From: Jim Brain (brain_at_jbrain.com)
Date: 2008-11-07 06:43:29
Bil Herd wrote: > As we found on the C128, a VIC is like a dump truck on the loose and it > don't stop for no blinking reset line. I had to laugh a few months back > when I pulled out the schematics for a c128D and saw that we conditioned > the reset line a little so the VIC wouldn't go clobbering the first > couple of cycles in a Z80 (before the Z starts paying attention to the > AEC) > I decided to just punt and buffer all of the lines, since the VIC might not be the only instance where the bus is "dirty" and cannot be trusted. Here's a shot of what I am working on: http://www.jbrain.com/vicug/gallery/softrom It's a small board that houses a 32KB RAM that takes the place of a JEDEC 28 pin ROM/EPROM. It fits in the same space as a 28 pin EPROM. On powerup, an onboard uC will isolate the RAM from the bus, holding the target machine in a RESET state, and load an image into the RAM from a small 4MB Flash ROM or an SD card. When finished, the RAM will be connected to the bus and the RESET line de-asserted. The uC is connected to the bus as well, and I hope to allow the target machine to "communicate" the the unit by sending a specially crafted sequence of addresses, which will switch the unit into "program mode", where the lower 8 bits of the address lines would be the data patch and the upper 5 lines would be commands. It's early in the design, but I wanted to try to lay it out first, as it makes little sense to move forward if I couldn't get all of the parts into a 28/.600" footprint. As you can see, I'm still struggling with some signals, and it's a first layout for this design. I have two goals. A Software ROM that can be loaded with kernals of your choice, and a cart ROM emulator, which would allow one to select a game from an SD card, and then the unit would reset and the game would be ready to play. Maybe it's wishful thinking, dunno. But, it looks like a neat challenge, and I have all of the SD/FAT code available from sd2iec. Jim Message was sent through the cbm-hackers mailing list
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