After all the helpful tips here in the forum, I've come up with a first parts list for the Übersquirrel Mark I, code named "Blue Maiden", a 65816 machine that I'm going to try to run with 3 V instead of 5 V to use more modern and larger chips. The idea is to primary run a still to be written 16-bit version of Forth ("Liara Forth") but to be flexible enough to boot other things with the swap of an SD card. Note there is still lots of hand-waving involved.
Core parts:CPU: W65816-S6PG-14 (duh) I already have the DIP version here.
RAM: AS7C316096A 2 Mb SRAM 48 pin TSOP. Starts at Bank 0, glue logic magic maps ROM into lower 8 Kb.
ROM: AT27BV256 32 Kb PLCC 32 pin OTP. Use only 8 Kb, but with possible option (reprogram glue CPLD, jumpers?) to expand it to 16 or 32 Kb. Note this is one-time programmable only, but at less than three euros a chip, we can live with that; I'll just have to test the code in an emulator a lot. Contains boot routine, which loads OS to Bank 1 from SD card / mass storage (see below) or fails to a small monitor routine, and then only handles interrupts.
Clock can: 8 MHz or as fast as it will go with testing.
Glue Logic: ATF2500C-20PU Atmel CPLD 20 ns 24 pins in 40 pin DIP. This is the
really hand-wavy part of the design, with a big label "stuff happens here" on it.
I/O:I/O: XR68C192CJ DUART 44-pin PLCC. This has an additional 8-pin output to drive a LCD screen, see below. No use for the 7-pin (sic) input I can think of yet. Gets its own 3.6 MHz clock can so we can fool around with the system clock.
NHD-0420DZ-NSW-BBW LCD Display 20 x 4 with parallel input, hitched to the DUART. There is also a SPI version that could be attached to the 65SPI, but we have the DUART output port sitting around anyway and can put the SPI lines to other uses. Note this is a 5 V part, will have to figure out how to attach it. If it's too much trouble in the end, this can go.
6522 VIA for later expansion, all outputs/inputs go to sockets. Already have the DIP version here.
65SPI 44 pin PLCC for the peripherals, see next section.
SPI peripherals (8 lines available)Mass Storage: S70FL01GS 128 MB Serial Flash RAM SPI. Get two of these (using four SPI lines) for total of 256 MB.
SD card sockets. Two of them, because life was always better with two floppies back in the day. One SPI line each. Since my "real" computers have SD card access, I should be able to write code there, stick the card in this computer and boot without all the messy EEROM stuff I'm doing now.
Real Time Clock. Haven't settled on the exact make yet. One SPI line.
One SPI line left for expansion, led out to a socket.
Case/outside:I found a nice glass dome at during a trip to the furniture store with a big hole for warmth at the top to hold the computer. It's probably hard to judge the scale here, but it should fit one eurocard easily:
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The cables would come in through the back/bottom, possibly with a shield on the back. Not sure yet, but my desk is all dark wood, so I'd try to make it fit in. (Yes, I bought two domes for when this one breaks. You learn to buy spares when you have kids and a cat in the house.)
You'll notice there is nothing here for graphics except for the LCD screen, which will be more an indicator if included at all. While experimenting with the ÜSq Mark Zero, I learned that for some reason I have no interest at all in graphics, and so everything will be via DUART to a "terminal" (my old white plastic MacBook, which is expendable if I fry everything).
So that's the idea so far. I'd be most grateful for any feedback and suggestions. Will try to get a schematic in the next weeks, but probably won't happen before May. Back to the garden ...
(Oh, about the names: I like blue/white LCDs. Also, the "Tali" in Tali Forth has pretty much become a reference to Tali'Zorah vas Normandy, so at some point I started thinking of the "better, more advanced" 16-bit Forth as "Liara" Forth after Liara T'Soni, who of course is an Asari and therefore blue. If none of this makes any sense to you because you haven't spent hundreds of hours of your life playing
Mass Effect, the summary is it's a horrible video game joke.)