30 May
2013
30 May
'13
3:40 p.m.
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.
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.
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.
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.
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.
Allison