Issue 131 June 2001
Wireless Data Link

Start • Hardware Architecture • Transceiver Chip • Transceiver Chip Operation • Hardware Construction • Antenna Considerations • Radio Software • Software Design Considerations • Software Implementation • Software Enhancements •Compliance Issues • Your Turn • Sources & PDF

Hardware Architecture

Scout’s hardware and software were designed to be modular. The wireless data link is physically partitioned onto two separate boards, one containing a PIC processor for radio control, message formatting, and data encoding (see Figure 1). The other board contains the RF hardware, consisting of the RFM TR3000 chip and supporting discrete components (see Figure 2). By separating the two boards, we were able to keep the digital noise and trash away from the radio. Although unnecessary, we could have wrapped a little metal shield around the radio.

The PIC processor hardware design is minimal. The PIC16F877 provides 8 KB of flash programmable program memory, and 368 bytes of RAM in a surface-mount package. Obviously, a socketed part was out of the question because of space constraints. The radio controller PIC’s hardware UART lines are connected directly to the corresponding pins on the Scout’s main PIC processor for a tightly linked asynchronous serial interface.

On the radio side, we used discrete PIC lines to connect to the RFM chip’s TxD, RxD, and two control lines. The radio controller PIC uses an 8-MHz crystal, providing low-power (approximately 2- to 4-mA) operation. The choice of crystal frequency was a trade-off between power consumption and data bandwidth. At 8 MHz, the PIC is only able to keep up with a 2400-bps encoding rate using our data encoding scheme.

The radio board hardware is a copy of the RFM development kit design, repackaged for our application. We required a lower data rate than the development kit supported. So, we had to recalculate component values. We also stripped out many of the development kit components that were unnecessary in our application. These unnecessary components were in the manufacturer’s design to demonstrate and support all of the capabilities of the chip, however, we wanted kept the design as simple as possible. There was limited time to pull off this effort, so it was best not to stray far from a proven design.

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