68040/ bus query

[beachycove]

What happens if you plug, say, a 25MHz 68040 chip into a logic board meant for a 33MHz 68LC040 chip (LC575)?

Would you get a bus error/ crash, by any chance, due to the speed differential?

Explanation: I bought what was supposed to be a 33MHz 68040 off EBay a year ago, but could never get it to work as expected. The system (a 33MHz Color Classic "Mystic" running 8.1) booted, typically, but quickly crashed as reported. When it stayed alive long enough, though, it did report having a 33MHz 68040 in the System Profiler.

I figured at the time that the chip was bad, especially as it had arrived with badly bent pins, courtesy of the postal service and poor packaging. But I had a brainwave today: what if it were a slower chip?

Why not look and see, you say? Well, unfortunately, the previous owner epoxied a heat sink onto the chip, so that I cannot read the specs to confirm them. But if it were a slower chip, and I was sold the wrong item (I seem to recall that the seller had a couple of 25MHz 68040s for sale at the same time), would that explain the problem?

 

[Quadraman]

It depends on the chip. Some chips take to being overclocked better or worse than others due to variances in production. You also have to remember that any 040 chip you find is going to be 15+ years old, and the electrical pathways break down over time. It's a crapshoot, really. Sometimes you can use the stepping number to tell you if a particular run is more or less prone to overclocking based on reports from other overclockers using that version, but not always. If you are going to be putting a 25mhz chip into a 33mhz socket, then plan on digging up a heatsink, a tube of thermal compound, and possibly even a cooling fan.

 

[Patrickool93]

I have don the smae thing with my C650 '040 in my Performa 630. It worked fine and was stable. It ran and reported as a 33mhz.

 

[beachycove]

Well, on the strength of that (and having tried it in a Q605), I hereby declare the processor dead.

 

[TylerEss]

+1 you just overclocked it, which can cause problems but it should work for at least a few seconds.