SensorPath bus Description

The single wire SensorPath interface was developed by National Semiconductor in 2003 to implement thermal management on PC Mother Boards.
The SensorPath interface shows up as the SWD pin on National ICs. The term SWD standards for Single Wire Data. SensorPath uses 3.3V single-ended signaling, with a pull-up resistor and open-drain low-side drive. The SensorPath Bus ICs interface to the SuperI/O family of controllers. The pull-up resistor seems to be optional with the value selected depending on the bus capacitance. The Digital Temperature Sensor LM95010 for example requires a 1.25k pull-up to 3.3 volts. If the Master already contains an internal pull-up, an external resistor is not required.
The maximum allowable output load current is 4mA. Maximum capacitive load is 400pF.

SensorPath I/O

The Single Wire Data line uses Pulse Width Modulation [PWM] to send data over the line.
Five bit types are used; Data Bit 0, Data Bit 1, Start Bit, Attention Request, and Reset.

Companies producing SensorPath components:
National produces a number of devices with the SensorePath bus, a few are listed below.

National Semiconductor
{LM32, Dual Thermal Diode Temperature Sensor, Digital Temperature Sensor with SensorPath Bus}

TI Texas Instruments
{Digital Out Temperature Sensor with Single-Wire SensorPath Bus}

The partial schematic diagram shows the I/O ports as FET drivers and not TTL ports. However the I/O is TTL compatible.
The devices found to date appear to all be 3.3 volt devices, which of course is the new standard voltage rail [surpassing 5 volts].

Editor Note Not all electrical interfaces need to be determined when doing circuit design.
In some cases the use of an interface between devices is assumed based on the parts selected for other reasons and not based on the interface at all.
For example a part selected for it's performance might define the usage of the SWD because of its presence on the IC.
That is, a particular single wire interface might be used because it resides on an integrated circuit already being used for other reasons.
That's not saying anything for or against SWD, just that the primary function of the IC is what's normally being selected.
However, there may be those times when an IC may be produced in two versions, one with a particular interface and one with another interface.

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Modified 7/16/2015
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