When the MCU wakes up from sleep modes deeper than Idle (with the exception of the two standby modes), the system clock source must stabilize before the CPU starts to operate. This introduces a short delay, which depends on the selected clock source. If an internal RC oscillator or external clock is used, the start-up delay is 6 cycles. This is in addition to the RC oscillator start-up time. If the XTAL oscillator is used, the start-up delay is configurable. If frequency stability is wanted, it is recommended with start-up delays of 1,000 cycles for ceramic resonators and 16,000 cycles for quartz crystals respectively. This is in addition to the oscillator start-up time, which will depend on the resonator and load capacitances. In addition, there is a 13 cycle minimum delay before an Interrupt Service Routine (ISR) starts executing after wake-up. This is due to, e.g. the program counter being pushed on the stack and the jump to the ISR.
During the start-up delay, the power consumption is close to the power consumption in Idle, and thus represents “inefficient” power. If possible, it is therefore recommended to wake up as seldom as possible and rather “do more” every time the device wakes up.
To minimize the wake-up delay and conserve power, use an RC oscillator or external clock source, and wake up as seldom as possible.