# Wake-up Time

The normal wake-up time for the device is six main clock cycles (CLK_PER), plus the time it takes to start the main clock source and the time it takes to start the regulator if it has been switched off:
• In Idle sleep mode, the main clock source is kept running to eliminate additional wake-up time
• In Standby sleep mode, the main clock might be running depending on the peripheral configuration
• In Power-Down sleep mode, only the internal 32.768 kHz oscillator and the Real-Time Clock (RTC) clock source may be running. These are used by the Brown-out Detector (BOD), Watchdog Timer (WDT) or Periodic Interrupt Timer (PIT). All the other clock sources will be OFF.
Table 1. Sleep Modes and Start-Up Time
Sleep Mode Start-Up Time
Idle Six clock cycles
Standby Six clock cycles + one OSC start-up
Power-Down Six clock cycles + one OSC start-up

The start-up time for the different clock sources is described in the CLKCTRL - Clock Controller section.

In addition to the normal wake-up time, it is possible to make the device wait until the BOD is ready before executing the code. This is done by writing 0x3 to the BOD operation mode in the Active and Idle (ACTIVE) bit field in the BOD Configuration (FUSE.BODCFG) fuse. If the BOD is ready before the normal wake-up time, the total wake-up time will be the same. If the BOD takes longer than the normal wake-up time, the wake-up time will be extended until the BOD is ready. This ensures correct supply voltage whenever code is executed.