The terminal of the coil is connected to the GP4 port. In order to transmit data to the RFID reader, we have to modulate the low-frequency carrier by changing the coupling between the reader and tag antennas. We can achieve this by switching the GP4 as input
The IO pins and the external coil form an LC resonant circuit and act as an antenna. This antenna will pick up the carrier generated by the RFID reader.
The recovered carrier is rectified thanks to the bridge formed with the clamping diodes, feeding back the resulting signal to the power supply of the microcontroller.