FAQ2.pdf

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Use of PC-type Clock Generators

Q: Can I use a standard PC-type clock synthesizer with my MPC106-based PowerPC

system?

No. The standard PC-type clock generators allow anywhere from 1 to 4 ns skew to occur between

PCI clocks (typically 33 MHz) and the CPU clock (bus frequency, average of 66 MHz). Since the

MPC106 uses the PCI clock to generate a matching CPU clock, any skew between the two causes

the MPC106 and the PowerPC processor to be out of sync by 1-4 ns. The setup and hold times of

the two devices cannot tolerate this.

Any MPC106 based design must use a skew-controlled clock generator. Many companies make

these, including Motorola (MPC980, MPC972), ICWorks (W42B972), AMCC(SC3506), TI

(CDC2586) as well as others.

NOTE:

It is not possible to compensate for this skew by altering trace lengths or using delay

lines. All known parts allow part-to-part variation of 1-4 ns, so this technique would fail.