| Developers' Corner : Real Applications from Real People |
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Unit Design
The Swim Display units are large numerical displays designed for use as a swim meet scoreboard. Each display is organized as two rows of eight seven-segment LED digits. (Typically organized as one digit for the lane number, one digit for the place, and six digits for the time.) A primary goal of this project was to be able to make a lower cost display in low volumes, thus the desire to use off-the-shelf components wherever possible. A primary component of the cost, as expected, was the single digit displays, but the power supply and enclosure were also significant cost contributors.
The initial plan was to have several different versions of the display: from a single line version, up to a four line version, thus reducing the per-line cost of the power supply and enclosure. A simple control board was designed, to control one line of eight digits, in a manner that allows these control boards to be daisy-chained together to control multiple lines. However finding anyone to make the enclosures at all, let alone at a low cost, and with low volumes, proved much more difficult than expected. This drove the decision to go with only a single two line enclosure, instead of a family. In line with the goals of low cost for low volumes, the control boards were designed with through-hole components, with a pitch of no less that 0.1” so that boards could be easily hand assembled. There were a large variety of suitable microcontrollers to choose from, but not so many in DIP packaging. Freescale’s MC9S08QG8CPBE was selected for the controller, which has worked out well. It needs no external crystal or oscillator, so that the only IC’s on the board are the microcontroller, an RS232 line driver chip, and a regulator. The only other board components required were connectors and capacitors.
The only downside with this controller is that it is not 5V tolerant. Although the display boards were designed for 5V signaling, they seem to do okay with 3.3V, provided the signals are clean. Freescale’s free IDE and configuration wizard works well, and allowed for rapid development in C, on a processor family I had not used before. Programming is done in-circuit, using an inexpensive little prototyping board. The only thing “missing” for this product line, is a low cost cable and software that could program the chip directly from the serial or USB port at a customer installation, as these displays aren’t cheap to ship!
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Bruce Cochrane has been developing software for embedded devices for over 20 years. He was exposed to competitive swimming about ten years ago when his daughter joined a local swim team. At that time there was only a single provider of swimming timing equipment, and the equipment was very expensive. About five years ago, a promotion by NEC sparked interest in building a lower cost display for these timing systems. At that time, he built a working prototype, using display panels with 5” high LCD digits, but they weren’t sufficiently visible. (Some pools are 50 m long.) More recently, another manufacturer’s promotion, this time by Freescale, prompted him to revist this idea. This time, larger LED panels, with excellent visibility, were used. These panels are controlled serially rather than in parallel, which also means that several panels can be controlled with just a few pins, eliminating the previous requirement of an FPGA or CPLD to manage all the segment controls.
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