Interfacing a 68000 to RAM

On Tue, Jun 2, 2009 at 9:53 AM, dwight elvey <dkelvey at hotmail.com> wrote: OK... that RAM is plenty fast enough, but you also need to ensure any memory-mapped peripheral I/O chips are also fast enough if you want to ground DTACK. We had 4MHz 8530 UARTs, so we _had_ to use DTACK in the "expected" way. Simple designs can ground DTACK, more complex designs tend to use DTACK normally, especially to be able to use less expensive memory. These days, if I were building a 68000 design, I'd look at fast memory and fast I/O chips and just do it in the simplest way possible - when you ground DTACK, what you are doing is telling the CPU that at the time in the cycle that it would check (and wait on) DTACK, the transfer has already happened (Data Transfer ACKnowledge) - the CPU doesn't have to wait _ever_. In a mixed-speed design, if some component is fast enough, DTACK will be grounded at that point in time anyway, but through handshaking, not because it was permanently tied to ground. Are you trying to do ROM development? Why not use a plug-in ROM emulator? (I like the PromICE, but it's made by someone that I've known and worked with, starting 25 years ago, so if you have a favorite or get one for free, by all means, use it). You don't have to do any target-machine hacking - you get a rig that matches your target (28-pin cables, 24-pin cables, 32-pin cables, 256Kx8, 128Kx16, whatever) - and the PromICE is somewhat configurable in that respect - then compile the code and download it. It could be tricky if you need the target machine to compile your code because of the tools needed, but you could have the Cat make the code, transfer it to another machine via serial port or network, then keep ROM images on the other machine so you can try new code and revert back to working code if you compile up a dud. I would hesitate to make recommendations on how to hack into an existing design without schematics in front of me, but if all you are trying to do is get writable ROMs, I'd seriously recommend a ROM emulator that's externally loadable (built or bought, makes no difference there). -ethan