The HCS412 combines the patented KEELOQ? code hopping technology and bi-directional transponder challenge-and-response security into a single chip solution for logical and physical access control. High security learning mechanisms make this a turnkey solution when used with the KEELOQ? decoders. When the HCS412 is used as a code hopping encoder device, it is ideally suited to keyless entry systems, primarily for vehicles and home garage door openers. It is meant to be a cost-effective, yet secure solution to such systems. The HCS412 can also be used as a secure bi-directional transponder for verification of a token. The HCS412 has a sensitive transponder input that increases the communication range beyond traditional transponder systems. This makes the HCS412 ideal for passive keyless entry, secure access control and identification applications. A single HCS412 can be used as an encoder for Remote Keyless Entry (RKE) and a transponder for immobilization in the same circuit and thereby dramatically reducing the cost of hybrid transmitter/transponder circuits.