Replace 200mhz Dreamcast SH-4 with 240mhz version?

There's a faster version of the sh4 available.

What do you think would happen if you swapped cpu?

 

whats tied to CPU speed on the DC

 

If you overclock the Dreamcast cpu to 240 MHz, which I did a few years ago, the vmu can get corrupted unless it's overclocked by the same ratio. I think the powervr gets overclocked too. Seems it can be overclocked, but it's not on the same clock as the SH-4.

Some games would be smoother, some would simply be 20% faster. Depends on how they're programmed. I believe Shenmue II would be smoother and Quake would be faster. It also improves homebrew like divx player and snes emulation.

Edit, in case it can be useful:

OC is done on ic305, and it has been reported a few cases where 280 MHz was stable. In my case only 240 MHz was stable and 280 MHz didn't even try to boot.

You might have more success with your preproduction Dreamcasts.

 

There is the SH7750R which is apparently pin compatible with the SH7750 in the Dreamcast. It comes with quite a few improvements over the older generation and more efficient. I have no idea if its as simple as doing a straight swap with the existing CPU to get it working.

http://www.renesas.com/products/mpumcu/superh/sh7750/sh7750r/index.jsp

Ted.

 

I had one my my JPN DC overclocked many years ago for that reason alone. Indeed the Shenmue II ran much better although other games had issues like running too fast and/or the speech sped up way too fast.

 

are you really sure it is SH7750 ?
AFAIK Dreamcast/Naomi/etc uses custom SH4 version - SH7091 which is somewhere between SH4(7750) and SH4A

 

What needs to be done to ic305? Would be cool to see a toggle switch, Can it be changed on the fly?

 

I'll let this old schematic speak for itself:

And you can't switch on the fly unless you're some kind of ninja and can do it in under 4.17 ns, between two tics of the clock.
And even then, there might be timing issues with the other components.

It does generate more heat, so that has to be taken into account if you play for extended periods of time with the overclock enabled. e.g. Shenmue II.

 

I'm going off what I have read across the interwebs, a bit like this article...

http://www.segatech.com/technical/cpu/index.html

Ted.