Смотря в каком LPC- в новых эту "фичу" пофиксили. On the LPC2141/2/4/6/8 (unlike earlier Philips ARM devices) the SSEL0 pin can be
used for a different function when the SPI0 interface is only used in Master mode. For
example, pin hosting the SSEL0 function can be configured as an output digital GPIO
pin and used to select one of the SPI0 slaves.