Building a computer on a 68000 CPU

[aka]

Hi all,

We want to build a new computer, but to keep it cheap, we'd like to use the 68000.

As I'm a programmer, and do not know so much about electronics and how to connect the hardware, I'd like to ask you guys, if you can provide me with some answers that would just pop out of your mind.

I'm very fond of the Atari ST(e), that's basically why I've chosen the 68000.

The hardware we'd like to include is...
68000, 10 or 12MHz (perhaps a $5 CPU)
DMA for playing sounds (connects to 24bit DAC)
DMA for recording sounds (connects to 24bit ADC)
RTC (probably DS12887)
IDE harddisk interface
RAM, at least 1MB, preferrably upgradable to 15.x MB
EPROM, 16k would be enough, but preferrably 256k

What I'm hoping for, is that we can make a simple, fast, and low cost design. I'd prefer, though, ease and speed over cost.

Now, for the questions...

1: Approximately how many 16bit words can I transfer per second using a 12MHz CPU ?

2: (Roughly) how is hardware connected - If I want, for instance, to wire the DMA's registers to address $fffffc10, how would this preferrably be done?

[w_wegner]

Hi,

if you are a programmer and do not know how to connect things, I think it is a bit too risky wanting to build your own computer. You will learn much, but probably spend more money than if you get something that is already complete.

If you know about 68k architecture and want to stick with it, I would suggest using an evaluation board (or single board computer) of one of the newer Coldfire processors; then you have at least processor, RAM and Flash on a board that is working and you can concentrate on your other peripherals and software.

MCF527x/528x are quite low-cost and you can get IDE working quite easily - for audio DMA you have to use some external logic. MCF532x/537x have the synchronous serial interface that lets you connect an external ADC/DAC in I2S mode with no external logic if you can live with both sampling clocks being synchronous at any time (no different sampling rates for playback and recording at the same time).

There is still work to do, but it is feasible for somebody with little experience in electronics. And, in case everything works fine and you have the fantastic product to make you gazillions, you still have the schematics of the evaluation board and can have a real product made. :smileywink:

Regards,
Wolfgang

[aka]

Regarding building a computer: I'm willing to learn.

I'm not so fond of using microcontrollers for this one.

-Anyway, the plan is to make more than one, so it might pay spending some time on the design.
I've looked at the schematics of the Atari, and they use a PAL22v10; when I get the datasheet, it says I should use GAL instead.
-Does the PAL connect the CPU to the memory and hardware?
I've also been looking at another IC, the DP84332, but it seems to only support up to 8MHz, so if I want 12MHz, I believe I couldn't use this one, right?

Are there any schematics for the evaluation board available for download? -I know there is a serial interface on it, and perhaps a parallel interface, but I'm not interested in any of those, so they'd be taken off.

[aka]

Hi all,

Now I learned something about designing a 68000 computer from scratch.

It's not really so complicated, when you know how to code already.

Anyway, my most important question is...

If I make a basic design with...

M68000 CPU, SRAM, ROM, GLUE and a custom made DMA, how fast would I be able to move data?

Eg:

How many longwords would I be able to read per second?

How many longwords would I be able to write per second?

How many longwords would I be able to copy per second?

-I would need something like 4.5MB (1.1ML) per sec, so how fast a CPU would I need?

Would 8MHz or 12MHz be enough, or would I need 16MHz or above?

If calculating it the simple way, it'd be...

12MHz: 12000000/4 (4 clock periods to move a longword) = 3ML per second, which then again would be 12MB per second. Now some of these clock periods would go to the CPU for instruction-fetch, say 1/3, so the above would be the absolute maximum, right?

-But I must be missing something here, because I have an old computer from 1988, which can only read 300kB per second and write 300kB per second, copy 230kB per second (unoptimized code).

If you have a board, where you could do a test, it'd be awesome, however, an approximated answer would do fine for me. =)

- aka a.k.a. aka -