Remote controlled garage door opening system

A radio frequency transmitter for remotely communicating with a receiver which is operable to transmit a command signal to cause the opening and closing of a garage door, said radio frequency transmitter being one of a set of multiple radio frequency transmitters having associated therewith a manufacturer's key designating a particular manufacturer;

• (a) said radio frequency transmitter having stored therein a transmitter identifying code comprising a serial number code portion unique to that transmitter, a secret key code portion unique to that transmitter, a synchronization value code portion which increments each time the transmitter is actuated, and a non-linear function generator for generating a multi-bit hopping code which changes each time the transmitter is actuated, said transmitter transmitting a coded signal representative of the hopping code and the serial number code portion;
• and
• (b) said receiver being of the type comprising a microprocessor operable between a learn mode and an operate mode;
• (i) said receiver further having said manufacturer's key stored in said receiver at a factory, the receiver receiving the hopping code and the serial number code portions transmitted during said learn mode, with said stored manufacturer's key and the received serial number code portion associated with said radio frequency transmitter used to independently generate in said receiver a secret key, and thereafter a synchronization value, corresponding to the secret key code portion and synchronization value of the particular transmitter;
• (ii) processing circuitry within said receiver storing, during said learn mode, said so-generated secret key and so-generated synchronization value, randomly in an unused discrete memory location, or if all said discrete memory locations are used, then by replacing the information in the randomly chosen memory location with the new information;
• (iii) said receiver, during said operate mode, performing a non-linear decoding function on said intercepted hopping code using one of said stored secret keys, thereby to generate a second synchronization value, comparing said second synchronization value with said stored synchronization value and generating said command signal when said second synchronization value bears, and is within a window of, a predetermined relationship with said stored synchronization value, in the absence of said predetermined relationship, performing another non-linear decoding function on the intercepted hopping code using a different one of said stored secret keys to generate another second synchronization value, and continually repeating the sequence until a second synchronization value is found which bears the said predetermined relationship with said stored synchronization value.