US Patent 7340059 Programmable scrambler and De-scrambler for digital telephony equipment

A scrambler circuit for digital telephony equipment receives a sequence of input bits, generates the sequence of output bits based on the input bits and stores the sequence of output bits. The generating of the sequence of output bits includes selecting at least one of the stored output bits in accordance with contents of a mask register, and applying a logic operation to a current input bit and the selected at least one stored output bit to provide a current output bit.