PowerMac 6100: 40MHz underclock?

[TylerEss]

I'm considering a crazy 6100/Portrait Monitor project and would really like to run my G3 upgrade on at least a 40MHz bus. I know there have been a (very few) few 6100s overclocked to run at 100MHz or more, so it seems likely enough that I could get 40 or more out of the bus... but I don't want to push the 601 that hard, or even let it become the bottleneck. Changing the multiplier to 1 would allow bus overclocks limited only by what the bus itself could handle, and 45MHz PowerPC would still be fine for getting the Sonnet or Newer extension loaded. [}] ]'>

Anybody know how to change the bus multiplier on the 6100?

 

[TylerEss]

Hokay... the PowerMac 6100/60 has a 30MHz crystal oscillator onboard, which means it's suppling the bus clock. Page 488 of the PowerPC 601 Users' Guide (http://stuff.mit.edu/afs/sipb/contrib/doc/specs/ic/cpu/powerpc/mpc601.pdf) makes it look like BCLK_EN controls the core/bus ratio by oscillating appropriately. In particular, it looks like tying it to ground would cause the CPU clock to equal the bus clock.

Let's talk about examples in a 6100/60, for concreteness' sake...

Would the chip be smart enough to run at 30MHz if BCLK_EN was tied to ground, despite the presence of the 60MHz and 120MHz inputs? In effect, there'd be a wait-state after every instruction.

Or, would this cause the CPU to think that the bus speed was actually 60MHz, in disagreement with reality? Would it be necessary to tie the 60Mhz output of the clock-chip to the 2X_PCLK and the 30MHz to PCLK_EN, in addition to tying BCLK_EN to ground?

I *really* wish I had a fast, dual- or tri-trace scope... :-(

 

[Bolle]

I just had that question flying around in my head yesterday...

good that I dont understand any word of what you´re talking but it sounds like it could make sence

 

[TylerEss]

Lots of chips on the Dark Side just have one clock input, the font-side-bus input. Then they've got a multiplier input that tells them how much faster than the bus to run. I was hoping the PowerPC 601 would be like that, but it's a little more complicated.

Any ideas?

 

[Bolle]

i guess its too hard just to try out the stuff you said above?

 

[TylerEss]

I could try it, but I don't have a 6100 kicking around. Also, without an oscilloscope to view the timing signals in question, I wouldn't be able to get nearly as far with it as someone who did.