Girsberger Elektronik AG’s avalanche rescue training transmitters make efficient use of board space and power, thanks to Xilinx CoolRunner-II CPLDs.

Every year in Switzerland, approximately 27 winter sports enthusiasts lose their lives in avalanches. If caught in an avalanche, your life depends upon the speed of your companions as rescuers to locate you and dig you out. After only 30 minutes, your chance of survival is just 40%. Speed is of the essence.

One rescue tool is an avalanche beacon, a small portable unit that transmits a beep about every second. Using one significantly increases the chances of being quickly and successfully rescued if buried by an avalanche. Your companions need only to switch their units to "receive" mode and locate your beeps.

As with any rescue and survival skill, the correct use of avalanche beacons requires education and practice. However, for use in classroom situations, the signals from regular avalanche beacons are too strong to allow for a meaningful demonstration of the basic search principles.

At Girsberger Elektronik AG, we developed a special reduced-strength transmitter for search training (Figure 1). It meets our key requirements: low cost, battery efficient, and compatible with the various timing signals (that is, the interval and duration of beeps) of all beacon brands on the market. We use a 73.768 KHz watch oscillator followed by a Xilinx CoolRunner™-II device to generate the timing signals (Figure 2). This provides an optimal solution, saving on both power consumption and PCB real estate.

I/O Port Expanders
The best practical training application for beacons is, of course, to bury the transmitters into mountain snow. However, changing the location of the transmitters to build new search scenarios cost valuable training time. By designing a control console (Figure 3) for as many as 16 remote transmitters activated via radio link, we enabled trainers to quickly and easily simulate new accident situations.

In order to handle 16 remote transmitters we required more I/O lines than what is available on a microprocessor. We chose to use two Xilinx CoolRunner-II devices, which provide a compact solution with low console battery consumption.

PCM Codec to RAM Interface
We also found CoolRunner-II CPLDs effective in reducing transmission loss. To initiate transmission over a private mobile radio, you must press a PTT (push to talk) button and wait before the transmitter is ready for you to start speaking. If you’re under stress, there’s a strong tendency to start talking before the transmitter is ready, and the beginning of your transmission will be lost. You may have to repeat the whole transmission in order for the receiver to make sense of it. This is inconvenient and, especially in emergency situations, can cause additional stress and frustration.

In some cases the PTT button is replaced by a voice activity detector, which has an inherent turn-on delay, meaning that the beginning of your transmission will always be lost.

To eliminate loss at the beginning of transmissions, we combined a PCM codec with a static RAM to implement a digital audio delay line, which holds up the voice signal (Figure 4).

The PCM data transfers in to and out of the codec over a high-speed 2.048 MHz serial bus (Figure 5). Encoded PCM samples are converted to parallel data and written into a ring buffer in the RAM. By reading parallel data from the RAM location with a fixed offset (modulo the size of the ring buffer) to the write address, the output is delayed. The parallel data is then reconverted and fed into the codec over a serial bus. One PCM sample reads/writes every 125 microseconds. The delay can be adjusted by modifying the read-to-write address offset.

These operations, in particular the emulation of the serial bus, cannot be performed by a standard microprocessor, which would be too slow. We chose a Xilinx CoolRunner-II device with 64 macrocells for the interface between the codec and the RAM. This solves our speed problem and also saves PCB real estate that would have otherwise been taken by standard logic components.

The Development Process
Excellent support from Impact Memec, the local Xilinx representative, kept the development start-up time extremely short. We used ISE WebPACK™ software for all of the designs, and found it especially helpful for the following reasons:

• Access to error messages on the Xilinx website provided instant in-depth information about the particular message, often with hints on how to solve the problem.
• The designs were simulated using ModelSim XE Starter, leading to some design modifications and allowing flaws to be fixed early in the development process. Some problems were due to signal spikes on clock lines, which were properly indicated by the simulator.

Conclusion
Xilinx CoolRunner-II devices are well suited for our applications. They provide a reliable technical solution combined with a short development time and a low start-up cost.

Based on this positive experience, Girsberger Elektronik AG is planning to incorporate Xilinx CoolRunner-II devices in such products as:

• The upgrade and replacement of an old avalanche beacon ASIC to eliminate the high NRE cost.
• A beacon to locate stolen cars. By taking over part of the training transmitter CPLD design, we will save significant development time.
• A data over voice interface for use on ISDN B-channels. The CPLD beats the speed of any microprocessor in manipulating the bitstream on an industry-standard IOM bus.

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