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FSB speed limit

Posted on 2002-03-18
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Last Modified: 2011-09-20
Currently, the FSB is 333Mhz , why can it go faster. Is it because the current bus it's not true 32 bit and it's  only 16+16 bit bus, hence the limitation to speed up to 1.0Ghz or something else.



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Question by:hhheng
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pjknibbs earned 200 total points
ID: 6879201
I think you're asking, "Why can't FSB go above 333MHz?". Actually, the fastest FSB around is currently 166MHz--to get 333MHz they use trickery to transfer data on both the rising and falling edges of the clock pulses.

As for the reason, it's nothing to do with the number of bits, and PCI is true 32-bit anyway. (SDRAM is actually 64-bit in both its single and double data rate forms). The limitation is this: you have 64 separate data lines running between the CPU and main memory. Whenever you send signals down these lines you're turning them into tiny radio transmitters, and adjacent lines can act like aerials and pick up these signals, thus corrupting their own data. This effect gets worse as frequency rises, so basically there comes a point where you can't reliably transmit signals down a parallel arrangement.

This is why RDRAM was invented--since RDRAM is a serial system and uses very few wires, there's less crosstalk and the signals can go faster.

Oh, and if you're wondering how CPUs can go as fast as they do without encountering this problem internally, it's because the problem diminishes with the size of the wires involved--this is at least part of the reason why the trend in CPU design has been to smaller and smaller transistors.
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by:hhheng
ID: 6879426
Wow.. you really know your stuff well.
Thanks and cheers..
Heng
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