned value is the calculated value, not the actual

PWM-OUT pin duty cycle which is forced to 0%.

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