Replacing DRAM with SRAM in a 264 system.

Hello,

In a C64 replacing the DRAM with an SRAM is no longer an issue, but so 
far there was no working design to do the same in a 264 (C16/116/+4).

I was bored yesterday evening and decided to change that. I still had an 
Alliance AS6C1008 (55ns) around and since TED supplies its address lines 
unmultiplexed most of the wiring is rather simple:

A0-A15, D0-D7 are connected 1:1.

/OE on the RAM is tied low, CS2 and A16 are tied high.

/CS on the RAM is /CAS from TED.

The only problematic signal was R/W, that one has to be routed through 
an inverter and then NAND'ed with PHI0 before being connected to the RAM.

So all you need besides the RAM is a single 74LS00.

My hacked together test circuit has so far survived all software I 
tested it with, including some demos. So if you can't find 64Kx4 DRAMs, 
you now have an alternative.

While checking why the RAM didn't run with R/W connected directly I 
noticed that TED uses a different RAS/CAS timing than VIC. VIC will take 
/RAS and /CAS  HIGH on the rising or falling edge of PHI0. TED on the 
other hand will take both signals HIGH about 100ns _after_ the rising or 
falling edge of PHI0. No problem with a read cycle, but on a write 
cycle, that'll cause problems.

  Gerrit





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