US Patent 7328122 Speed sensor

A speed sensor includes a magnetic pickup generating a cyclical output based upon passing teeth of a rotating gear. A counter counts the number of clock pulses that are generated during a measurement period of the cyclical output. The measurement period, in terms of cycles of the cyclical output, is varied based upon the number of pulses that were counted in a previous measurement period. Thus, at lower speeds, the number of clock pulses generated during fewer cycles is counted, while at higher speeds the number of clock pulses generated during more cycles is counted.