Can the NSS pin of CP240x be connected to GND?
No. CP2400/2 devices have a 4-wire Serial Peripheral Interface (SPI) which provides access to the internal registers and memory. The active-low slave-select (NSS) signal allows support for multiple slave devices on a single bus. It is also used to detect the start and end of a SPI transfer for CP2400/2, and should be connected to the SPI master instead of connection to a static low level signal, e.g. GND. The SPI transaction ends when NSS is de-asserted.