The T5557 is a contactless R/W IDentification IC (IDIC ) for applications in the 125 kHz frequency range. A single coil, connected to the chip, serves as the IC's power supply and bi-directional communication interface. The antenna and chip together form a transponder or tag. The on-chip 330-bit EEPROM (10 blocks, 33 bits each) can be read and written blockwise from a reader. Block 0 is reserved for setting the operation modes of the T5557 tag. Block 7 may contain a password to prevent unauthorized writing. Data is transmitted from the IDIC using load modulation. This is achieved by damping the RF field with a resistive load between the two terminals Coil 1 and Coil 2. The IC receives and decodes 100% amplitude modulated (OOK) pulse interval encoded bit streams from the base station or reader.