Remote High Limit

RDTS Measurement

TIME

LM99 ALERT pin

Status Register: RTDS High

End of Temperature

conversion

ALERT mask set in

response to reading of

status register by

master

LM99

SNIS129D – JANUARY 2003 – REVISED MARCH 2013

www.ti.com

Figure 6. ALERT Output as an Interrupt Temperature Response Diagram

ALERT Output as an SMBus ALERT

When the ALERT output is connected to one or more ALERT outputs of other SMBus compatible devices and to

a master, an SMBus alert line is created. Under this implementation, the LM99'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 resolving which part generated an interrupt and

service that interrupt while impeding system operation as little as possible.

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

together. The ARA is a method by which with one command the SMBus master may identify which part is pulling

the SMBus alert line LOW and prevent it from pulling it 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, first, send

their address to the master and second, release the SMBus alert line after recognizing a successful transmission

of their address.

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

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

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

“disengaging of SMBALERT” requirement prevents locking up the SMBus alert line. Competitive parts may

address this “disengaging of SMBALERT” requirement differently than the LM99 or not at all. SMBus systems

that implement the ARA protocol as suggested for the LM99 will be fully compatible with all competitive parts.

The LM99 fulfills “disengaging of SMBALERT” by setting the ALERT mask bit (bit D7 in the Configuration

register, at address 09h) after successfully 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 STATUS REGISTER, at address 02h, 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 LM99 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 STATUS REGISTER is read and

fan started, setpoint limits adjusted, etc.

8. Master resets the ALERT mask (D7 in the Configuration register).

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

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