LM64 datasheet(10/29 Pages) NSC

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1.0 Functional Description (Continued)

1.2.2 ALERT Output as an Interrupt

The LM64’s ALERT output can be implemented as a simple

interrupt signal when it is used to trigger an interrupt service

routine. In such systems it is desirable for the interrupt flag to

repeatedly trigger during or before the interrupt service rou-

tine has been completed. Under this method of operation,

during the read of the ALERT Status Register the LM64 will

set the ALERT Mask bit in the Configuration Register if any

bit in the ALERT Status Register is set, with the exception of

Busy and Open. This prevents further ALERT triggering until

the master has reset the ALERT Mask bit, at the end of the

interrupt service routine. The ALERT Status Register bits are

cleared only upon a read command from the master (see

Figure 5) and will be re-asserted at the end of the next

conversion if the triggering condition(s) persist(s). In order

for the ALERT to be used as a dedicated interrupt signal, the

Comparator Mode bit in the Remote Diode Temperature

Filter and Comparator Mode Register must be set low. This

is the power-on default state. The following sequence de-

scribes the response of a system that uses the ALERT

output pin as an interrupt flag:

Master senses ALERT low.

Master reads the LM64 ALERT Status Register to deter-

mine what caused the ALERT.

LM64 clears ALERT Status Register, resets the ALERT

HIGH and sets the ALERT Mask bit in the Configuration

Master attends to conditions that caused the ALERT to

be triggered. The fan is started, setpoint limits are ad-

justed, etc.

Master resets the ALERT Mask bit in the Configuration

1.2.3 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 us-

ing 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 inter-

rupt.

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 re-

ceived 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 rou-

tine and then reset the ALERT Mask bit in the Configuration

The following sequence describes the ARA response proto-

col.

Master senses SMBus alert line low

20065507

FIGURE 4. ALERT Output as Temperature Comparator

Response Diagram

20065508

FIGURE 5. ALERT Output as an Interrupt Temperature

Response Diagram

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