The P0 port is an open drain output in most 8051 derivatives. I suppose you use an external pull-up resistor. Even so, the output needs a not negligible amount of time to reach the high level. One single setb P0.3 might not be long enough (at 48 MHz). Try a pull-up resistor of about 2k2. Alternatively, try a different port, say P1.

The '1' to '0' transition is much faster, because the pull-down is performed by an active MOS transistor within the micro.

Alfredo del Rio.