Low power digital pdm microphone interfacing for always-on applications

Abstract

An encoding technique for reducing the power of PDM microphones is disclosed. Digital MEMS microphones utilize a modulation technique called Pulse Density Modulation (PDM), where a single data line (PDMDAT) is used to convey the digital information from the microphone source to a receiver. A characteristic of PDM is that a low noise signal will produce the most transitions, a zero signal will produce an alternating bitstream of logic-1s and logic-0s, and low noise bitstreams will be rich in singleton and doubleton 1s/0s. Typically, CMOS drivers transmit the PDM bitstream signal. CMOS drivers consume power primarily when they transition, so a bitstream rich in singletons and doubletons will increase power consumption. Differential encoding with an XNOR function is used as a singleton-suppression encoder, and a differential encoding with an XOR function is used as a doubleton-suppression encoder. In some embodiments, such as a dual PDM microphone configuration, the microphones alternate sending data on the rising (transition to logic-1) and falling (transition to logic-0) edges of PDMCLK. In other embodiments, a Voice Activity Detection (VAD) function may be added. In some other embodiments, a suppressed clock pulse duration modulator may be added.