TABLE 1. Temperature vs Register Reading (Continued)

Temperature

Reading (Dec)

Reading (Hex)

(SENSOR ERROR)

80h

4.3 Register 28-2Fh: Fan Tachometer Reading

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

28h

29h

R

R

Tach1 LSB

Tach1 MSB

7

15

6

14

5

13

4

12

3

11

2

10

LEVEL1

9

LEVEL0

8

N/A

N/A

2Ah

2Bh

R

R

Tach2 LSB

Tach2 MSB

7

15

6

14

5

13

4

12

3

11

2

10

LEVEL1

9

LEVEL0

8

N/A

N/A

2Ch

2Dh

R

R

Tach3 LSB

Tach3 MSB

7

15

6

14

5

13

4

12

3

11

2

10

LEVEL1

9

LEVEL0

8

N/A

N/A

2Eh

2Fh

R

R

Tach4 LSB

Tach4 MSB

7

15

6

14

5

13

4

12

3

11

2

10

LEVEL1

9

LEVEL0

8

N/A

N/A

The Fan Tachometer Reading registers contain the number of 11.111 µs periods (90 kHz) between full fan revolutions. The results

are based on the time interval of two tachometer pulses, since most fans produce two tachometer pulses per full revolution. These

registers will be updated at least once every second.

The value, for each fan, is represented by a 16-bit unsigned number.

The Fan Tachometer Reading registers will always return an accurate fan tachometer measurement, even when a fan is disabled

or non-functional.

The least two significant bits (LEVEL1 and LEVEL2) of the least significant byte are used to indicate the accuracy level of the

tachometer reading. The accuracy ranges from most to least accurate. [LEVEL1:LEVEL2]=11indicates a most accurate value,

[LEVEL1:LEVEL2]=01 indicates the least accurate value and [LEVEL1:LEVEL2]=00 is reserved for future use.

FF FFh indicates that the fan is not spinning, or that the tachometer input is not connected to a valid signal. These registers are

read only — a write to these registers has no effect.

When the least significant byte (LSByte) of the LM85C 16-bit register is read, the other byte (MSByte) is latched at the current

value until it is read. This is required to ensure a valid reading. The LM85C will update the Fan Tachometer Reading registers at

the start of an LSByte read. Therefore, reading the MSByte register twice in a row will yield the same data.

When the LSByte of the LM85B 16-bit register is read, the other byte (MSByte) is latched at the current value until it is read. At

the end of the MSByte read the Fan Tachometer Reading registers are updated.

During spin-up, the PWM duty cycle reported is 0%.

4.4 Register 30-32h: Current PWM Duty

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

30h

R/W

Fan1 Current PWM Duty

7

6543210

N/A

31h

R/W

Fan2 Current PWM Duty

7

6543210

N/A

32h

R/W

Fan3 Current PWM Duty

7

6543210

N/A

The Current PWM Duty registers store the current duty cycle at each PWM output. At initial power-on, the PWM duty cycle is

100% and thus, when read, this register will return FFh. After the Ready/Lock/Start/Override register Start bit is set, this register

and the PWM signals will be updated based on the algorithm described in the Auto Fan Control Operating Mode section.

When read, the Current PWM Duty registers return the current PWM duty cycle. These registers are read only unless the fan is

in manual (test) mode, in which case a write to these registers will directly control the PWM duty cycle for each fan. The PWM

duty cycle is represented as shown in the following table.

Current Duty

Value (Decimal)

Value (Hex)

0%

0

00h

0.3922%

1

01h

.

.

.

.

.

.

.

.

.

25.098%

64

40h

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