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387 oscillators

iulianv

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Jan 5, 2012
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On 386 mainboards (at least on the ones I have, or have had so far) the CPU frequency is half of the frequency of the oscillator installed on the board. On some boards there is also a socket for a second oscillator, intended for a math coprocessor of a different frequency than that of the CPU. The question is: does the same "1/2" rule apply in this case? I mean, in order to install a 33MHz NPU do I need a 66MHz oscillator?
 

orion24

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Oct 19, 2011
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On 386 mainboards (at least on the ones I have, or have had so far) the CPU frequency is half of the frequency of the oscillator installed on the board. On some boards there is also a socket for a second oscillator, intended for a math coprocessor of a different frequency than that of the CPU. The question is: does the same "1/2" rule apply in this case? I mean, in order to install a 33MHz NPU do I need a 66MHz oscillator?

Never seen a board with a socket for a second one so far. More likely it should be the same, but the mainboard manual should clearly state, in case they intended the end-user to plug an oscillator in there. Basically I dough they had that in mind. Maybe these are "cut down" versions of boards that support both sync and async FPU operation and had an oscillator there as well. The rest should be jumper-configurable.

Do you have any pics BTW? The whole "oscillator" story has my interest lately, as an attempt to overclock 386 systems, where more voltage can only be done via mods and the frequency increasing steps (25-33-40-50) are too big to expect CPUs to handle them.
 

Old Thrashbarg

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not sure about the oscillator speed, but ONLY intel 387's can run async. cyrix, iit, ulsi, etc have to run sync.

I have a Cyrix x87DLC running async (at 33mhz, from a 66mhz oscillator) in my 386DX40 system... doesn't seem to have any problems.
 

iulianv

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I have a Cyrix x87DLC running async (at 33mhz, from a 66mhz oscillator) in my 386DX40 system... doesn't seem to have any problems.

So it is 1/2 after all... do you happen to have a pic of the mainboard? :)

Also, which is the most accurate benchmark/sysinfo software in detecting/measuring the frequency of a 387 NPU?
 

Old Thrashbarg

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No, don't have any pictures, but there's really not much to see anyway... it's just another oscillator the same as the one for the main CPU, except 66mhz rather than 80mhz. It's set with a jumper... with the jumper in one position the NPU uses the same oscillator as the CPU, and with the jumper in the other position the NPU uses its own dedicated oscillator.
 
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