LM82
1.0 Functional Description
1.4 POWER ON RESET DEFAULT STATES
(Continued)
the address select pins after the first read or write to any
device on the SMBus will not change the slave address of
the LM82.
1.6 TEMPERATURE DATA FORMAT
Temperature data can be read from the Local and Remote
Temperature, T_CRIT, and HIGH setpoint registers; and writ-
ten to the T_CRIT and HIGH setpoint registers. Temperature
data is represented by an 8-bit, two’s complement byte with
an LSB (Least Significant Bit) equal to 1˚C:
Temperature
+125˚C
Digital Output
Binary
0111 1101
0001 1001
0000 0001
0000 0000
1111 1111
1110 0111
1100 1001
+25˚C
+1˚C
0˚C
−1˚C
−25˚C
−55˚C
Hex
7Dh
19h
01h
00h
FFh
E7h
C9h
LM82 always powers up to these known default states:
1. Command Register set to 00h
Local Temperature set to 0˚C
Remote Temperature set to 0˚C until the LM82 senses a
diode present between the D+ and D− input pins.
4. Status Register set to 00h.
5.
Configuration Register set to 00h; INT enabled and all
T_CRIT setpoints enabled to activate T_CRIT_A.
6. Local and Remote T_CRIT set to 127˚C
1.5 SMBus INTERFACE
The LM82 operates as a slave on the SMBus, so the
SMBCLK line is an input (no clock is generated by the LM82)
and the SMBData line is bi-directional. According to SMBus
specifications, the LM82 has a 7-bit slave address. Bit 4 (A3)
of the slave address is hard wired inside the LM82 to a 1.
The remainder of the address bits are controlled by the state
of the address select pins ADD1 and ADD0, and are set by
connecting these pins to ground for a low, (0) , to V
CC
for a
high, (1), or left floating (TRI-LEVEL).
Therefore, the complete slave address is:
A6
MSB
and is selected as follows:
Address Select Pin State
ADD0
0
0
0
TRI-LEVEL
TRI-LEVEL
TRI-LEVEL
1
1
1
ADD1
0
TRI-LEVEL
1
0
TRI-LEVEL
1
0
TRI-LEVEL
1
LM82 SMBus Slave
Address
A6:A0 binary
001 1000
001 1001
001 1010
010 1001
010 1010
010 1011
100 1100
100 1101
100 1110
A5
A4
1
A2
A1
A0
LSB
2.
3.
1.7 OPEN-DRAIN OUTPUTS
The SMBData, INT and T_CRIT_A outputs are open-drain
outputs and do not have internal pull-ups. A “high” level will
not be observed on these pins until pull-up current is pro-
vided from some external source, typically a pull-up resistor.
Choice of resistor value depends on many system factors
but, in general, the pull-up resistor should be as large as
possible. This will minimize any internal temperature reading
errors due to internal heating of the LM82. The maximum
resistance of the pull up, based on LM82 specification for
High Level Output Current, to provide a 2.1V high level, is
30kΩ. Care should be taken in a noisy system because a
high impedance pull-up will be more likely to couple noise
into the signal line.
1.8 DIODE FAULT DETECTION
Before each external conversion the LM82 goes through an
external diode fault detection sequence. If D+ input is
shorted to V
CC
or floating then the temperature reading will
be +127 ˚C, and the OPEN bit in the Status Register will be
set. If the T_CRIT setpoint is set to less than +127 ˚C then
the D+ input RTCRIT bit in the Status Register will be set
which will activate the T_CRIT_A output, if enabled. If a D+
is shorted to GND or D−, its temperature reading will be 0 ˚C
and its OPEN bit in the Status Register will not be set.
The LM82 latches the state of the address select pins during
the first read or write on the SMBus. Changing the state of
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