Rainbow Electronics LM32 - Manual (Page 9)

Typical Performance Characteristics
Remote Diode Temperature Reading Sensitivity to Diode
Filter Capacitance
Thermal Diode Capacitor or PCB Leakage Current Effect
on Remote Diode Temperature Reading
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1.0 Functional Description
The LM32 measures 3 temperature zones. The LM32 uses a
∆V
be
temperature sensing method. A differential voltage,
representing temperature, is digitized using a Sigma-Delta
analog to digital converter. The digitized data can be re-
trieved over a simple single-wire interface called Sensor-
Path. SensorPath has been defined by National Semicon-
ductor and is optimized for hardware monitoring. National
offers a royalty-free license in connection with its intellectual
property rights in the SensorPath bus.
The LM32 has one address pin to allow up to two LM32s to
be connected to one SensorPath bus. The physical interface
of SensorPath’s SWD signal is identical to the familiar indus-
try standard SMBus SMBDAT signal. The digital information
is encoded in the pulse width of the signal being transmitted.
Every bit can be synchronized by the master simplifying the
implementation of the master when using a micro-controller.
For micro-controller’s with greater functionality an asynchro-
nous attention signal can be transmitted by the LM32 to
interrupt the micro-controller and notify it that temperature
data has been updated in the readout registers.
To optimize the LM32’s power consumption to the system
requirements, the LM32 has a shutdown mode and supports
multiple conversion rates.
1.1 SensorPath BUS SWD
SWD is the Single Wire Data line used for communication.
SensorPath uses 3.3V single-ended signaling, with a pull-up
resistor and open-drain low-side drive (see Figure 3). For
timing purposes SensorPath is designed for capacitive loads
(C
L
) of up to 400pF. Note that in many cases a 3.3V standby
rail of the PC will be used as a power supply for both the
sensor and the master. Logic high and low voltage levels for
SWD are TTL compatible. The master may provide an inter-
nal pull-up resistor. In this case the external resistor is not
needed. The minimum value of the pull-up resistor must take
into account the maximum allowable output load current of
4mA.
1.2 SensorPath BIT SIGNALING
Signals are transmitted over SensorPath using pulse-width
encoding. There are five types of "bit signals":
• Data Bit 0
• Data Bit 1
• Start Bit
• Attention Request
• Reset
All the "bit signals" involve driving the bus to a low level. The
duration of the low level differentiates between the different
"bit-signals". Each "bit signal" has a fixed pulse width. Sen-
sorPath supports a Bus Reset Operation and Clock Training
sequence that allows the slave device to synchronize its
internal clock rate to the master. Since the LM32 meets the
± 15% timing requirements of SensorPath, the LM32 does
not require the Clock Training sequence and does not sup-
port this feature. This section defines the "bit signal" behav-
ior in all the modes. Please refer to the timing diagrams in
the Electrical Characteristics section (Figure 1 and Figure 2)
while going through this section. Note that the timing dia-
grams for the different types of "bit signals" are shown
together to better highlight the timing relationships between
them. However, the different types of "bit signals" appear on
SWD at different points in time. These timing diagrams show
the signals as driven by the master and the LM32 slave as
well as the signal as seen when probing SWD. Signal labels
that begin with the label Mout_ depict a drive by the master.
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FIGURE 3. SensorPath SWD simplified schematic
LM32
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