LM64EVAL National Semiconductor, LM64EVAL Datasheet - Page 10

• Manufacturer Part Number: LM64EVAL
• Description: BOARD EVALUATION LM64
• Manufacturer: National Semiconductor

Specifications of LM64EVAL

• Sensor Type: Temperature, Fan Controller
• Sensing Range: 0°C ~ 85°C
• Interface: SMBus (2-Wire/I²C)
• Sensitivity: ±1°C
• Voltage - Supply: 3 V ~ 3.6 V
• Embedded: Yes, MCU, 8-Bit
• Utilized Ic / Part: LM64
• Lead Free Status / RoHS Status: Not applicable / Not applicable

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1.0 Functional Description
1.2.2 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 rou-
tine 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 de-
scribes 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 deter-
3. LM64 clears ALERT Status Register, resets the ALERT
4. Master attends to conditions that caused the ALERT to
5. Master resets the ALERT Mask bit in the Configuration
FIGURE 4. ALERT Output as Temperature Comparator
mine what caused the ALERT.
HIGH and sets the ALERT Mask bit in the Configuration
Register.
be triggered. The fan is started, setpoint limits are ad-
justed, etc.
Register.
Response Diagram
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(Continued)
10
1.2.3 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 LM64’s ALERT should be operated us-
ing 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 inter-
rupt.
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 re-
ceived 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 rou-
tine 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.
1. Master senses SMBus alert line low
FIGURE 5. ALERT Output as an Interrupt Temperature
Response Diagram
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