Overclocking Apollo 1240/40 to 50MHz?

[PMC]

It depends on a lot of things...  If the CPU is a rc model, then it's ceramic coated which makes it better at conducting heat.  Is the CPU relatively new?  The older the CPU generally the less tolerant to overclocking it will be.

40Mhz is the upper design limit for an '040.  I managed to safely and successfully overclock my 25Mhz unit to that speed, but it wouldn't go any further.  

If I were you I'd either invest in an '060 accelerator or if you are determined to overclock I'd suggest going in steady increments of say 2Mhz over and above the rated speed.

You might see Mac '040s advertised as being 50Mhz units.  This is because as you know the processor cycles every second clock pulse.  I don't know of anyone getting more than 44-45Mhz out of an '040 but you can always try...

[PMC]

@Tj

It's not just heat, it's current too.  You'll need to source a pokey PSU to supply the required juice for the A1200, HDD (required to load an os containing '040 specific libraries), CPU and extra memory.  

The A500's stock PSU struggled on my 25Mhz system.

On the plus side, my 25Mhz '040 was fitted with a heatsink between the fan and surface of the chip and ran happily at 40Mhz without problem.  I tried maxing it to 50Mhz, but all I got was a black screen, but the system was none the worse after being reset to 40Mhz again.

[PMC]

tomekm wrote:
ok, let`s verify some things:

-afaik the fastest version of 040 is 50MHz.

-My 040 is the RC revision.

No, the fastest version of the '040 is 40Mhz.  Some Mac expansions were shown as beiing 50Mhz becuase the 25Mhz 68040 needs a 50Mhz oscillator as it's timed to cycle on every second oscillator pulse.  If you check your CPU you will find that an 80Mhz oscillator is fitted, thus giving 40Mhz chip speed.

Seeing as your system is in a tower with a fan and a pokey PSU, you have scope to dissipate the heat and power your system.  However, running an '040 reliably at speeds higher than 40Mhz is difficult because it's right at the limit of the design capability.  It's up to you, but I wouldn't take it beyond 44Mhz.  To try and run it at 50Mhz is asking it for more than a 20% increase in speed which is likely to be more than your '040 can handle.  

At best, you'll have paid for a 100 Mhz oscillator and be greeted with a black screen on bootup.  At worst, your CPU will destroy itself, possibly taking the accelerator card and memory with it.  

[PMC]

Yeah, I too ran my 25Mhz '040 at 40Mhz without issue.  

Amiga Format ran an article back in 1998 when they explained the pros and cons.  They explained it a little like this.

Because the 68K series is getting quite old, normally only one set of dies is used to produce the CPU.  These are then tested at varying speeds, and rejects are quickly dumped.  These chips are pressed into the ceramic cases and the speed ratings are applied.  It's quite possible that all 68040 CPUs produced since are manufactured to 40Mhz tolerances, but down rated accordingly.

http://www.cpu-world.com/CPUs/68040/

Note that according to CPU-World, the 68040 was maxed at 40Mhz.  I've never heard of a 68040 being rated at a higher speed.

Try it by all means, but I offer no guarantees that you won't suffer system damage as a result.