Hello,
the shift register I'm writing into (74ACH595) is edge triggered. The following code does not work for MCU running 48MHz while works fine in 12MHz mode:

... ; init serial input
SETB P0.3 ;clk up
CLR P0.3 ;clk down


The modified snippet works for 48MHz also:
... ; init serial input
SETB P0.3 ;clk up
SETB P0.3 ;clk up
SETB P0.3 ;clk up
CLR P0.3 ;clk down