Presto 040, replace processor?

[ArbysTPossum]

A simple question, but I can't find any information, so maybe no one has tried. If I were to replace a 68LC40 on a Sonnet Presto 040 card with a full 68040 (With FPU), would the card accept the processor, and would the system (An LCII) be able to see/use the FPU? Or are there additional components that would need to be populated on the board for this to work?

I imagine the benefit of the FPU would be minor on a 16Mhz bus, but is it possible is the bigger question.

 

[joshc]

Yeah it should work, but actually swapping it would be difficult because those cards do not have a socketed processor. The processor is soldered directly on the board.

 

[Bolle]

Can confirm… FPU just works when a full 040 is installed.

 

[ArbysTPossum]

Excellent! Just what I wanted to hear. Totally gonna do this.

 

[olePigeon]

If you're replacing a 25MHz LC040, it may be possible to install a 33MHz or faster CPU and swap out the crystal on the card for an extra boost in speed.

 

[ArbysTPossum]

If you're replacing a 25MHz LC040, it may be possible to install a 33MHz or faster CPU and swap out the crystal on the card for an extra boost in speed.

Is this possible? If the bus is 16Mhz, would there be an issue with a 33Mhz processor on a 16Mhz bus? I haven't done the proc swap yet, but swapping the crystal sounds very interesting. I wonder how hard I could push this.

 

[olePigeon]

@ArbysTPossum People like @Bolle would know for certain. I think most accelerator's CPU do not use the host system's clock to determine the speed, but instead use its own crystal. But there are exceptions like some of the Quadra accelerators.

You'd want to determine how the crystal is utilized. Sometimes they're 1:1, but often they're 2:1 ratio (i.e. 20 MHz crystal for a 40 MHz chip.) It can be as easy as simply swapping the crystal out for a faster one. Other times it may require modifying resistors, installing faster components (such as for the cache), etc.

It varies from accelerator to accelerator.

Maybe you can post a picture of your accelerator?

 

[Bolle]

I haven’t actually tried how far you can go on the LC Presto040. Some cards came with 55MHz oscillators from the factory running the CPU at 27.5MHz.
I don’t think much more is possible because with the CPU sitting upside down in a tiny space with next to no airflow it’s already getting toasty at 25MHz.

 

[ArbysTPossum]

Follow-up necro-post. I did the thing, and it works. I didn't do a CPU socket or anything fancy. There was a plastic spacer under the CPU to keep it away from the logic board, I removed that in favor of some layers of kapton tape, which allowed the CPU to sit lower on the boar,d allowing a taller heatsink to be put on the CPU itself. However, I don't think the CPU needs a heatsink, or at least doesn't need a very good heatsink. It's only running at 25Mhz, and I think the CPU i have was rated to 30Mhz if I'm reading it right.

tl;dr Did it, works, runs great, happy. :]

 

[tt]

Nice! I thought about doing this, where did you end up finding the replacement CPU? Did you test it before committing to soldering it to the board?