Remote High Limit

RDTS Measurement

TIME

LM89 ALERT pin

Status Register: RTDS High

End of Temperature

conversion

ALERT mask set in

response to reading of

status register by

master

Remote High Limit

RDTS Measurement

LM89 ALERT Pin

Status Register: RTDS High

TIME

LM89

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SNIS128D – AUGUST 2002 – REVISED JUNE 2014

Feature Description (continued)

Figure 5. ALERT Comparator Temperature Response Diagram

9.3.2.2

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 REGISTER

bits are cleared only upon a read command from the master (see Figure 6) 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 (BFh)

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 determine what caused the ALERT

3. LM89 clears STATUS REGISTER, resets the ALERT HIGH and sets the ALERT mask bit (D7 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 (D7 in the Configuration register).

Figure 6. ALERT Output As An Interrupt Temperature Response Diagram

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