PDA

View Full Version : 16MHz MC68000 + A500 = Will it blend?



luncheon
9th August 2012, 16:21
Hello to everyone!

I just joined this forum, being an Amiga owner since back in 91 or 92 IIRC.

I currently own two A500s, the one I own since new (1MB chip RAM, external Mtec AT-500 w/8MB fast RAM, 400MB HDD and Kickstart 3.1/40.63) and another one that I found on the streets a couple of years ago! Sadly it has no keyboard but the (R6A) mainboard looks good enough, and all custom chips run fine (tested them in my working A500).

So, I will try to resurrect that one first, and then source a nice case and keyboard, in order to do some HW hacking with it.

I found a working sample of a 16MHz MC68000, and I have it currently running at 7MHz on my main A500. However, I'd like to install it in the other A500 and extract more performance out of it.

Has anyone tried to implement this project from Aminet? It seems to have what I'm looking for:

docs/hard/14mhz.lha

If not, does anyone know if a 16MHz 68000 can handle 28MHz straight off the main clock, with the help of an adequate heatsink?

My aim is to have a 14 or 28MHz 68000 A500, with 1MB of chip RAM and 2MB of slow ram, together with a 2GB CF card (thru IDE68k) and KS 40.63. I think that should cover most of my retrogaming needs, using WHDload etc.

Thanks!

ptp170
9th August 2012, 16:38
Zetro posted this a couple of years ago if it helps :)

http://www.amibay.com/showthread.php?t=4934&highlight=68000+14mhz

luncheon
10th August 2012, 06:44
Thanks! I should search before asking... :double

Unfortunately, it seems that the pics are gone from that thread, so what seems to be a viable project will be kinda hard to implement.

I took a second look to the mainboard I found and found one good thing and two bad:

GOOD - It's a R6A board and it looks MINT so KS 40.63 and 1MB chip RAM should be doable.

BAD - It has a 8370 Agnus chip (???). Also, R309 (next to Paula) and the resistor that goes to CIA A are missing, so it's off to the components store before I can fire it up.

I just ordered a 8372A Agnus chip, I'll have to wait for it to arrive as well...

However, the question still remains: Can a 68HC000P16 or a 68000P16 handle 28MHz without disintegrating? Maybe with a heatsink?

AndyLandy
10th August 2012, 07:24
8370 Agnus? That's surprising for an r6a board, but of course it could quite easily be a retro-fit. Still, an 8372A should just be a drop-in replacement, with a couple of minor hacks to make it 1MB ChipRAM.

Zetr0 lost a lot of pics a little while back, it was all a bit lame. I'm not sure if he still has them archived somewhere, it may be worth prodding him and asking.

But overall, as I recall, the general consensus was "Yes, you can brute-force an A500 to 14MHz, but don't expect to be able to do anything useful with it"

Still, good luck with your project, all hardware hackery is good hardware hackery! :lol:

luncheon
10th August 2012, 13:34
8370 Agnus? That's surprising for an r6a board, but of course it could quite easily be a retro-fit. Still, an 8372A should just be a drop-in replacement, with a couple of minor hacks to make it 1MB ChipRAM.

Zetr0 lots a lot of pics a little while back, it was all a bit lame. I'm not sure if he still has them archived somewhere, it may be worth prodding him and asking.

But overall, as I recall, the general consensus was "Yes, you can brute-force an A500 to 14MHz, but don't expect to be able to do anything useful with it"

Still, good luck with your project, all hardware hackery is good hardware hackery! :lol:

Bear in mind I found this on the sidewalk, so as you say they may have used that A500 for spares for another, older computer (early NTSC 512k A500 comes to mind, we have a lot of those in Argentina) and then just chucked it away.

A 14MHz 68000 may be good enough to run most WB-based programs from the time OS 3.1 launched (92/93), and will help boost some of the CPU-heavy OCS games like F1GP. But that's why I'd like to try to take it to 28MHz, since I read somewhere that a 28MHz 68000 should be near a 14MHz 68020 performace-wise.

I ordered a spare 68HC000P16 to give the 28MHz boost a shot, I assume that in a worst-case scenario I will fry just the CPU...

I'll try to PM Zetr0 as soon as all components arrive home.

Thanks! :)

Mendel
10th August 2012, 13:49
They would probably blend but how would you use liquid computer and processor? :whistle:

Justin
10th August 2012, 13:51
what people are trying to tell you politely is no, it won't work, it will run workbench for a little while maybe, most software will crash upon booting.

so don't bother.

Toni.Wilen
10th August 2012, 14:10
(Some technical details why it is not worth the trouble)

Higher clocked 68000 without faster RAM (or separate small cache like in SupraTurbo 28) only helps if program uses heavily instructions that have internal non-memory fetch calculation cycles, mainly multiplication, division and long shifts.

Most other instructions are hardwired to memory access cycle timing (pipelining was introduced in 68020) = have about same speed as normal clocked 68000.

Yes, it probably helps with some 3d games/programs but 68020, even much lower clocked one would be much faster..

ptp170
10th August 2012, 14:27
They would probably blend but how would you use liquid computer and processor? :whistle:

Consume to become part man part machine?

#CueTerminatorSoundtrack

luncheon
10th August 2012, 14:52
Thanks for the feedback! I wasn't aware of the memory speed limitation. So if it's not worth the trouble I'll just wait for a decent accelerator card to show up for the A500. There are some interesting projects out there, I'll keep an eye on that.

For now I'll try to resurrect that A500 board I found so I can experiment with it.

NMI
10th August 2012, 15:44
One hurdle to overcome when you create an accelerator for the A500 (or A1000 & A2000) is that the 8520s are using 6800-style (not 68k) clock and synchronous access.
The 68000 has support for this build in and generates the clock signal for the 8520:s internally, but if you overclock the 68k processor you also overclock the 8520:s.
So I'm actually amazed that anyone has gotten this type of simple hack to work!

The 68020 and later cpu:s lacks this support for 6800-type peripherals so on the later Amigas it is handled by the fatter Gary I believe.

rkauer
10th August 2012, 23:37
@NMI: correct.

One A500 with an overclocked 68000 will work if you divide the E-clock (the pin that makes the CIA clock) by the same factor the CPU is overclocked, yes. Will work.

So to use a 14.18MHz clock (the 28.something MHz generated by the main crystal) you must halve the E-clock output.

To use full 28MHz, you must divide the E-clock signal by a four factor. That's divide the signal by two using a flip-flop and then divide it again.

The CIA clock is not tied to the CPU, but without some local bus FAST RAM this overclock exercise will become a futile effort.

Also, old games will not run correctly because they are (badly) programmed to use the full capacity of the original CPU.