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6.3.6 Open-Drain Outputs, Inputs, and Pull-Up Resistors

The SMBDAT, ALERT, T_Crit, GPIO and PWM open-drain outputs and the GPD, TACH, and A0 inputs are

pulled-up by pull-up resistors to VDDas suggested in the table below.

Pin Name

Pin Number

Suggested Pull-up Resistor

Range

Typical

SMBCLK

1 kΩ to 2 kΩ

1.5 kΩ

SMBDAT

1 kΩ to 2 kΩ

1.5 kΩ

ALERT

1 kΩ to 5 kΩ

3 kΩ

T_Crit

1 kΩ to 5 kΩ

3 kΩ

5 kΩ to 20 kΩ

10 kΩ

GPIOx

1-3;18,19

5 kΩ to 20 kΩ

10 kΩ

GPDx

20-24

5 kΩ to 20 kΩ

10 kΩ

PWM

See (1)

TACH

1 kΩ to 5 kΩ

3 kΩ

(1)

Depends on the fan drive circuitry connected to this pin. In the absence of fan control circuitry use a

1 kΩ pull-up resistor to VDD.

6.3.7 Diode Fault Detection

The LM64 can detect fault conditions caused by the remote diode. If the D+ pin is detected to be shorted to

VDD, or open: (1) the Remote Temperature High Byte (RTHB) register is loaded with 127°C, (2) the Remote

Temperature Low Byte (RTLB) register is loaded with 0, and (3) the OPEN bit (D2) in the status register is set.

Therefore, if the Remote T_CRIT setpoint register (RCS): (1) is set to a value less than +127°C and (2) the

ALERT Mask is disabled, then the ALERT output pin will be pulled low. If the Remote High Setpoint High Byte

(RHSHB) is set to a value less than +127°C and (2) the ALERT Mask is disabled, then the ALERT and T_Crit

outputs will be pulled low. The OPEN bit by itself will not trigger an ALERT.

If the D+ pin is shorted to either ground or D−, then the Remote Temperature High Byte (RTHB) register is

loaded with −128°C (1000 0000) and the OPEN bit in the ALERT Status Register will not be set. A temperature

reading of −128°C indicates that D+ is shorted to either ground or D-. If the value in the Remote Low Setpoint

High Byte (RLSHB) Register is more than −128°C and the ALERT Mask is Disabled, ALERT will be pulled low.

6.3.8 Communicating with the LM64

Each data register in the LM64 falls into one of four types of user accessibility:

1. Read Only

2. Write Only

3. Read/Write same address

4. Read/Write different address

A Write to the LM64 is comprised of an address byte and a command byte. A write to any register requires one

data byte.

Reading the LM64 Registers can take place after the requisite register setup sequence takes place. See Section

7.1.3.1.

The data byte has the Most Significant Bit (MSB) first. At the end of a read, the LM64 can accept either

Acknowledge or No-Acknowledge from the Master. Note that the No-Acknowledge is typically used as a signal

for the slave indicating that the Master has read its last byte.

6.3.9 Digital Filter

The LM64 incorporates a user-configured digital filter to suppress erroneous Remote Temperature readings due

to noise. The filter is accessed in the Remote Diode Temperature Filter and Comparator Mode Register. The

filter can be set according to the following table.

LM64

SNAS207B – MAY 2004 – REVISED JANUARY 2024

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