Rainbow Electronics LM84 - Manual (Page 10)

1.0 Functional Description
(Continued)
1.3 SMBus INTERFACE
The LM84 operates as a slave on the SMBus, so the
SMBCLK line is an input (no clock is generated by the LM84)
and the SMBData line is bi-directional. According to SMBus
specifications, the LM84 has a 7-bit slave address. Bit 4 (A3)
of the slave address is hard wired inside the LM84 to a 1.
The remainder of the address bits are controlled by the
address select pins ADD1 and ADD0, and are set by con-
necting 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 A3 A2 A1 A0
MSB LSB
and is selected as follows:
Address Select Pin State LM84 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 LM84 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
device on the SMBus will not change the slave address of
the LM84.
1.4 TEMPERATURE DATA FORMAT
Temperature data can be read from the Local Temperature,
Remote Temperature, and T_CRIT setpoint registers. Tem-
perature data can only be written to the T_CRIT 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
1.5 OPEN-DRAIN OUTPUTS
SMBData and T_CRIT_A outputs are open-drain and do not
have internal pull-ups. A “high” level will not be observed on
these pins until pull-up current is provided from some exter-
nal 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 local temperature reading errors due to self
heating of the LM84. The maximum resistance of the pull-up,
based on LM84 specification for High Level Output Current,
to provide a 2V high level, is 30 k
Ω
.
1.6 DIODE FAULT DETECTION
Before each remote conversion the LM84 goes through an
external diode fault detection sequence. If the D+ input is
shorted to V
CC
or floating then the temperature reading will
be +127˚C, bit 2 (OPEN) of the Status Register will be set. If
the Remote T_CRIT setpoint is set to less than +127˚C then
bit 4 (RTCRIT) of the Status Register will be set which will
activate the T_CRIT_A output, if enabled. If D+ is shorted to
GND or D−, the temperature reading will be 0˚C and bit 2 of
the Status Register will not be set.
LM84
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