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List:       classiccmp
Subject:    Re: [cctalk] Re: pdp11/93 mod
From:       "Cory Smelosky" <b4 () gewt ! net>
Date:       2013-05-30 23:29:03
Message-ID: 45089A43-44AB-402A-A749-9FF35EDC1901 () gewt ! net
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On 30 May 2013, at 18:40, "allison" <ajp166@verizon.net> wrote:

> 
> On 05/30/2013 12:26 PM, Cory Smelosky wrote:
> > 
> > On Thu, 30 May 2013, Dave McGuire wrote:
> > 
> > > 
> > > On 05/30/2013 07:32 AM, allison wrote:
> > > > That said I've pushed the clock on 11/23 board once too see and the 13mhz
> > > > clock was wound up to 25mhz using a external source.  I started seeing
> > > > errors above 18mhz but they were random, seems bus timing and memory
> > > > timing all had to be happy and I was pushing the margins. I eventually
> > > > put in a clock module for 15.8mhz as I had one.  For that it was a modest
> > > > speed up as the 11/23 is slower to start with.
> > > 
> > > Hey!  I too overclocked an 11/23.  My friend Ernie and I were hanging
> > > out late one night hacking on one of his systems, and I had brought a
> > > spare 11/23 CPU along for this purpose.  We got it up to 16MHz; we
> > > didn't take it any higher than that.  I'm amazed to hear that you got it
> > > up to 25MHz!!
> > > 
> At 25 a lot of things were funky and not running well,  UODT was about it.
> Oddly heat wasn't the issue.  Clearly propagation delays were.
> 
> > 
> > This became extremely interesting when I read this quickly and saw "GHz" instead \
> > of "MHz" ;)
> I had to look to see I didn't mistype mhz! ;)
> 
> At that time generating a signal above 512mhz  was outside my equipment capability.

Ah.

> 
> Over the years I've pushed silicon many times and often heat was not the issue, as
> it would quit working usually long be fore that became an issue. Though I did take \
> a 6mhz z80 to 10mhz and it would run there only if kept cooler than 100F, any \
> warmer and it would quit and Z80s (nmos) run warm to start so it was glued to a \
> Peltier cooler to get down to about 50f.  It wasn't overheating it was a matter of \
> propagation times shifting enough with temperature to not make it anymore.  FYI it \
> only worked because I had memory that was faster than 35ns, the bus and control \
> signal timing had far worse timing margins than the actual 10mhz part.

z80s are interesting little things. ;)

> 
> Cmos parts like the 1802 can be pushed too, and since they run stone cold in their \
> normal speeds it's more timing issues that get in the way.

Try doing that with modern PC chipsets that run 20 times hotter. ;)

> 
> I ran the same test on a PDP-8e once and found with fast semiconductor (not \
> commercial) memory it was far faster and could hit 2x, almost, at that point all \
> the IO cards and the bus started to get really unhappy and stop talking.

Interesting how the bus didn't handle the higher clock rates as well…although it can \
be expected due to how they'd need to be precision.

> 
> Most of the time internal clock and other timing distribution or bus switching is \
> more likely to stop the show before junction heating.  Its only the higher density \
> parts that are already warm for that reaaon and can switch that fast but getting \
> rid of the heat is their problem.  Then again you need to assure the rest of the \
> hardware can keep up as often that is more of a limiting factor.

Just cool everything with liquid helium or similar. ;)

> 
> Allison


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