TABLE 12. Hysteresis Settings (Continued)

Setting

HYSTERESIS

.

.

.

.

.

.

5h

5˚C

.

.

.

.

.

.

Fh

15˚C

4.21 Register 6Fh: Test Register

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

6Fh

R/W

Test Register

RES

RES

RES

RES

RES

RES

RES

XEN

00h

If the XEN bit is set high, the part will be placed into XOR tree test mode. Clearing the bit (writinga0tothe XEN bit) brings the

part out of XOR tree test mode.

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

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.

4.22 Registers 70-7Fh: Vendor Specific Registers

These registers are for vendor specific features, including test registers. They will not default to a specific value on power up.

4.22.1 Register 74h: Tachometer Monitor Mode

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?

74h

R/W

Tach Monitor Mode

RES

RES

T3/4-1

T3/4-0

T2-1

T2-0

T1-1

T1-0

00h

Each fan TACH input has 4 possible modes of operation. The modes for TACH3 and TACH4 share control bits T3/4-[1:0]; TACH2

is controlled by T2-[1:0]; TACH1 is controlled by T1-[1:0]. The result reported in all modes is based on 2 pulses per revolution. In

order for modes 2 and 3 to function properly it is required that the:

PWM1 output must control the fan that has it’s tachometer output connected to the TACH1 LM85 input.

PWM2 output must control the fan that has it’s tachometer output connected to the TACH2 LM85 input.

PWM3 output must control the fans that have their tachometer outputs connected to the TACH3 or TACH4 LM85 inputs.

Setting (Tn[1:0]) Mode Function

00

0

Traditional tach input monitor, false readings when under minimum detctable RPM

01

1

Traditional tach input monitor, FFFFh reading when under minimum detectable RPM

10

2

Most accurate readings, FFFFh reading when under minimum detectable RPM

11

3

Least effect on programmed PWM of Fan, FFFFh reading when under minimum detectable RPM

•

Mode 0: This mode uses the conventional method for fan tachometer pulse detection and does not include any circuitry to

compensate for PWM Fan drive. This mode should be used when PWM drive is not used to power the fan. This mode may

report a false RPM reading when under minimum detectable RPM as shown in the follwing table.

•

Mode 1: This mode uses the convertional method for fan tach detection. The reading will be FFFFh if it is below minimum

detectable RPM.

•

Mode 2: This mode is optimized for accurate RPM readings and activates circuitry that extends the lower side of the RPM

reading as shown in the following table.

•

Mode 3: This mode minimizes the effect on the RPM setting and activates circuitry that extends the lower side of the RPM

reading as shown in the following table.

PWM Frequency

Mode 0 and 1 Minimum RPM

Mode 2 and 3 Minimum RPM

10.01

841

210

15.02

1262

315

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