Если проц LPC23xx, как минимум,то на PORT0 у него выходит Emulation Trace Modul. И управляется он следующим образом
Only bit 3 of this register is used to control the ETM interface pins. The value of the RTCK I/O pin is sampled when the external reset is asserted. When RTCK pin is low during external reset, bit 3 in PINSEL10 is set to enable the ETM interface pins. When RTCK pin is high during external reset, bit 3 in PINSEL10 is cleared to disable the ETM interface pins. The ETM interface control pin can also be modified by the software.