Re: [stella] Re: 2600's TIA & the TV Boy
Subject: Re: [stella] Re: 2600's TIA & the TV Boy|
From: Kevin Horton <khorton@xxxxxxxxxxx>
Date: Sun, 19 Aug 2001 18:38:15 -0500
At 02:26 PM 8/18/01 -0700, you wrote:
Has this been posted anywhere? I would like to see that.
It's too large to post. It's one of the bonuses on the Stella v2 CD.
*nothing* is too big when you've got a cablemodem and can DL
Because it's very difficult to co-ordinate something like this; it has to
be done in-person and it cannot really be done over the 'net. Not to
mention, I want a good challenge.
Well, just be aware that there are at least 3 interested parties for a
miniaturized 2600 chipset.
There's only one way it's gonna be done and that's through an FPGA. It
would be impossible to make a true custom chip since the required
quantities are pretty low.
Getting a true custom chip made involves lots of work and tons and tons of
cash; we're talking $10K or more tooling, and minimum of 10K pieces (at
probably $3-$10 each).
Though, there was this company making test chips for people; you got a 1mm
by 1mm "Tinychip" (their term) housed in a package of your choice, and you
got to design it. The process is CMOS and it costs about $800 (I think
that's what it was). For that you got 10 or 12 chips (AFAIR) and none are
tested or sealed, and from what I read, 4-5 of them typically don't work.
I think an FPGA is a pretty good "fit" for this, and would work
well. Unfortunately, my FPGA dev package is good only for up to 10K gate
chips so I won't be able to integrate anything but the TIA, RIOT, and
bankswitching stuff. I can't afford the $5000 to upgrade it :-P. (I'm
using Xilinx Foundation, less than 8K gates version.)
Quick observations. I removed the "2600 on a chip" from my TV Boy awhile
back and made a small board up with a 2600 cart socket and A/V outputs on
it. It worked... sort of.
The TV Boy's chip isn't very 2600 compatible it seems, and I'm pretty sure
I know the reason behind it. The games run, but the sprites are all messed
up and I think it's because the TIA uses LFSR's for the counters (less
transistors this way) while the 2600 on a chip uses normal binary
counters. This would account for the totally whacked sprite movement on
games such as Pigs in Space.
Other than the TIA "problems" it seems to be fairly good otherwise. They
*did* implement the TV Type and skill switches, and the digital inputs are
acquired using a fairly "unique" setup- every clock cycle ( phi = low, the
time when the 6502 processing and not using the bus) the address lines are
tri-stated, and the joysticks and switch inputs are strobed. They pass
thru normal old 1N4148 diodes to the address lines directly. Because of
this, it breaks tons of carts that use bankswitching if the "address" (i.e.
combinations of buttons/skill switches) matches up to one of the bankswitch
addresses. I don't see much way around this, except maybe a latch on the
address lines between the chip and the cart, clocked by phi (on the rising
edge, when the 6502 requests data for the high cycle). Pitfall 2 sure was
cute through this chip though; a big screen full of random coloured garbage :-)
Question: Do any of the "pirate" 2600 clone consoles using "2600's on a
chip" have problems with carts?
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