Remote High Limit

RDTS Measurement

Time

ALERT mask set in

response to reading of

status register by master

LM63

ALERT pin

Status Register: RTDS High

End of Temperature

conversion

LM63

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SNAS190E – SEPTEMBER 2002 – REVISED MAY 2013

ALERT OUTPUT AS AN INTERRUPT

The LM63'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 routine has been completed. Under this method of operation, during the read of the ALERT

Status Register the LM63 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 8 ) 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 describes the response of a

system that uses the ALERT output pin as an interrupt flag:

1. Master senses ALERT low.

2. Master reads the LM63 ALERT Status Register to determine what caused the ALERT.

3. LM63 clears ALERT Status Register, resets the ALERT HIGH and sets the ALERT Mask bit in the

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 8. 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 LM63'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 LM63 or not at all. SMBus systems that implement the ARA protocol

as suggested for the LM63 will be fully compatible with all competitive parts.

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