Hi..

If you mean PULSE Generator, then why using 8051 ??
because anything you add to it will slow you down check
8038.
unless,
I Think, if you use its internal memory, and connect
Pin to GND, you get from pin31 Pulses (Crystal/12)
which about 1Mhz, but I'm not sure if you can count on it,
and what its reliability, I Never used it.
Also you might want to use other 8051 based, which can
reach to 40Mhz, I suppose there's even faster..
Hope Its helping.
Goodluck
OK