Problem
We needed a test device for development of a distributed self-configuring RF network. Our chief programmer asked our hardware staff to develop a prototype device with the following features:
- 915 MHz transceiver
- Microprocessor with a crystal timebase
- FLASH memory or non-volatile RAM
- Silicon serial number
- Analog inputs
- Digital I/O
- Solar power supply
And could we get it all on a circuit board no larger than 3cm x 3cm (1.2" x 1.2")?
Solution
Our circuit used a low power processor and had two positions for serial memory devices in the standard eight-pin SSOP package. It had six analog inputs and sixteen digital I/O connections plus on-board temperature and power measurement circuits. A RF transceiver IC provided the 915 MHz communications.
The power supply used a one Farad capacitor for charge storage. It was powered from an external solar cell. A self bootstrapping voltage regulator provided 3.3 VDC from capacitor voltages as low as 700 mV.
Power management was very important. Solar energy is unpredictable so the unit had to conserve power as much as possible. The processor went into "sleep mode" and was awakened periodically by an on-chip timer to perform data logging tasks. Power to all other circuits was switched off while the processor was asleep. If the capacitor charge was depleted to the point the power supply failed the unit would reboot automatically when solar energy recharged the capacitor.
The RF transceiver was a power hog, especially while transmitting. The processor monitored capacitor charge level, and when sufficient power was available it listened to communications on the network. On schedule or on demand it transmitted data packets to the network when power was available.
Results
We did manage to cram it all on a 3cm x 3cm board. This was one of the densest circuit boards we have made, averaging 98 components per square inch. Note: the prototype board shown in the pictures also had a wire-wrap pad array in the middle and a liquid crystal display driver on the other end.
We also made a RF communications unit that operated off of a battery or a low voltage AC supply that connected to a PC. This unit automatically became a node in the network and allowed remote data collection and programming for devices in the distributed network.