68K ISA project

From: "Ethan Dicks" <ethan.dicks at gmail.com> Sent: Tuesday, July 13, 2010 9:00 PM To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org> Subject: Re: 68K ISA project You're probably right. The 68EC000 is QFP (IIRC) isn't it? The 68000 DIL package is an impressive chip (taking a lot of PCB space). Jup. I was indeed thinking of byte-wide SRAM and EPROM (27512). To keep things simple (and code efficient / fast) the EPROM will sit in $FFFF0000-$FFFFFFFF (you need some ROM there for the reset vector) and the RAM sits at $00000000-$0000FFFF. If you know the 68000, those 2 regions are "zero page" and references are shorter which means less code bytes and less clock cycles. I was thinking of simple 6850 ACIA's and 6821 PIA's (or better 6522). The standard version runs at 1 MHz, the "B" version at 2 MHz (or was it 1.5 MHz?) With simple address decoding you can use the peripheral chips in combination with VPA*. When VPA* is asserted the 68000 goes into a synchronous mode at one tenth of its clock and you can interface the 6800 synchronous interface chips directly. Using the "B" version means that you can clock the 68000 at 12.5 MHz (maybe at 16). The FDC (2793) would be used to implement RX01 for the simulated pdp8/e. I have once written a "DOS" in 6809 assembler, and have that rewritten in 68000. I remember that I have used it, so that code must be somewhat working :-) EPROM and RAM at 100ns or 70 ns is easy to get. As everything goes on one single Euro card, there is no need for address or data buffers. Nice!! I will give this some thought ... you could connect the LCD via a parallel interface directly to the 68000 board and let the 68000 do the addressing. Later, for performance reasons, you could offload some tasks for the LCD to an AVR. 2x40 would be enough for the 2 rows of lights on the 8/e front panel. 4x40 would leave room for a (debug) output text line, and you could implement on the bottom row a "soft key" text line. Just beneath the LCD are a few rectangular push buttons. Its meaning depends on the state and is indicated on that 4th line of the display. I am thinking of this, because in my 6809-pdp8/e I implemented some of the HP64000 style debugging commands. The HP64000 also uses that soft key idea. Commands are built by pushing the buttons. The meaning of them changes as the debug command is constructed (eg: "trace until address" or "run until address"). Hmm, I'm getting carried away! Yes, 68000 articles always triggers me too :-) I have to brush up my knowledge of the 68000 hardware and see what is available (for low cost of course). - Henk.