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STEbus 65C02

65C02, 32K Flash ROM, 64K fast RAM, USB link to PC.
Runs BBC BASIC, at up to 14MHz.

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My favourite CPU on my favourite bus!

Boots from a 55ns Flash ROM, copies the BIOS and 16K BBC BASIC into the shadow RAM, then disables the ROM.

RAM is two 15ns 32K RAM chips, cache-RAM chips from old PCs.

It uses a novel asymmetric clock stretching circuit to access different devices at different speeds, on a cycle-by-cycle basis. When it accesses the STEbus, PHI2 high time can last indefinitely.

USB module is a fast replacement for RS232. Control bits were mapped to look like an Acorn Tube ULA, so I could use the firmware from a BBC micro second processor. Boot ROM includes BBC BASIC. This runs very fast through the USB link. Currently running at 8MHz, aiming for 16.

Prototype only does I/O cycles for now, but this is enough to prove the glue-logic concepts. Next stage is to move glue logic from two GALS to one CPLD. I prefer Xilinx but they are abandoning 5V versions. I can either change to all-3V onboard or use an Altera CPLD.
  • 1 × WDC 65C02-14 CPU
  • 2 × 32K SRAM, 0.3" wide, 15 to 25 ns access Lower RAM and Upper RAM (boot ROM is copied to upper RAM)
  • 1 × 74HCT688 Electronic Components / Misc. Electronic Components
  • 4 × 74HCT245 Electronic Components / Misc. Electronic Components
  • 1 × OSC, 16 MHz Master clock

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Discussions

Ralph wrote 12/21/2018 at 20:49 point

I like how this looks - is there a source for the eurocard-friendly PCB in the US?
I already have about 50 of the appropriate connectors and a P2 backplane.

  Are you sure? yes | no

Keith wrote 12/28/2018 at 15:34 point

Pardon the late reply. You would have to google for a US source, as would I, but I imagine European sources would ship to the US. Sourcing is global these days.  STEbus backplanes need termination to pull up some signals and absorb reflections and undershoots, but you can find circuits for that in the specification. This 65C02 design is very experimental. I've got it to the stage where I need to think out a fuller design and lay out a board. Wiring this many wires by hand is not fun.

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