Atari XE C25953 emmu unit replicated in GAL16v8 and GAL20v10
As the happy owner of an Atari 65XE I wanted to upgrade it and have fun with full 130kB of RAM. After browsing the internet I learned that C25953 is very hard to find. There are some GAL implementations, however those require an extra D latch. Unfortunately my motherboard doesn't support such a solution.
After quick investigation, looking into C25953 discrete IC implementation's schematic, it turned out that it was possible to fully replicate C25953 logic with cheap, old, yet still available GALs (GAL16v8 and GAL20v10). The only trick was to generate the missing latched HALT signal used in logic equations. Unfortunately due to the GALs' hardware restrictions 1:1 pin compatible replica was not feasible forcing external wirings.
Structure of the repository
gal16v8 - best to use with a custom PCB:
winculp - implementation in CUPL, jedec files, etc.
graphics - photos of the implementation
kicad - kicad schematics/pcb and gerber files
gal22v10 - can be directly mounted in the socket, however requires some basic wiring and soldering on top of the IC:
winculp - implementation in CUPL, jedec files, etc.
graphics - photos of the implementation and wiring diagrams