5v Fpga Hypothetical Question

[SpacedCowboy]

I think it'll work fine on the Atari 8-bit, the external hardware will be under software control (and be invisible if the software doesn't enable it). I'm not using any addresses normally accessible without hardware being added (there's a couple of pages dedicated to external h/w on the XL/XE). As for writing into the 8-bit's memory, all Atari 8-bits already halt the processor when Antic (the "gpu") wants control of the bus - there's a signal dedicated to that (/HALT). I'll have control of that signal too, and as long as I time my accesses to not conflict with Antic, the 6502 won't even know anything has happened. Memory is active however (because Antic needs to read RAM to draw the screen), it's just the 6502 in pause-mode.

I'm less sure about the C64, but have yet to look into it in any great detail. On the ST, everything will be memory-mapped into the cartridge space, so I'm not foreseeing too many problems there.

[TomS_]

Hmm. This is interesting - I've always regarded data sheets to be universal truth from on high, but what you seem to be saying is that if I have a slow bus (~2MHz, maybe as high as 10MHz), I could put series resistors on those 5v lines, and then feed it directly into a VCCIO=3.3v FPGA like the HX series ?

Only if the FPGA has built in over voltage protection. Check the datasheet.

e.g. some Altera CPLDs contain internal zener diodes that make some IO pins tolerant of higher voltage by using an external series resistors. But others don't, and the datasheet explicitly says that the user has to place external zener diodes if they want to work with higher voltages.

Depending on how many signals you need to interface with, lots of external components could end up taking up more board space than some level shifters.