The first thing I wanted to do with the new oscilloscope was to test the X-Y-Mode. Not with some lame Lissajous-figures but with arbitrarily adjustable positions. And the easiest way to to so, was to take an ATMega32 and two DAC’s.
I had two parallel-input DAC’s lying around, but i did not want to make a layout or to wire so many pins.
The next best alternative was to build two R/2R-resistor ladders. I found a large amount of 620kΩ resistors (can’t remember why I bought them. They seem to have been there for years.) and some perfboard.
I wired the resistor networks directly to Port A and B of the AVR, so I could directly assign 8-bit numbers to the port register. I wrote a simple program where an interrupt updates the ports at ca. 30Hz and displays some bouncing dots.