LM64 datasheet(14/42 Pages) TI2

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The SMBus alert line is connected to the open-drain ports of all devices on the bus, thereby AND'ing them

together. The ARA method allows the SMBus master, with one command, to identify which part is pulling the

SMBus alert line LOW. It also prevents the part from pulling the line LOW again for the same triggering condition.

When an ARA command is received by all devices on the bus, the devices pulling the SMBus alert line LOW: (1)

send their address to the master and (2) release the SMBus alert line after acknowledgement of their address.

The SMBus Specifications 1.1 and 2.0 state that in response to and ARA (Alert Response Address) “after

acknowledging the slave address the device must disengage its ALERT pulldown”. Furthermore, “if the host

still sees ALERT low when the message transfer is complete, it knows to read the ARA again.” This SMBus

“disengaging ALERT requirement prevents locking up the SMBus alert line. Competitive parts may address the

“disengaging of ALERT” differently than the LM64 or not at all. SMBus systems that implement the ARA protocol

as suggested for the LM64 will be fully compatible with all competitive parts.

The LM64 fulfills “disengaging of ALERT” by setting the ALERT Mask Bit in the Configuration Register after

sending out its address in response to an ARA and releasing the ALERT output pin. Once the ALERT Mask

bit is activated, the ALERT output pin will be disabled until enabled by software. In order to enable the ALERT

the master must read the ALERT Status Register, during the interrupt service routine and then reset the ALERT

Mask bit in the Configuration Register to 0 at the end of the interrupt service routine.

The following sequence describes the ARA response protocol.

1. Master senses SMBus alert line low

2. Master sends a START followed by the Alert Response Address (ARA) with a Read Command.

3. Alerting Device(s) send ACK.

4. Alerting Device(s) send their address. While transmitting their address, alerting devices sense whether their

address has been transmitted correctly. (The LM64 will reset its ALERT output and set the ALERT Mask bit

once its complete address has been transmitted successfully.)

5. Master/slave NoACK

6. Master sends STOP

7. Master attends to conditions that caused the ALERT to be triggered. The ALERT Status Register is read and

fan started, setpoints adjusted, etc.

8. Master resets the ALERT Mask bit in the Configuration Register.

The ARA, 000 1100, is a general call address. No device should ever be assigned to this address.

The ALERT Configuration bit in the Remote Diode Temperature Filter and Comparator Mode Register must be

set low in order for the LM64 to respond to the ARA command.

The ALERT output can be disabled by setting the ALERT Mask bit in the Configuration Register. The power-on

default is to have the ALERT Mask bit and the ALERT Configuration bit low.

Remote High Limit

RDTS Measurement

TIME

ALERT mask set in

response to ARA

from master

LM64

ALERT Pin

Status Register: Remote High

Figure 6-4. ALERT Output as an SMBus ALERT Temperature Response Diagram

LM64

SNAS207B – MAY 2004 – REVISED JANUARY 2024

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