LM40 datasheet(10/27 Pages) NSC

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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 LM40 hardware monitor measures up to 3 temperature

zones and 5 power supply voltages. The LM40 uses a

∆V

temperature sensing method. A differential voltage, repre-

senting temperature, is digitized using a Sigma-Delta analog

to digital converter. Internal scaling resistors allow direct

measurement of the +1.6V, +2.5V, +5V, +3.3V and +12V

power supply inputs. The digitized data can be retrieved over

a simple single-wire interface called SensorPath. Sensor-

Path has been defined by National Semiconductor and is

optimized for hardware monitoring. National offers a royalty-

free license in connection with its intellectual property rights

in the SensorPath bus.

The LM40 has one address pin to allow up to two LM40s 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 LM40 to

interrupt the micro-controller and notify it that temperature/

voltage data has been updated in the readout registers.

To optimize the LM40’s power consumption to the system

requirements, the LM40 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

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 LM40 meets the

±15% timing requirements of SensorPath, the LM40 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 LM40 slave as

well as the signal as seen when probing SWD. Signal labels

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FIGURE 3. SensorPath SWD simplified schematic

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