--- Richard Erlacher <edick_at_idcomm.com> wrote:
> > The first COMBOARD used 2114 SRAMs because they were more reliable than
> > Intel's DRAMs of the day (plus the XC68000 would occasionally enter a
> > microcode fault and reset, causing Z-80-style bus-driven refresh mechanisms
> > to pause too long, letting all the bits leak out).
> >
> Since the MC68000 and its kin didn't have a Z-80-style bus-driven refresh
> mechanism, this must have been implemented in external hardware that was
> turned
> off when an exception was encountered, or it was done in software, which was
> apparently not used in the exception handler.
This is slightly before my time, but I was told that the earliest 68000
designs that used DRAMS, used a derivative of the Z-80 scheme because
the designers of the time were used to it. I didn't mean to imply that
the 68K had some kind of magical in-built DRAM refresh support.
The microcode faults I was told about for the XC68000 (not MC68000) would
lock things up tight for dozens of clock cycles, then go right back at it,
as it nothing ever happened. It was strictly internal to the CPU and caused
by bugs in the first run. Your software never knew what hit it unless it
was checking timings against an external clock.
By the time I personally touched a 68000, these kinks were long worked out
(except for knowing that one needs to put low-ohmage resistors (30-60 Ohms)
in series with the refresh lines from a DRAM controller when attempting
to refresh several dozen DRAMs (4164s in this case). I chuckled when I
saw those same resistors in the front memory cartridge of an Amiga 1000.
They at least got them in there by the time their production started. I
added hundreds of 68 Ohm resistors to the first production run of COMBOARD-II
rev 0s. Once one of those came back from the soldering house, I think it
took 3 hours of bench time to apply all the ECOs. Good thing they were
already expensive.
-ethan
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Received on Sat Jul 07 2001 - 00:43:27 BST