SMT techniques (was Re: group buy for homebrew CPUs?)

vrs vrs at msn.com
Sat Aug 12 12:23:42 CDT 2006


From: "Dave McGuire" <mcguire at neurotica.com>
>    A variation on this theme might be to build a very small number of
> different types of boards, but with a CPLD on each one...a small, cheap
> one like a Xilinx XC9536 (something like $3/ea qty. 1)...then have a
> JTAG connector on each board, and squirt the "personality" into the
> board that way.  I've not had a chance to work closely with
> FlipChip-based machines (unfortunately) so I don't know how much
> consistency there is on things like power pins and such, so I don't know
> if this'd be practical, but I think it's an interesting idea.

The good news is that there are at least a dozen flipchips I can think
of where that approach should work fine (provided your CPLD uses TTL
levels).
The bad news is that those flipchips are the most common ones.  (Several
are on ePay at the moment for $6+shipping.)

The problem that I see, is nearly every device made out of flipchips
uses one (usually more) of the harder to find flipchips.  Worst are
the green-handled Gxxx series, which typically have custom linear
circuits on them.  Also problematic are about half the Mxxx series,
the ones that use open-collector outputs, negative logic voltage
levels, etc.  But much of my gear is looking for one or another of
these "special" modules.

Of course, the older Rxxx/Sxxx modules also use the negative levels,
making them more difficult to do with modern logic components.  (The
Wxxx are a mixed bag; a little of everything, including simple connectors.)

    Vince




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