Rainbow Electronics LM99 - Manual (Page 10)

1.0 Functional Description
(Continued)
Remote temperature data is represented by an 11-bit, two’s
complement word with an LSB (Least Significant Bit) equal
to 0.125˚C. The data format is a left justified 16-bit word
available in two 8-bit registers:
Actual vs. LM99 Remote Temperature Conversion
Actual Remote Diode
Temperature,˚C
LM99 Remote Diode
Temperature Register, ˚C
Binary Results in LM99
Remote Temperature Register
Hex Remote
Temperature
Register
120 +104 0110 1000 0000 0000 6800h
125 +109 0110 1101 0000 0000 6D00h
126 +110 0110 1110 0000 0000 7100h
130 +114 0111 0010 0010 0000 7200h
135 +119 0111 0111 0000 0000 7700h
140 +124 0111 1100 0000 0000 7200h
Output is 11-bit two’s complement word. LSB = 0.125 ˚C.
Actual vs. Remote T_Crit Setpoint
Actual Remote Diode
T_Crit Setpoint,˚C
Factory-Programmed
Remote T_CRIT High
Setpoint, ˚C
Binary Remote T_CRIT High
Setpoint Value
Hex Remote T_CRIT
High Setpoint Value
126 +110 0110 1110 71h
Local Temperature data is represented by an 8-bit, two’s
complement byte with an LSB (Least Significant Bit)
equal to 1˚C:
Local
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, ALERT 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
provided by some external source, typically a pull-up resis-
tor. Choice of resistor value depends on many system fac-
tors 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 LM99. The maximum
resistance of the pull-up to provide a 2.1V high level, based
on LM99 specification for High Level Output Current with the
supply voltage at 3.0V, is 82 k
Ω (5%) or 88.7 kΩ (1%).
1.8 DIODE FAULT DETECTION
The LM99 is equipped with operational circuitry designed to
detect fault conditions concerning the remote diode. In the
event that the D+ pin is detected as shorted to V
DD
or
floating, the Remote Temperature High Byte (RTHB) register
is loaded with +127˚C, the Remote Temperature Low Byte
(RTLB) register is loaded with 0, and the OPEN bit (D2) in
the status register is set. As a result, if the Remote T_CRIT
setpoint register (RCS) is set to a value less than +127˚C the
ALERT and T_Crit output pins will be pulled low, if the Alert
Mask and T_Crit Mask are disabled. If the Remote HIGH
Setpoint High Byte Register (RHSHB) is set to a value less
than +127˚C then ALERT will be pulled low, if the Alert Mask
is disabled. The OPEN bit itself will not trigger and ALERT.
In the event that the D+ pin is shorted to ground or D−, the
Remote Temperature High Byte (RTHB) register is loaded
with −128˚C (1000 0000) and the OPEN bit (D2) in the status
register will not be set. Since operating the LM99 at −128˚C
is beyond it’s operational limits, this temperature reading
represents this shorted fault condition. If the value in the
Remote Low Setpoint High Byte Register (RLSHB) is more
than −128˚C and the Alert Mask is disabled, ALERT will be
pulled low.
Remote diode temperature sensors that have been previ-
ously released and are competitive with the LM99 output a
code of 0˚C if the external diode is short-circuited. This
change is an improvement that allows a reading of 0˚C to be
truly interpreted as a genuine 0˚C reading and not a fault
condition.
LM99
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