Hello everyone..
I would like to know how fast AT89C51 can count the external signals with the internal clock. I saw the datasheet http://www.keil.com/dd/docs/datashts/atmel/at89c51_ds.pdf but I do not understand How fast it can count external signals

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I believe, the maximum count rate is 1/24th of the OSC Clock Frequency
and the State of the Input Pin should be held steady for 12 clock cycles ( 1 machine cycle ),
to guarantee that current State is sampled.

Specifically ...
a) The Counter Input Pin is sampled during the S5-P2 execution state.
b) The Count register is updated on the following S3-P1 execution state..