Can I write/encode an NFC tag using the BL65x?

The NFC capability of the BL652 is NFC-A Listen Mode, Type 2. It is exposed as a passive interface. Therefore, the BL652 NFC functionality currently allows for read-only (type 2) tags which can be read from an Active NFC reader, such as a smartphone or Arduino-based shield. It is not able to write or encode external tags.

More information on NFC capability on the BL652 can be found in section 7.1, "Near Field Communication (NFC) on page 314 of the BL652 smartBASIC Extensions User Guide

For the BL654, more information can be found in section 6.1 on page 344 of the BL654 smartBASIC Extensions User Guide