Remote High Limit

RDTS Measurement

TIME

ALERT mask set in

response to reading of

status register by master

LM64

ALERT pin

Status Register: RTDS High

End of Temperature

conversion

LM64

SNAS207A – MAY 2004 – REVISED MARCH 2013

www.ti.com

Configuration Register.

4. Master attends to conditions that caused the ALERT to be triggered. The fan is started, setpoint limits are

adjusted, etc.

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

Figure 6. ALERT Output as an Interrupt Temperature Response Diagram

ALERT Output as an SMBus ALERT

An SMBus alert line is created when the ALERT output is connected to: (1) one or more ALERT outputs of other

SMBus compatible devices, and (2) to a master. Under this implementation, the LM64's ALERT should be

operated using the ARA (Alert Response Address) protocol. The SMBus 2.0 ARA protocol, defined in the SMBus

specification 2.0, is a procedure designed to assist the master in determining which part generated an interrupt

and to service that interrupt.

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.

12

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