The microSCOPE with TV Video out by 12f675

The oscilloscope is still one of the most important measurement tools of the electronic engineer. With the advent of the often very reasonably priced USB scopes, such an instrument is now within reach of every body. Twenty five years ago that was quite a different story. A (good) oscilloscope was then a very expensive instrument available only to a happy few. As a result many electronics hobbyist made their own scope. The popular electronics magazines in the sixties and seventies were full of scope designs almost always based on vacuum tubes. This period was closed in 1975 when the Dutch electronics magazine Elektuur (in the rest of the world known as Elektor) published their fully transistorised “Elektorscoop”. Despite all this, the purchase of the expensive cathode ray tube and Special high voltage transformer remained too expensive for many people.

The 12f675 is one of the smallest members of the Microchip micro controller family. In the 8 pin package we find a 14-bit processor core, 1k word FLASH program memory, 64 bytes RAM and 128 bytes data EEPROM. Next to the standard peripherals such as two timers, a watchdog timer etc. the 12f675 is also equipped with a 10-bit AD converter with sample and hold, a comparator and a programmable voltage reference. Al these components can be configured under software control and Fig. 2 depicts how this is done for the &microSCOPE. Pin 6 is used as the analogue input. Internally this pin is connected via the sample and hold to the AD converter. Since the supply voltage is used as the reference voltage for the AD converter, the measurement range is exactly from 0 to 5V. Pin 6 is also connected to one of the inputs of the comparator to implement the trigger function of the &microSCOPE. The trigger level is determined by the programmable voltage reference which is connected to the other input of the comparator. Since it is possible to invert the output of the comparator by software, it is possible to trigger on a positive or negative slope.

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