An apparatus and method for waking up a main processor (MP) in a low power or ultra-low power device preferably includes the MP, and a sub-processor (SP) that utilizes less power than the MP to monitor ambient conditions than the MP, and may be internalized in the MP. The MP and SP can remain in a sleep mode while an interrupt sensor monitors for changes in the ambient environment. A sensor is preferably an interrupt-type sensor, as opposed to polling-type sensors conventionally used to detect ambient changes. The MP and SP may remain in sleep mode, as a low-power or an ultra-low power interrupt sensor operates with the SP being in sleep mode, and awakens the SP via an interrupt indicating a detected change. The SP then wakes the MP after comparing data from the interrupt sensor with values in storage or with another sensor.