The confusion might arise from the fact, that ALL contemporary '51 derivatives (at least those which are pin compatible with the original '51, e.g. the popular AT89C5x family) are specified for maximum clock frequencies well beyond the "traditional" 12MHz (usually at least for 20MHz); yet most of the textbooks still mention 12MHz as the '51 design limit (which was true some 15-20 years ago).

So, as Andy said, consult the datasheet of the particular derivative (and name that derivative here, too).

I add a caveat, increasing the mcu's clock you are likely to change also the timing of any circuits attached to it, so you must watch out for them too (check in their datasheets).



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Jan Waclawek