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LM89-1CIMM Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM89-1CIMM Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 20 page 1.0 Functional Description (Continued) 1.2.2 ALERT Output as an Interrupt The LM89’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 undesirable for the interrupt flag to repeatedly trigger during or before the interrupt service routine has been completed. Under this method of operation, during a read of the STATUS REGISTER the LM89 will set the ALERT mask bit (D7 of the Configuration register) if any bit in the STATUS REGISTER is set, with the exception of Busy (D7) and OPEN (D2). This prevents further ALERT triggering until the master has reset the ALERT mask bit, at the end of the interrupt service routine. The STATUS REG- ISTER bits are cleared only upon a read command from the master (see Figure 4) 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, bit D0 (the ALERT configure bit) in the FILTER and ALERT CONFIGURE REGISTER (xBF) 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 a interrupt flag: 1. Master Senses ALERT low 2. Master reads the LM89 STATUS REGISTER to deter- mine what caused the ALERT 3. LM89 clears STATUS REGISTER, resets the ALERT HIGH and sets the ALERT mask bit (D7 in the Configu- ration register). 4. Master attends to conditions that caused the ALERT to be triggered. The fan is started, setpoint limits are ad- justed, etc. 5. Master resets the ALERT mask (D7 in the Configuration register). 1.2.3 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 LM89’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 de- signed 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. Com- petitive parts may address this “disengaging of SMBALERT” requirement differently than the LM89 or not at all. SMBus systems that implement the ARA protocol as suggested for the LM89 will be fully compatible with all competitive parts. The LM89 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 proto- col. 20041531 FIGURE 3. ALERT Comparator Temperature Response Diagram 20041528 FIGURE 4. ALERT Output as an Interrupt Temperature Response Diagram www.national.com 8 |
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