CDP1861 PIXIE graphics chip replacement for COSMAC Elf

[Eric Smith]

In development. Inspired by the Spare Time Gizmos STG1861, but not
based on that design. Rev. 0, not yet ready for production:

    https://www.flickr.com/photos/22368471@N04/sets/72157667299828132

It is 2.0 inch by 0.7 inch, with a 24-pin round-pin DIP header to plug
into a normal 24-pin DIP socket (vs. more the common square pins that
won't work with normal IC sockets).

The surface-mount components were assembled onto the board by a
commercial service, which does not do through-hole, so I had to solder
the DIP header by hand.  I had to make the pads for the DIP header
very small to squeeze the TQFP CPLD between the rows, so it turns out
to be unsuitable for hand assembly by novices. Since I am not willing
to do the hand assembly for other people, I'm not sure whether this
board would actually be worth selling; I might have too many customers
that aren't able to assemble it successfully.

The CPLD programming is done by a "Tag Connect", which uses pogo pins
to contact the ten gold pads seen on the top of the board.  There are
holes near those pads for the Tag Connect's steel alignment pins;
while there is enough clearance on the top of the board, I failed to
consider that the frame of the DIP header on the bottom of the board
would prevent two of the alignment pins from extending far enough. I
had to cut out part of the DIP header frame.

The CPLD code has been written but has not yet been debugged.