SNAS207B May 2004 – January 2024 LM64
PRODUCTION DATA
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.
The ARA, 000 1100, is a general call address. No device should ever be assigned to this address.
The ALERT Configuration bit in the Remote Diode Temperature Filter and Comparator Mode Register must be set low in order for the LM64 to respond to the ARA command.
The ALERT output can be disabled by setting the ALERT Mask bit in the Configuration Register. The power-on default is to have the ALERT Mask bit and the ALERT Configuration bit low.