Two Wires Carry Power And Data: Backstory

Oct. 1, 2009
At times, designers face wire and/or cost limitations in linking MCUs with a remote device such as a sensor. This classic Idea for Design from October, 1996 describes a UART-based two-wire interface that fills the bill nicely.

This application is as useful today as it was when I described it in the Oct. 1, 1996 issue (see "Two Wires Carry Power And Data"). The deployment of sensors has increased significantly, resulting in an increased demand for efficient interconnect techniques between sensors and a host system. The use of only two wires often is attractive from both a cost and reliability standpoint. Sensing applications are no different than others—power and size are being driven smaller while faster operation is desirable and sometimes necessary.

The original application utilized what was the latest analog IC technology at the time. The data decoder employed the LMC7211 comparator, which was one of the first comparators to include the positive voltage rail in its input range (called common-mode input range). The device is provided in a SOT-23 package, which was considered “extremely small” in its day.

Today, we find many “rail-to-rail” input comparators at faster speeds and in much smaller packages. A later comparator and good choice for this design is the National Semiconductor LMV7239 ultra-low-power rail-to-rail input comparator. This device has much faster propagation delay (45 ns versus 4 µs) for just a little more supply current (65 µA typical at 5 V) and is available in a smaller SC70 package.

The improved propagation delay will allow for faster potential data flow and better symmetry in the recovered data stream. The LMV7239 can also be employed on the host side for the same performance improvements when bidirectional communication is needed. As the speed of the data transfer is increased, the distortion caused by the sensor side decoupling capacitors must be considered. As discussed in the original text, the decoupling capacitors should be limited to obtain higher data rates.

Microcontrollers have also made significant advances. Many ultra-low-power devices are suitable for this application. One may find it necessary to turn off unnecessary peripherals and limit software execution during data transfer and then limit data transfer during any analogto- digital conversion. This limits unwanted noise and the potential for data and/or analog measurement errors.

The optional low-dropout linear regulators (LDO) discussed are used to drive the sensor circuitry and help filter inherent data noise. The devices originally discussed are now considered old technology, and much better devices exist today. A good example for this application is the LP3990 150-mA regulator, which is available in an extremely small uSMD package (1.0 by 1.3 mm) or the larger SOT-23 package.

This device is available in many fixed output voltages and maintains its output voltage when the input voltage is only 200 mV higher (worst case over all conditions). Compare this to the LP2951, which only delivers 100 mA and the input must be 600 mV above the output. The LP3990 provides the ability to run the slave circuit more efficiently (lower loss) and at higher voltages than older devices. For higher-current slave circuits, try the 500-mA LP38691 regulator, available in a 3- by 3-mm leadless leadframe package (LLP) as well as other packages.

For higher resolution and accuracy, a remote sensor circuit may need to be biased at higher voltages than available with this circuit. This same concept can easily be applied to higher voltages sent across the wire, resulting in higher bias voltages available for a remote slave.

By carefully studying your design requirements and properly selecting component values, this two-wire system can be an ideal solution for many applications.

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