This code emulats NFC tags (to be accurate ISO 14443-3 compatible type 2 tags). It's a "software-only" nfc-tag-emulator for 8 bit microcontrollers. Nothing more than 4 resistors, 3 capacitors and 1 diode are required to connect the RFID antenna to a microcontroller. The code is developed for an ATiny84 microcontroller.
Details on the circuit: https://www.nonan.net/nkruse/simplenfc