1) i,m using AT89c52, but 89C5115 and 87C51FA have PCA, and these two devices are similar to AT89c51 which is also exact the same like AT89c52 , so i thinking they are the same as they have the same series.

2) so now if AT89c52 don`t have PCA, means i have to use another register to run the stepper motors at full speed, so what register you suggest.

1)That is what datasheets are for, assuming that one derivative is 'similar to another' without consulting the datasheet is fools errand.

2) the best solution is to get hold of a derivative with a PCA, but I have seen solutions using timer2 which you can check in the datasheet to see if your ACTUAL chip has.

ALL the letters and no misspellings is a request for the actual and FULL name of the derivative you are using.

Erik