Video DVM circuit

  
Video-DVM is a very cheap DVM that shows how an output as complex as a videocomposite signal can be generated entirely in software: two I/O pins and three resistors are all the hardware required. Connected to any TV set it displays voltages, included max and min peaks, using both giant digits and an analog bar-display . A serial data output for computer data logging is provided, too. The micro is the Atmel’s AT90S1200, ideally suited for hobbysts thanks to its 512 words flash memory, public programming protocols, free assembler and simulator available at www.atmel.com. More recently, other tools as free Basic and C compilers appeared on the Internet.
Video DVM circuit - schematic

The circuit is not only a working project but also a guideline for any application using a tv set as giant display: all the hard work (interrupt driven, time balanced display software) is ready made, letting even novices to modify the code to build anything ranging: from a multimeter... to a frequency meter.... to a game scoreboard.... to a watering timer.... to a video pattern generator.... to a weight scale.... The detailed flash programming protocol is available on the web site, too. The chip has 32 bytes of RAM, 64 byte of EEPROM, 512 words of program memory, and an 8 bit timer. The instruction set is coincise and very well balanced, and thanks to an Harward RISC architecture even a complex task as the one described here is accomplished using only about 400 instructions. The instruction cycle time is very short, letting software video synthesis possible. A simple two bits, asymmetrical DAC built around three resistors feeds the composite video signal at a standard level of 1 Vpp suitable to be input to any TV set with a SCART connector or an AV (VCR) input. Even the serial datastream (ASCII data at 1200 baud, no parity, 8 data bits, 1 stop bit) is generated by software. The level is TTL: most personal computers work with TTL levels as well as with standard RS232 levels, provided that the connecting cable is not too long, so I found a voltage translator like the MAX232 not necessary. If...



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