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68040 clock doubling



I was reading in here about Motorola erroneously saying some of its 040 
chips run at 80MHz when their true speed is 40MHz and ditto 33/66 MHz 
systems etc.

Why is this different from clock doubling as in the 486DX/2 CPU?

I understand that most 68040s have two clock signals a PCLK and a BCLK, 
the BCLK runs at 1/2 the speed of the PCLK.

And the BCLK is used for all bus timing, whereas the PCLK is used for 
internal processor logic timing.

Now as far as I can tell the only difference between this and the DX/2, 
is that the DX/2 only has one clock and uses a clock 2x multiplier to 
derive the internal processor speed. Why do the two clocks on the '040 
_not_ have the same effect as clock doubling?

Can the two run out of sync for instance?

Bye...


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