In the PDF at the end of this post, I have captured some notes associated with an abandoned project to use an ATMEL AVR – ATMEGA16 in place of a National MM5740 keyboard decoder. Both parts come in 40 pin DIP packages, which makes the AVR substitution idea work pretty well.
avr encoder
The initial idea was to check to see if the AVR could be used as a plug in replacement, with some socket stacking and rewiring. I was also considering building a Datantetics Apple 1 and Apple 2 compatible keyboard using Cherry key switches and an AVR as the encoder. The reason why I stopped this project is that costs would probably have forced a selling price of close to $200. These notes are associated only with mapping the pinouts. I had assumed that if I could make a reasonable mapping that the software would be pretty straight forward.
It turns out that there are just enough pins to do the job without any loss in functionality. The strobe control, Output enable (OE) and shift lock functions are not used on Apple II/IIplus or earlier Datantetics keyboards. The Bounce Mask function can be accomplished in the AVR’s software. The clock of the AVR can be generated by either the internal RC oscillator or an external crystal input.
The PDF follows:
MM5740-to-AVR-mapping