A deep idle mode for electronic devices is described, which provides significant power savings while allowing significantly shorter resumption times than experienced with a suspend mode. During deep idle mode, a root clock such as the microcontroller unit phase-locked loop (MPLL) is scaled or gated entirely and other clocks such as the processor, memory, and general purpose timer clocks may be scaled. To maintain functionality while these clocks are scaled or gated, an external clock source couples to the processor, memory, and a general purpose timer.