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Functional Description (Continued)

4.19 Registers 6A-6Ch: Absolute Temperature Limit

Address

Read/

Write

Name

Bit 7

(MSB)

Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

(LSB)

Default

Value

Lock?

6Ah R/W Zone1 Absolute Temp Limit 7 654321 0 64h U

6Bh R/W Zone2 Absolute Temp Limit 7 654321 0 64h U

6Ch R/W Zone3 Absolute Temp Limit 7 654321 0 64h U

For the LM85B in the Auto Fan mode, if a zone exceeds the temperature set in the Absolute Temperature Limit register, all of the

PWM outputs will incresase its duty cycle to 100%. This is a safety feature that attempts to cool the system if there is a potentially

catastrophic thermal event. If set to 80h (-128˚C), the feature is disabled. Default=100˚C=64h

For the LM85C in the Auto Fan mode, if a zone exceeds the temperature set in the Absolute Temperature Limit register, only the

PWM output associated with the Absolute Temperature Limit will go to 100%.

These registers become Read Only when the Ready/Lock/Start/Override register Lock bit is set. Any further attempts to write to

these registers shall have no effect. After power up the default values are used whenever the Ready/Lock/Start/Override register

Start bit is cleared even though modifications to these registers are possible.

TABLE 11. Absolute Limit vs Register Setting

Temperature Reading (Decimal) Reading (Hex)

−127˚C −127 81h

−50˚C −50 CEh

0˚C 0 00h

50˚C 50 32h

127˚C 127 7Fh

4.20 Registers 6D-6Eh: Zone Hysteresis Registers

Address

Read/

Write

Name

Bit 7

(MSB)

Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

(LSB)

Default

Value

Lock?

6Dh R/W Zone1 and Zone2 Hysteresis H1-3 H1-2 H1-1 H1-0 H2-3 H2-2 H2-1 H2-0 44h U

6Eh R/W Zone3 Hysteresis H3-3 H3-2 H3-1 H3-0 RES RES RES RES 40h U

If the temperature is above Fan Temp Limit, then drops below Fan Temp Limit, the following will occur:

– The fan will remain on, at Fan PWM Minimum, until the temperature goes a certain amount below Fan Temp Limit.

– The Hysteresis registers control this amount. See below table for details.

These registers become Read Only when the Ready/Lock/Start/Override register Lock bit is set. Any further attempts to write to

thses registers 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.

TABLE 12. Hysteresis Settings

Setting HYSTERESIS

0h 0˚C

LM85

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