Start-up Time

All crystals have a start-up time, which is the time it takes for the crystal output to become stable. The start-up time of a given crystal is specified in the crystal data sheet. There are multiple choices on the tinyAVR 1-series and megaAVR 0-series for setting the start-up time, with the maximum value being 65535. This means that the device counts 65536 cycles with a frequency of 32.768 kHz, which translates to a start-up time of two seconds. When the start-up time has expired, the XOSC32K Status bit in the Main Clock Status register (MCLKSTATUS.XOSC32KS) will be set.

Note: The 32.768 kHz crystal will not automatically be started when the Enable bit in the 32.768 kHz Crystal Oscillator Control A register (XOSC32KCTRLA.ENABLE) is written to ‘1’. It will only be started when the 32.768 kHz clock is requested by a peripheral, for example the main clock or RTC, or if the Run Standby bit in XOSC32KCTRLA (XOSC32KCTRLA.RUNSTDBY) is set.

Note: MCLKSTATUS.XOSC32KS will not be set unless the 32.768 kHz clock is requested by a peripheral. This is also true when XOSC32KCTRLA.RUNSTDBY is set.