17 Jun
2008
17 Jun
'08
7:18 a.m.
Holger Veit wrote:
Roy J. Tellason schrieb:
for Z80 the consideration is clock speed (parts exist to 10mhz) and wich
process
(Nmos, NmosII or CMOS).
I'm not familiar with many 6800 designs, but
I was somewhat
surprised to see how limited some parts are, like the 8085 in the
"8085 Cookbook" where you could really get away with very little.
OTOH, the c64 doesn't use any buffering _at all_ and yet the CPU in
there seems to have little trouble driving 3 ROM chips, a set of 8
4164s, plus all the peripherals. I'm guessing that the Z80 is
probably somewhere in between, and that the datasheet probalby won't
give me the whole story anyhow
The plain 6800 is not a good example to compare to
the 8085; from its
generation it rather relates to the 8080 three-chip system. Compare
the 8085 to the 6802, and you'll get a minimum 2-chip system 6802-6846
like the 8085-8355 pair. Using EPROMs rather than mask ROMs will
expand the chipcount similarly in both ways.
The C64 does not use the 1st generation 6502, but rather a modified
8502 CPU with different electrical specifications, so I guess it was
explicitly designed to be able to drive its special C64 peripherals.
It will, admittedly run into problems though with own extensions, e.g.
replacements of the onboard ROMs and additions to the external
expansion port.
The Z80 has also undergone several modifications throughout its first
version, which may or may not resulted in higher fanout.
For all of them, one also has to consider that
nowadays one won't use
the circuitry that was used 20/30 years ago any longer, but use modern
chips. This starts with low power HCT TTL, one will avoid DRAMs of
that time like a plague now, using monolithic 512KB SRAMs instead, and
maybe replace glue logic entirely with LP-CPLDs which can drive much
more loads.
Eactly. The biggest consideration is capacitive loading as CMOS draws no
static current but the input still has capacitance that has impact with
dynamic
changes. so If you run the cpu slower than spec you can often
"overload" the bus
capacitively and have it work. Use care and make sure all the ground
paths are
very heavy for best results.
For practical small systems using z80, 6502 or 8085 and modern memory
buffering
is generally not needed unless your driviing cable or going off board.
Allison
Regards