> From owner-stella@xxxxxxxxxxx Mon Apr 19 19:03:29 1999
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> Subject: [stella] Cartridge read timing problems?
> URL: http://www.cae.wisc.edu/~desmet/
> Hexcode: 50-69-6E-65-20-69-73-20-66-75-6E-21
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>
> If many of you recall, I'm working on the great atari development
> system/game server. Currently, mainly on the game server.
>
> I am having slight quirks, and maybe someone out there may know why.
>
> The hardware:
> Attached to the atari bus(the A+D line out of cartridge socket), is a
> screensaver ROM(it is just an atari rom that John Harvey wrote for me),
> and the output of a octal 3-state buffer.(I'm using a 74HCT541) The
> address lines go both the the screensaver ROM, and into a MUX(3x 74HCT157.
> The output of the address mux's and the input of the 3state buffer go to a
> couple other roms(and later other stuff). It is configured so that only
> the screensaver, or the buffer reacts to the atari's reads(atari A12).
> When setup for the screensaver, runs fine. when setup for the other
> roms(through the 3state buffer & MUX's), only 'mostly' runs. In this
> state, the output of the 3 state is only enabled when the atari reads(and
> in this state), and the mux's select the atari address input to be passed
> on to the other roms.
>
> What happens:
> The ROMs are currently a pile of 'commerial' software. The problem is
> that the games run improperly. All games at least start up. Some games
> seem to run, and crash later. Others display wierd graphics. For
> instance, Adventure, the power up screen looks like this:
>
> #################
> #x x#
> #x x#
> # #
> # #
> # 1 #
> # #
> # #
> # #
> # #
> ####### #######
>
> where the x's are a patch of boarder that appears and dissapears. It
> flickers on and off irratically. Boths sides flicker at exactly the same
> time, but it does not appear to be periodic. The patch is exactly the
> same width as the edge(2 playfield blocks, I think).
>
>
> I believe the problem has to do with the 3 state buffer(the other rom's
> are 150ns, and should be plenty fast enough). When changed to an ALS,
> games by and large behave better, but still crash and have funny graphics
> problems. My best guess is that because the atari doesn't give the
> cartridge a real read signal, but instead just makes the cart(and hence my
> buffer) base it's output on the address, that my buffer happens to either
> hold the lines driven too long, or more likely not long enough.
>
> Any ideas about timing?
> Any ideas about what else may cause such software actions?
>
> Mark De Smet
Have you tried eliminating the 3 state buffer altogehter? I am not quite clear on exactly what you are doing, but if you just want to selectivly enable each of the other ROMS you should just hook the address and data busses from the cart slot directly to all the roms then just enable the appropriate one onto the data bus using it's chip enable line. The 2600 seems to have pretty good bus drive capabilities since the Supercharger has 3 RAMS, 1 ROM, and the SC custom chip on the bus. If address bus drive becomes a problem you could just throw in an bus buffer IC that is always on to give the lines some extra drive power.
Dan Boris
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