[FoRK] Cheap Electronics Dissection Project
Kragen Javier Sitaker
kragen at pobox.com
Tue Oct 10 20:23:29 EDT 2006
On Sun, 17 Sep 2006 19:12:57 +0200, Dave Long wrote:
> This is an interesting approach for a dot clock: use the SPI interface
> to serially push pixels out, yielding 8 pixels per load and allowing
> one to "unroll" the display update.
That's a pretty nifty idea --- I had been wondering if I could make
that work, in particular for audio output. (I can't find my notes on
that at the moment, though.)
I was just reading about how Steve Wozniak used a similar approach for
his TV terminal and for the Apple ][ color video; from
My only affordable output device was my Sears TV because it was
free. Going to characters on the screen from a game on the screen
was a natural step. I already had the TV timing circuits minimized
and just had to use the horizontal and vertical counters to drive
some memory with the characters stored. I chose what I figured was
the absolute least cost memories, some dynamic shift
registers. They were old (PMOS) and came in tiny 8-pin packages,
and were half the size of 14-pin chips. My design goal was to have
the fewest chips in the end. More correctly it was to have the
smallest board space used. So these tiny serial shift registers
were the winners for me. Some low level engineers or designers
might be scared off by dynamic parts, but I just thought it all
out and made sure that they never stopped shifting.
"Dynamic" here means that the memory circuit was astable --- if you
let it just sit there for a couple of propagation delays, it would
forget everything. In Jeri Ellsworth's lecture last year about
reverse-engineering old hardware (presumably in this case she meant
the 6502), she had a few words about how this sort of thing is kind of
a pain to reverse-engineer --- you can't single-step it!
But a VERY significant role was how my hardware BREAKOUT
experience at Atari led me to a unique and creative goal for my
Apple ][. I'd designed this machine to have color, which was
totally based on an idea that I'd had one night working on
BREAKOUT over at Atari. The idea was to use digital chips, or more
accurately a single chip, to rotate a code around at a multiple of
the color subcarrier rate of NTSC TV signals. From 16 starting
codes, you got totally different digital patterns out of this
shift register. If it was synchronized with the TV via the color
burst technique of NTSC then it would be 'similar to' true color
info. I was pleasantly amazed on the Apple ][ when it worked.
That doesn't sound too hard, does it? Although that only gives you 16
Some of this information was also in the Woz "Founders At Work" page:
> Better yet, the following sites show that despite a small LCD display,
> a machine resembling a glorified Speak'n'spell with a built-in BASIC
> can be compelling for (at least the geekier elements of) the videogame
> The company that made these is still around, and currently offers a
> "vtech Genius Notebook"*, with a ~64x32 bitmap LCD, for $30. Looks
> like they have devices with screens about half the resolution at $15.
> BASIC no longer seems to be included, perhaps due to lack of demand (or
> lack of desire to field the support calls?), but considering that they
> also offer manufacturing services:
> they might be perfectly happy to build cheap programmable boxes if
> someone else were willing to shoulder the market risk.
That sounds pretty cool. Maybe they can teach me a thing or two.
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