[stella] Fpga2600 Status Update

Subject: [stella] Fpga2600 Status Update
From: adam@xxxxxxxxxxxxxxxx
Date: Sat, 19 Mar 2005 22:26:07 -0500
Hash: SHA1

[regarding http://sourceforge.net/projects/fpga2600/ ]

It's been a while, so I thought I'd update.

As you know, the RIOT code is complete and tested.

I've been pounding away at the TIA code recently, mostly spurred on by
releases of various commercial projects and another similar open source
effort.  (The author of that other effort hasn't released any code yet,
but promises he'll make it open source when he's done).  At this point
I don't care so much about finishing first as I do about finishing;
I picked this project up to learn something new, and I'd like to see
it through.  (This is where you can make me feel good by cheering me on,
if nothing else).

Regarding the most recent tia.vhd checked into CVS on sourceforge...
(that's http://sourceforge.net/projects/fpga2600/ for those of you
playing along at home).

Analog output on the audio pins is a bit problematic for an FPGA.
I've replaced the analog AUDIO0 and AUDIO1 pins with 4 bits each.
I'll feed these into a cheap resistor ladder DAC and then into my 2600,
as well as a pair of RCA jacks.

Similarly, I've decided NTSC video is a bit of a pain; the current
code outputs 4 bits of color, instead of the single phase shifted
colorburst signal.  I plan to feed these plus the three luminance bits
into a lookup table (basically a ROM) to get 4 bits each red, green,
and blue, then use these to drive another set of cheap resistor ladder
DACs which will drive the component colors on a VGA monitor.  I expect
the picture to be sharp. :)

As you'd probably guess, this means the DELAY pin is completely ignored.
I'll leave true NTSC up to someone who's up enough on their analog to
produce the correct phase shifted colorburst signal from those 4 bits
of color.  I suspect, however, given the current target audience and
expected usage of this code, that it will never actually happen, which
is fine with me.  Now that we live in the 21st century, I think it's ok
for the relic of NTSC to die.  This project is all about preserving the
look and feel of the old games using new technology anyway.

Simulations of the code look good, and I've ordered parts to interface
my developer board to my 2600, and to a VGA monitor.  I'm hoping that it
will go as smoothly as my RIOT testing went.  If it does, then I should
have some good screenshots to share.

Once that's done, the next step will be to evaluate the 6502 core I dug
up way back when...

- -- 
adam@xxxxxxxxxxxxxxxx        http://cuddlepuddle.org/~adam
KG6GZR                       http://cuddlepuddle.org/~adam/resume.html

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