Posted By
TLC
on 2010-08-18
13:56:53
|  Re: 7501 vs 8501 - Is one more reliable?
Hi!
Just wanted to comment on your experiences with 8501 CPUs... I could only ever see one single 7501 CPU in my whole life :-D, and that one was broken, too  , so I can't really comment on whether 7501s are actually more stable. I'd suppose (from that, which actually matches your findings  ) that the vast majority of 264 machines came out with 8501s anyway. ...As to "why" these are that prone to faults -- that's been discussed for a couple of times as I guess... the story must go back either to badly designed layout, underestimated power dissipation (and local overheat on the chip as a consequence), or bad quality silicone die. It's an interesting matter because earlier Commodore chips (the original 1MHz 6502 and its counterparts) are usually really stable, even though some of them are the first representatives of the "cheap" manufacturing process invented by MOS Technology itself (which involves fixing and packaging a great percentage of faulty silicone chips)... The 8501 is basically a 2MHz 6502 core with some small additions... it obviously dissipates more heat than a 1MHz one. Did they underestimate that?... I don't know. The "way" these chips die is said to be due to cracks starting up from silicone die irregularities, and is triggered by heat (dilatation); some cracks would eventually cut vital traces on the silicone die, resulting in a fault. (That may be the answer to your second question: whether some crack cuts some vital trace, or just something that causes partial problems, is completely unpredictable). It's suggested (but not proven) that applying some simple cooling (like heatsinks) could prevent, or delay the, well, sad end...
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