The MCS 51 Microcontroller Manual states:
"There are no restrictions on the duty cycle of the external input signal, but to ensure that a given level is sampled at least once before it changes, it should be held for at least one full machine cycle."
For a controller that uses 12 clocks per cycle I would surmise that the input frequency would have to be something less than 1 MHZ, to make sure the required level is there at sampling time (for 8052 it is at S5P2). There are 6 States with 2 Periods each. S5P2 would be very near the end of each cycle.

Hope this helps

Hal