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Keith Howell

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About Keith Howell

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    Combat Commando
  1. Aye, there's the rub. A lot of projects get mostly done, but not completely done. I heard some guys got most of the BBC micro done, but that was a decade ago and still nobody has done it completely. It is a bit like doing a crossword. We can quickly get all the easiest bits done, then the last clues take most of the time and effort.
  2. Oxidisation of socket contacts and dried-out electrolytic caps are the main failure mode in old machines. My XL only has the a few chips socketed, and always works perfectly. The BBC micro had most chips socketed, and as they aged they tended to need hitting to restore contacts. If your machine starts failing, it should be repairable.
  3. Hi guys! I haven't had much free time since I started working 12-hour days five years ago, but a workmate recently pointed out the Visual 6502 project where they photographed a 6502 die and reverse engineered a transistor-level model. They also did the TIA, which led me to this ANTIC recreation thread. I guess getting a chip exposed and photographed is now commercially available and affordable by amateurs like us. I see there has been discussion about the Atari data sheets. I downloaded them from Curt's website ages ago, converted them into HTML pages, and put them on my website. Alas that went offline in 2007 when I moved out of my cable service's range. Actually, they are just not spending on new cable to new homes :-< I converted quite a few Atari documents into HTML, as they original scans were fairly shabby and hard to read. I've taken the liberty of adding your ANTIC die photo to the ANTIC data sheet, and the TIA die photo to the TIA manual. Since people are still interested in Atari stuff, and AtariAge seems to be their main meeting place, I think I should make my stuff available again. So I've just joined AtariAge to get involved with retrocomputing. Is there any place I can upload them and edit them to correct any errors that get spotted? Incidentally, I am interested in cloning the Atari 800XL in FPGA. This needs many things to be working before one even gets a boot-up screen, so I started by cloning the simpler Acorn Atom. I deliberately designed the basic video timing to match the Atari, so that I would not need to start from scratch. See: Acorn Atom in an FPGA Polymorph computer in an FPGA Since I designed the video circuitry, it is easier for me to modify than if I had created a transistor model from a die photo. I already have a POKEY model in VHDL from MikeJ's FPGA Arcade. I had the Atom programming the VHDL POKEY and getting sounds out of it. I've not used an Atari keyboard matrix scanner, but I have written VHDL code to get a PS/2 keyboard manipulating bits in a virtual keyboard matrix. My VHDL video modules output RGB so they can easily interface to a video DAC or LCD. Outputting perfect colour means Atari colour artefacts won't appear. The original ANTIC/GTIA work together in a cunning way to implement player-missile graphics (sprites). I've already implemented a cursor in hardware, so sprites can be easily implemented with the same code. Simply add more cursors and program them to look like space invaders instead of an arrow. This isn't as frugal as the Atari method, but these days one doesn't have to be.
  4. FPGAs use around 30 times the the number of transistors that a custom ASIC would use, so they cost more in silicon area. The development boards don't sell in vast numbers, so they don't get economies of scale. I don't use FPGA for functions I can already get off the shelf. For example, I can buy ROM, RAM and a 65C02 very cheaply. I use FPGA for specialised functions, glue logic and functional copies of chips that are no longer made (e.g. 6847, 6809, POKEY, ANTIC, GTIA etc). On the positive side, you can use one FPGA development board to develop many projects.
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