Merging data and power onto a single wire is not a trivial task. Powering an extra device at the endpoint, while minimizing PCB real-estate and cost, complicate matters significantly. The circuit in Figure 1 outlines an easy method for achieving this task.
Figure 1: A comparator, reference and a handful of standard resistors are all that is required to transmit data and power using only a single wire.
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A comparator, reference, small-signal MOSFET and a handful of inexpensive resistors are all that are required to modulate and demodulate the power line. The microcontroller transmits data, as shown in Figure 2, by modulating the power line via MOSFET NPD.
Figure 2: Data is transmitted (CH1) by the master microcontroller and received on the other side of the wire by the slave microcontroller (CH2). Each low-power micro is powered by the 2.048 V low-noise reference (CH3). The transceiver combines/separates power+signal (CH4) onto and off of a single wire.
(Click on image to enlarge)
The modulations in the power line are decoded into received data by the LMP7300, a micropower precision comparator and precision reference with adjustable hysteresis. The integrated 2.048 V reference of the LMP7300 powers a low-voltage, low-power microcontroller with up to 1 mA of current. The circuit provides robust operation at data rates up to 5 kbps and can be used in either a point-to-point link, or on a multi-drop bus. The circuit is tuned to work with an injected voltage of 5 V. Changing the values of RPDv, RHT, RINT and RvINB allows the circuit to function with injected voltages other than 5 V.
Handshaking, timing, and protocol related issues must be handled inside the firmware of the microcontroller. A simple synchronous-master/slave arrangement ensures that no two devices transmit data at the same time. Rise and fall times on the Tx pin should exceed 250 ns to prevent unwanted noise injected into the power/signal line. If necessary, slow down the Tx rise/fall times with a series resistor on the gate of RPD. Minimize standby current by ensuring that the default transmitter output at power up is a logic-low.
The LMP7300 is available in a tiny 3x3mm MSOP-8 package, minimizing PCB real-estate requirements. When inactive, the circuit draws a miniscule 22 μA of current from the power supply. Cost of the design in volume is well under $1. The solution is well-suited for low-cost, space-constrained, battery-powered applications.
About the Author
Donald Schelle is a staff field applications engineer for National Semiconductor Corp., Ontario, Canada. He received a bachelor's of engineering and a diploma in electrical engineering from Lakehead University, Thunder Bay, Ontario, Canada, and a diploma in electrical engineering-computer control from Confederation College, Thunder Bay. You can reach him at Donald.Schelle@nsc.com
