Remote High Limit

RDTS Measurement

LM64 ALERT Pin

Status Register: RTDS High

TIME

LM64

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SNAS207A – MAY 2004 – REVISED MARCH 2013

THE ALERT OUTPUT

When the ALERT Mask bit in the Configuration register is written as zero the ALERT interrupts are enabled.

The LM64's ALERT pin is versatile and can produce three different methods of use to best serve the system

designer: (1) as a temperature comparator (2) as a temperature-based interrupt flag, and (3) as part of an

SMBus ALERT System. The three methods of use are further described below. The ALERT and interrupt

methods are different only in how the user interacts with the LM64.

The remote temperature (RT) reading is associated with a T_CRIT Setpoint Register, and both local and remote

temperature (LT and RT) readings are associated with a HIGH setpoint register (LHS and RHS). The RT is also

associated with a LOW setpoint register (RLS). At the end of every temperature reading a digital comparison

determines whether that reading is above its HIGH or T_CRIT setpoint or below its LOW setpoint. If so, the

corresponding bit in the ALERT Status Register is set. If the ALERT mask bit is low, any bit set in the ALERT

Status Register, with the exception of Busy or Open, will cause the ALERT output to be pulled low. Any

temperature conversion that is out of the limits defined in the temperature setpoint registers will trigger an

ALERT. Additionally, the ALERT Mask Bit must be cleared to trigger an ALERT in all modes.

The three different ALERT modes will be discussed in the following sections.

ALERT Output as a Temperature Comparator

When the LM64 is used in a system in which does not require temperature-based interrupts, the ALERT output

could be used as a temperature comparator. In this mode, once the condition that triggered the ALERT to go low

is no longer present, the ALERT is negated (Figure 5). For example, if the ALERT output was activated by the

comparison of LT > LHS, when this condition is no longer true, the ALERT will return HIGH. This mode allows

operation without software intervention, once all registers are configured during set-up. In order for the ALERT to

be used as a temperature comparator, the Comparator Mode bit in the Remote Diode Temperature Filter and

Comparator Mode Register must be asserted. This is not the power-on default state.

Figure 5. ALERT Output as Temperature Comparator Response Diagram

ALERT Output as an Interrupt

The LM64'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 LM64 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 5) 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 LM64 ALERT Status Register to determine what caused the ALERT.

3. LM64 clears ALERT Status Register, resets the ALERT HIGH and sets the ALERT Mask bit in the

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