15 Jul
2010
15 Jul
'10
10:32 a.m.
From: "Ethan Dicks" <ethan.dicks at gmail.com>
Sent: Wednesday, July 14, 2010 8:19 PM
To: "General Discussion: On-Topic and Off-Topic Posts"
<cctalk at classiccmp.org>
Subject: Re: 68K ISA project
I guess the ECC in my brain fails :-) Indeed, reset and all other vectors
are
in the low range, opposite to the "old" 8- bit Motorola chips. IIRC, the
8080
and 8085 already had the reset vector at 0.
Good idea. Will cost a few 20-pin DIL packages space on the board though.
It's hard to add buffers later after you wire wrapped or soldered wires. 16
for
databus and 14 or so for address bus. Address lines 14 and up are mostly
for the address decode logic, so they go to 1 or 2 input pins.
Yes. Trivially (2-3 address bits and 8 data bits and perhaps 2 gates
more than the upper address select logic.
Ahhh, you connect the LCD directly to the address and databus of the 68000.
I was thinking of wiring the LCD to pins of the PIA. I am not sure whether
the
LCD would be fast enough to connect directly to the 68000 busses.
Just need a use for this setup :-) The rest is assembler (which I like).
On 7/14/10, Henk Gooijen <henk.gooijen at
hotmail.com> wrote:
I have a clip that you can "clamp" on the DIL package. I clamp the small
clips
up to 24 pins or so with one hand, but the 64-pin clip needs both hands,
because of the three springs to clamp the clip firmly on the chip!
From: "Ethan Dicks" <ethan.dicks at
gmail.com>
It's more impressive covered in logic prove leads. [...snip...] I don't recall seeing anything in a DIP
package badged faster than
12.5MHz... (68EC000?)
The 68000 DIL package is an impressive chip (taking a lot of PCB space). To keep things
simple (and code efficient / fast) the EPROM will sit
in $FFFF0000-$FFFFFFFF (you need some ROM there for the reset
vector)
ISTR the reset vector is low - there's a fiddly circuit in the Amiga
for that, and we had our own "BOOT BIT" that got hammered to migrate
the ROM from low to high. I think the "modern" technique is to mirror
ROM at $0 and $FFnnnnnn and have a circuit that monitors A23 - as soon
ass A23 goes high, clobber the ROM mirror at $0 and open it up to RAM.
> 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. 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.
Indeed, though I think even our single-board COMBOARD had address
buffers - dunno if that'd still be necessary with 6-8 modern chips on
the bus. We had two EPROMs, a DRAM chip, some internal logic for
Unibus or Qbus DMA, and 2-3 I/O peripherals (Z8530, MC6821, etc). I'd
prepare for the possibility that you'd need to have a couple of '245s
there, but for a prototype, you could jumper across the '245 pins and
see how the address lines hold up. Something that might be an interesting alternative to
a VT220 might be
an HD44780-type textual LCD...
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. 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.
Interesting idea - I think that would be cool. 2x40 displays are much cheaper than 4x40 displays,
though. I've
picked them up attached to Satellite TV receiver front panels (with
LEDs and pushbuttons and software-controllable contrast) from
companies like BG Micro for as little as $8. For a bit more, you can
get 2x40 VFDs that you could see across the room. The only thing that
isn't immediately apparent to me is the switch-to-switch dimension to
line up switches with the display bits (if you wanted to do that sort
of thing - even simulated lights with just a "boot" switch would be
interesting to watch as the machine ran through its paces).
I will check the local auction sites. I have seen LCDs which lite up blue
and white if I am not mistaken). A few buttons underneath the LCD is
easy, but if you want to implement a switch register plus the other
switches (LOAD, CONT, DEP etc.) the would be quite small. Do you
have a pencil sharper for your finger tips? :-)
Cheers,
-ethan
- Henk.