Hi,
The LPC55S16 does not support I/O expander feature. For the LPC55S16, almost every signal is multiplexed with GPIO function, but the total GPIO pin number is totally dependent on package.
If you want to expand I/O cell, you have to use the chip for example PCAL9554B or PCAL9554C which use I2C to expand I/O.
https://www.nxp.com.cn/docs/en/data-sheet/PCAL9554B_PCAL9554C.pdf
it is okay to use a MCU with large package for example LPC54628, it has 208 pins, of course, a lot of GPIO pins.
https://www.nxp.com/products/processors-and-microcontrollers/arm-microcontrollers/general-purpose-mc...
with each GPIO pin of LPC family, you can set or clear the pin logic in output mode, or input the pin logic in input mode.
Hope it is helpful
BR
XiangJun Rong
