12.5MHz Overclock

[neff]

Yes, I'm aware most of those how-tos out there say just use a 10MHz oscillator.

But I only had a 25MHz one on hand, and a 74LS175, so I went ahead and built a 12.5MHz board for my Genesis (MK2 VA1 USA)

 

Halt works, the 68000 seems to handle 12.5Mhz clock with no issues. Sonic 1,2, and S&K are "playable", although it seems the VDP is being loaded with random garbage at the higher speed. You still have backgrounds, tiles, sprite movement, etc... but you can't really see anything but patterns of random noise moving around the screen, instead of the backgrounds and sprites you'd expect.

 

Now, I know some people have successfully done 12MHz overclock, and that Sonic 1 and 2 have been tested and found working.

 

Some of these older ROM chips have a minimum access time around 100ns, which is fast enough for 10MHz operation, but that's the limit before they become unreliable. I can understand why this would create a problem... and why it might result in a running system, but the VDP being loaded with garbage.

But how are others able to exceed 10MHz without the same problems I'm having? Do they all use Everdrives, or are there further mods that must be made to the system board?

 

[+-^CrossBow^-]

I've also seen people with these extreme overclocks, upgrade the ram on the genesis to faster speed ram as well.

 

I'm more interested in just getting a Mil Spec'd 10mhz 68000 to put into my Genesis just because...hehe

[neff]

Ah, you're correct... the VRAM in the model 2 has a minimum access time of 100ns. Looks like 10MHz is a hard limit without doing some part swaps.

 

I got a hold of some 40MHz oscillators, so I'll just divide that down to 10MHz (and quite likely 5MHz as well... I don't know how else you're supposed to beat BTTF 3).