A processing system operates in a first power domain and includes a first memory, a memory access circuit, and a first processing circuit. The first memory stores sound data detected by a microphone. The memory access circuit transfers the sound data to a second memory according to a first command, in order to store the sound data as voice data. The first processing circuit outputs a second command according to a human voice detection signal. The second command is for enabling a second processing circuit, in order to determine whether the voice data in the second memory matches a predetermined voice command. One of the first and the second processing circuits outputs the first command. The second processing circuit operates in a second power domain. A power consumption to which the first power domain corresponds is lower than a power consumption to which the second power domain corresponds.