Datasheet

Remote High Limit
RDTS Measurement
Time
Temperature
ALERT mask set in
response to reading of
status register by master
LM63
ALERT pin
Status Register: RTDS High
End of Temperature
conversion
LM63
www.ti.com 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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