68k homebuilts

On 12/12/08, John Floren <slawmaster at gmail.com> wrote: I have a bit of experience with the 68008 and 68000/68010 from the design end (they were used in various released and unreleased COMBOARD products) A simple design with a real 68K processor isn't that hard to whip up or debug - in fact not much harder than a simple 6502 or Z80 system. SRAM, of course, is simpler to implement than DRAM, but DRAM can be easier to expand over a couple of MB (for 1MB or less, I'd stuff in a couple of JEDEC RAM sockets for a 16-bit RAM field). Let me amend "simple" - simple is peripheral chips and memory that can keep up with the cycle time of your processor so that you don't have to handle wait-states, etc., and can therefore tie DTACK to ground (as advocated by a famous newsletter, "DTACK Grounded". At 4MHz to 8Mhz, it shouldn't be a problem these days unless you have a particular VLSI chip you want to use for I/O (6821, 6522, etc.) and it isn't fast enough. ROM and RAM that are fast enough shouldn't be an issue, even with inexpensive parts. I haven't handwired a 68K project, but I've worked on a few from design to prototype. One thing we ran into with one design was that UDS and LDS got swapped. Things mostly worked, but the one thing that failed was byte-wide reads. We didn't do those in too many places in our firmware, so it took a little work to figure out what the problem was. Once I got a trace over a screenful of instructions, the flaw was obvious. Other problems include byte-swapping between the 68K and the Unibus or Qbus it was mounted in (not a problem for most designs) and CAS/RAS undershoot between the DRAM controller and the RAM field (the solution is now widely known to put 30-39Ohm resistors in-line with the CAS/RAS lines). Our design was not one I'd call simple, so these sorts of problems are less likely to crop up with a homebrew. As pointed out, there are several varieties of 68K-core MCUs. Some of those have so many I/O peripherals integrated that I think you could get away with the MCU, some external ROM and RAM and little else. Your design would look, perhaps, like a early Palm Pilot, but wouldn't be hard to wire up and would look, to the software, like any other mix of ROM, RAM and I/O, integrated or not. I guess it comes down to what you want it to do when you are done. Is it going to use terminals and run a custom OS or OS68K? Is it going to have a framebuffer video of some kind? (I would not call that a simple design at that point) IDE disks? CF cards? Networking (ethernet?) -ethan