Quad Channel 1°C Temperature Sensor with Beta Compensation and 1.8V SMBus Communications
Datasheet
SMSC EMC1184 21 Revision 1.0 (07-11-13)
DATASHEET
5.5.2 ALERT / THERM2 Pin ComparatorTHERM Mode
When the ALERT / THERM2 pin is configured to operate in comparator mode, it will be asserted if any
of the measured temperatures exceeds the respective high limit, acting as a second
THERM function
in. The
ALERT / THERM2 pin will remain asserted until all temperatures drop below the corresponding
high limit minus the Therm Hysteresis value.
When the ALERT / THERM2 pin is asserted in comparator mode, the corresponding high limit status
bits will be set. Reading these bits will not clear them until the
ALERT / THERM2 pin is deasserted.
Once the ALERT pin is deasserted, the status bits will be automatically cleared.
The MASK_ALL bit will not block the ALERT / THERM2 pin in this mode; however, the individual
channel masks (see Section 6.11) will prevent the respective channel from asserting the ALERT/
THERM2 pin.
5.6 Temperature Measurement
The EMC1184 can monitor the temperature of up to three externally connected diodes.
The device contains programmable High, Low, and Therm limits for all measured temperature
channels. If the measured temperature goes below the Low limit or above the High limit, the ALERT
pin can be asserted (based on user settings). If the measured temperature meets or exceeds the
Therm Limit, the THERM pin is asserted unconditionally, providing two tiers of temperature detection.
5.6.1 Beta Compensation
The EMC1184 is configured to monitor the temperature of basic diodes (e.g., 2N3904) or CPU thermal
diodes. It automatically detects the type of external diode (CPU diode or diode connected transistor)
and determines the optimal setting to reduce temperature errors introduced by beta variation.
Compensating for this error is also known as implementing the transistor or BJT model for temperature
measurement.
For discrete transistors configured with the collector and base shorted together, the beta is generally
sufficiently high such that the percent change in beta variation is very small. For example, a 10%
variation in beta for two forced emitter currents with a transistor whose ideal beta is 50 would contribute
approximately 0.25°C error at 100°C. However for substrate transistors where the base-emitter junction
is used for temperature measurement and the collector is tied to the substrate, the proportional beta
variation will cause large error. For example, a 10% variation in beta for two forced emitter currents
with a transistor whose ideal beta is 0.5 would contribute approximately 8.25°C error at 100°C.
For the EMC1184 the External Diode 2 and External Diode 3 channels do not support Beta
Compensation.
5.6.2 Resistance Error Correction (REC)
Parasitic resistance in series with the external diodes will limit the accuracy obtainable from
temperature measurement devices. The voltage developed across this resistance by the switching
diode currents cause the temperature measurement to read higher than the true temperature.
Contributors to series resistance are PCB trace resistance, on die (i.e. on the processor) metal
resistance, bulk resistance in the base and emitter of the temperature transistor. Typically, the error
caused by series resistance is +0.7°C per ohm. The EMC1184 automatically corrects up to 100 ohms
of series resistance.
5.6.3 Programmable External Diode Ideality Factor
The EMC1184 is designed for external diodes with an ideality factor of 1.008. Not all external diodes,
processor or discrete, will have this exact value. This variation of the ideality factor introduces error in
the temperature measurement which must be corrected for. This correction is typically done using
programmable offset registers. Since an ideality factor mismatch introduces an error that is a function
