The e5561 is a member of Atmel's IDentification IC (IDIC) family for applications where information has to be transmitted contactlessly. The IDIC is connected to a tuned LC circuit for power supply and bi-directional data communication (Read/Write) to a base station. Atmel offers an LC circuit and a chip assembled in the form of a transponder or tag. These units are small, smart and rugged data storage units. The e5561 is a Read/Write crypto IC for applications which demand higher security levels than standard R/W transponder ICs can offer. For that purpose, the e5561 has an encryption algorithm block which enables a base station to authenticate the transponder. The base station transmits a random number to the e5561. This challenge is encrypted by both IC and base station. The e5561 sends back the result to the base station for comparison. As both should possess the same secret key, the results of this encryption are expected to be equal. Any attempt to fake the base station with a wrong transponder will be recognized immediately. The on-chip 320-bit EEPROM (10 blocks of 32 bits each) can be read and written blockwise by a base station. Two or four blocks contain the ID code and six memory blocks are used to store the crypto key as well as the read/write options. The crypto key and the ID code can be protected individually against overwriting. Likewise, the crypto key cannot be read out. 125 kHz is the typical operational frequency of a system using the e5561. Two read data rates are programmable. Reading occurs through damping the incoming RF field with an on-chip load. This damping is detected by the field-generating base station. Data transmission starts after power-up with the transmission of the ID code and continues as long as the e5561 is powered. Writing is carried out with Atmel's writing method. To transmit data to the e5561, the base station has to interrupt the RF for a short time to create a field gap. The information is encoded in the number of clock cycles between two subsequent gaps.