[SPARCbook] Overclocking 3GS (was: Reviving a Dead SparcBook 3GS - stuck booting)

[Matt Evans]

"Barry A. Dobyns" wrote:
> 
> Well, I was just thinking, I have the 3GS (85MHZ) and I know that the 3GX (110Mhz) is basically the same guts with a faster CPU
> daughter card.  So if I only cranked up the clock to 110Mhz, same as the 3GX, I wouldn't be exceeding any design limits in the
> system.
> 
> We'll concentrate on making it all work again first tho...

that is your assumption, however, when you talk about different
implementations of a chip in the same microarchitecture, typically its more
than just putting in a faster clock.  It's not like the SS5/85 has an 85mhz
cpu because it took them a while to discover a source of 85mhz clocks.  I
would put good money on the feature size of the 70 being larger than the
85, and the 85 being larger again than the 110.  

Typically to up the clock freq, you shrink the feature size and the voltage
swing.  You are trying to up the clock without doing either of the other
two things.  This means that you'll run closer to the slew rate of the
logic, and you'll throw off way more heat.  

The more heat is obviously bad.  Overdriving the chip's feature size will
likely make the thing crash, as the technology size has physical
constraints on it that may make 110mhz simply too much to bear.  After all,
its over a 25% increase in clock freq.  Do you think the entire 85mhz chip
has over 25% of leeway in all of its timings ? It might or it might not. 
YMMV, but i'd advise against it on a microsparcII.  

-- 
Matt Evans     email: mevans at cse.unl.edu      web: www.loathe.com/~bmajik
"And God saw that the wickedness of man was great in the earth, and that 
every imagination of the thoughts of his heart was only evil continually."
	- Genesis 6:5