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TEST& MEASUREMENT GBDSO Convert a Nintendo games console into a portable oscilloscope By Steve Willis Gameboy Digital Sampling Oscilloscope (1) The application of dedicated games consoles to more varied functions other than just playing games is fraught with difficulties. Not only do the consoles contain highly integrated and custom-made components, there will often be no publicly available development tools or applications manuals. However the Nintendo Gameboy© is one such system which has received considerable attention from the hobby designer and enjoys a large selection of both technical and software support through a network of websites. 34 Elektor Electronics 10/2000 TEST& MEASUREMENT Key Features: Dual trace display Sampling Rate: DC to 1 Msps Time Base: 100 s to 5 µs/Div Inputs: AC/DC 1 MegOhm Input gain: 50 mV to 10 V/Div Line or chart recorder trace modes Real-time FFT mode with dB scale Variable persistence XY mode PC link for screen or data transfer 5 hrs operation from NiMH batteries Averaging and Auto trigger functions Reference trace storage FFT -- the basics The following details the specific implementation of the GBDSO FFT for those requiring a greater explanation of its operation. Initially, the input data is sampled using the standard scope capture programs. The data is then passed through a Hamming window to reduce artefacts caused by the finite sample points (rectangular window). A Hamming window was chosen as it gives a good compromise between main lobe and side lobe widths. The windowed data is recorded using a bit-reversal algorithm to suit the decimation in time (DIT) process of the FFT. The data is then processed by a discrete Fourier transform (DFT), which is implemented using a 256-point radix-2 fast Fourier transform (FFT). 16-bit arithmetic is used for the majority of the calculations, but since the GB has no multiply instruction this has to be calculated longhand by assembler software. The complex real and imaginary values of the DFT are combined to produce 128 x 32 bit values. Finally, a logarithm is taken (1 bit = 6 dB) and the data is displayed on the screen. The entire processes (excluding sampling) takes 0.8 seconds of which the FFT accounts for 90% of the time requiring 4096 signed 16-bit multiplications and 6144 signed 16-bit additions. The attraction of using a mass produced console soon becomes clear when the time and cost involved in producing a general purpose portable instrument with processor, LCD display and user interface, not to mention the moulded case, sound system and serial port, are considered. The Gameboy Oscilloscope The GameBoy `GB' digital sampling oscilloscope `GBDSO' converts a GBpocket or GBcolour into a multipurpose test instrument which should prove invaluable to the hobby designer. Design of GBDSO posed many contrasting requirements: high speed, low power, low cost, small size and versatility. In order to simplify the hardware as much as possible, real-time software is used to p