Allen P. Drummond wrote:
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On some 8051 derivatives there is a PCA counter which will advance every clock cycle giving twelve times improvement in resolution.

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Actually, the PCA in that microprocessor allows you to select the PCA counter clock source as OSC/12 (ie normal) or OSC/4 (three times improvement) or and overflow or external source.

Three times improvement, not twelve.

Of course, the usefulness of this resolution depends upon what you are going to do with it.

Usually the solution in a design is to identify your most critical timing issue and select the crystal and clock solution accordingly. And remember, microprocessor crystals vary quite a bit with temperature so don't be deceived by your calculations... the real world can be a disappointment.

The best RX+/RX2 class micro I've seen is the Temic(Now Atmel) T89C51RD2. They have the internal 12 or 6 clock mode select as in the Philips RX2, BUT they allow you to select the fast or slow clocks to any feature onchip device requiring a clock source. This is a wonderful addition.

aka j