Overclocking the Megadrive

[Aidan]

Can anyone tell me why the Signetics 68000 (SCN68000) doesn't overclock? I've monitored it's startup, and the thing appears to get up and running, but something goes awry somewhere. Actually, it runs for about 183uS before stopping.

Also, connecting it to the Z80 clock has the same effect, although it does run for about 2.3seconds before stopping.

[Aidan]

Ok, I've managed a whole 600mS worth of operation before the SCN68000 crashes. That's with some extra cooling (no freezer spray yet though), and some additional components.

[Guest_robivy64]

The signetics 68K is a cheap korean knock-off of a 68000. Sega only used these in the early genesis/megadrive, perhaps because they realized it was far too crappy . Ive had no luck overclocking one, ive even replaced a signetics cpu with a motorola 68K.....overclocked to the moon

-R

[Adeptus]

Are later CPUs in the 68000 series (eg, anything up to 68060) fully compatible?
I know Amigas could take these as upgrades (on separate PCBs though)
Can't imagine what a Megadrive would do with all the extra power B)

[hippy dave]

only the 68010 is a pin-compatible replacement. 68020 and up are 32bit and interfaced differently.

[kendrick]

Rob Ivy actually did try putting a 68010 into a Mark-1 Genesis/Megadrive, but his web site is down so I don't blame anybody for not seeing it on a web search. The result of the swap out is cleaner audio sampling and less animation lag. But since the 68010 performs some of the older chip functions through emulation, a handful of games aren't compatible. Do I have a list? Yes I do...

Sonic 3
Sonic and Knuckles
Street Fighter II
Desert Strike

On this topic, does anybody know a good source for those old Motorola chips? You'd think that the big-M would still be manufacturing them, but no....

-KKC, who was surprised to see that JVC still sells parts for the old Wondermega/X'eye hardware. But who's going to pay $80 for a controller?

EDIT- D'oh, that was Rob posting three messages ago. Hi, Rob!

[Aidan]

The 68010 was designed to be compatible with the 68000 down to the pins. However, one of the changes made was a hardware implementation of some of the looping. This provides a speed boost when certain sets of instructions are executed. This might be the reason for the incompatibility.

Later chips are significantly different in that they provide the full 32bits of address space, rather than only 24bits. There's also things like caching in place, as well as an external data bus that is a full 32bits wide, rather than only 16bits. The 68010 was probably the last drop-in replacement for the 68000. Later chips would need a bunch more electronics to make them fit in!

Rob: I did try replacing the Signetics 68000 with a non-motorola one. This helped a bit, but did not significantly improve overclockability. One thing that does make me think however is the !DTAK generation. It's entirely possible that speeding the 68000 up doesn't help that much, as it has to wait for the custom chips to generate it's !DTAK signal. In that case, the scratchpad RAM would be better off being local to the processor.

Does anyone know if the customchips access the scratchpad RAM for any purpose?

[Aidan]

Found somewhere selling SGS Thompson 68000 - probably pulls, but cheap enough. I've seen some places trying to sell 68000P8 (8MHz version) for US$80.

[RobIvy64]

My original 68010 machine fell victim to lightning a short time after the article was written for UCM. I am now teaming up with Deven Gallo to help revive www.epicgaming.us  with some new content, including my new 68010 machine. These cpus are not easy to find . I just scored a sister machine with the exact same IC manufacturers and RAM latency, so a direct comparison will be performed. I've also enlisted the help of TailChao to whip of a benchmarking app that we can run on both machines in order to have hard numbers.

-R

[Aidan]

Benchmarking app would be interesting to have as a 'public release' for those who are interested in testing their machines...