Rainbow Electronics LM82 - Manual (Page 10)

1.0 Functional Description
(Continued)
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 A5 A4 1 A2 A1 A0
MSB LSB
and is selected as follows:
Address Select Pin State LM82 SMBus
Slave Address
ADD0 ADD1 A6:A0 binary
0 0 001 1000
0 TRI-LEVEL 001 1001
0 1 001 1010
TRI-LEVEL 0 010 1001
TRI-LEVEL TRI-LEVEL 010 1010
TRI-LEVEL 1 010 1011
1 0 100 1100
1 TRI-LEVEL 100 1101
1 1 100 1110
The LM82 latches the state of the address select pins during
the first read or write on the SMBus. Changing the state of
the address select pins after the first read or write to any de-
vice 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 Digital Output
Binary Hex
+125˚C 0111 1101 7Dh
+25˚C 0001 1001 19h
+1˚C 0000 0001 01h
0˚C 0000 0000 00h
−1˚C 1111 1111 FFh
−25˚C 1110 0111 E7h
−55˚C 1100 1001 C9h
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 re-
sistance of the pull up, based on LM82 specification for High
Level Output Current, to provide a 2.1V high level, is 30k
Ω
.
Care should be taken in a noisy system because a high im-
pedance 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.
LM82
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