hello

i designed hardware for calculating rpm of a motor using 89c2051. i provided input through 4n35 to a pin of 89c2051.

the problem is that the system works fine under 15000 rpm but when rpm exceeds 15000 then starts behaving abnormal.

i want to know that is there any maximum limit for the pin of 89c2051 of sensing high low inputs.

thanks a lot

regards

hasan