Amiga.org
Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: Phantom on October 27, 2009, 05:57:56 PM
-
Hi. I've recently acquired a 68000 with 12Mhz, although I read that you have to change the oscillator. Although looking at the motherboard I can find one two oscillators set to 28MHz. Any ideas where is this oscillator for the CPU?
-
There isn't a separate oscillator for the CPU.
-
You can't just change the (main) oscillator to upclock the CPU, everything is derived from the 28 Mhz quartz.
If you're thinking of clocking the CPU asynchronously you'll have to provide for bus sync logic.
-
That means that installing the 68000 12MHz CPU will be 12MHz or 7MHz?
-
The 12MHz one is capable of running faster, but unless you provide it with a different clock it will run at the same speed as the old one did.
-
And how I can achieve that?
-
See Zac67's post...
Ed: Net result is likely to be a CPU that spends at least 3/4 of its time waiting for the bus to become available, rather than just 1/2 of it as is normally the case. Can the CDTV take any FastRAM? If not then it's a largely pointless exercise anyway.
-
Better off going with a 68010, at least it'll give you 10% more speed or so without having to change the clock speed. Incompatibilities be damned!
-
I didn't understand that much, but anyway thanks.
-
The Amiga's quite a unique design: in the standard config the CPU lives on the same bus as the chipset (gfx and such) where it has to share memory access and is thus slowed down at times. However, it is possible to detach the CPU (and the 'fast' memory dedicated to that side) from the chip bus and let it run on its own.
This is very nice when high load is put on the chipset bus but it even gets much better once you get the chance to run the CPU bus faster than the chipset bus - this is what an accelerator board does.
Every now and then the CPU will have to access RAM or registers on the chip bus, so it'll have to slow down to its speed, so that the memory cycles start and end at the same time. This is the required synchronisation I was referring to.
It is fairly easy (little extra logic required) to run the CPU at an exact multiple of the chip bus, but without cache there's not much use.
To make it short: there's no (simple) way you can upclock your CPU, whether it's an 8, 12 or 20 MHz type. A CPU with significant cache might get away with running on a slow bus, but the 68000 is not designed for that I'm afraid.
-
This board saw better times... sigh...
Here goes some hacks to speed up the 68000 CPU without much hassle:
http://aminet.net/package/docs/hard/14MhzA500
http://aminet.net/package/docs/hard/14mhz
http://aminet.net/docs/rview/Accelerator14M.txt
http://aminet.net/package/docs/hard/new14acc_31
Good reading & hacking!:hammer:
-
Thanks for the links and to all of you.