[SunRescue] Sparc Overclocking....

[James Lockwood]

On Wed, 15 Mar 2000, Fleet Captain Druaga wrote:

> There are two crystals on it, one runs at approx 92mhz and the other at
> appprox. 105mhz.  Which one of these is the CPU clock crystal?  I know that
> either one has got to be close because the system runs at 25mhz and
> quartering either crystal frequency comes close.
> 
> I'd like to try overclocking this system but don't know which crystal to
> change. (I'm curious enough to possibly toast my $40 IPC.  At $40 for an IPC
> or $100 approx. for a used SS5 I feel it's worth the risk to change the
> crystal.)
> 
> Anybody have any info on this?

Bad Things will happen.

The IPC locks itself at a 1:1 ratio with the SBus, which has a designed
limit of 25MHz.  Bump the clock up higher and the CPU may be able to
tolerate it, but the rest of the system will not.  The SBus of the sun4c
systems was literally the system and memory bus, it wasn't segmented at
all.

A much better target for overclocking is a SS2 or IPX.  Both run at 40MHz
and run a 20MHz SBus at 2:1, going to 50MHz/25MHz is worth the effort.
Some clone vendors did exactly that, selling a "SS2+".

A used SS5, even a 70MHz version will be far faster than your IPC:

System            CPU        ClkMHz  Cache      SPECint SPECfp  Info
Name              (NUMx)Type ext/in  Ext+I/D      92      92    Date
================= ========== ======= ========== ======= ======= =====
Sun SS/IPC        FJMB86902  25      64           13.8    11.1  Nov92
Sun SS5/70        MicroSP2   70      16/8         57.0    47.3  Mar94
Sun SS5/85        MicroSP2   85      16/8         65.3    53.1  May95
Sun SS5/110       MicroSP2   110     16/8         78.6    65.3  May95

This is largely due to the much faster independant memory bus.  You might
just barely be able to squeak 20% more performance out of your IPC, but
that's a long way from the 300% increase a cheap SS5 would get you.

On the flip side, SS5's are more overclockable, but only the slower ones.
70MHz models easily run at 85MHz and some will make it to 110MHz.  The
programmable memory timing helps considerably.  By the time you make it to
110MHz the memory bus is maxxed out, going faster gains you little unless
you have cache (such as the 160/170MHz TurboSparcs).

Keep in mind that most Suns were designed to run near their limits
reliably from the factory as balanced systems.  You may get speed
improvements, but most of the time they will not be significant.  If they
could have run faster reliably they would have been sold that way.

-James