Data Sheet

185
Atmel | SMART SAM D21 [DATASHEET]
Atmel-42181G–SAM-D21_Datasheet–09/2015
Notes: 1. Number of cycles for the start-up counter.
2. Number of cycles for the synchronization delay, before PCLKSR.XOSC32KRDY is set.
3. Start-up time is n OSCULP32K cycles + 3 XOSC32K cycles.
z Bit 7 – ONDEMAND: On Demand Control
The On Demand operation mode allows an oscillator to be enabled or disabled depending on peripheral clock
requests.
In On Demand operation mode, i.e., if the ONDEMAND bit has been previously written to one, the oscillator will
only be running when requested by a peripheral. If there is no peripheral requesting the oscillator s clock source,
the oscillator will be in a disabled state.
If On Demand is disabled the oscillator will always be running when enabled.
In standby sleep mode, the On Demand operation is still active if the XOSC32K.RUNSTDBY bit is one. If
XOSC32K.RUNSTDBY is zero, the oscillator is disabled.
0: The oscillator is always on, if enabled.
1: The oscillator is enabled when a peripheral is requesting the oscillator to be used as a clock source. The oscilla-
tor is disabled if no peripheral is requesting the clock source.
z Bit 6 – RUNSTDBY: Run in Standby
This bit controls how the XOSC32K behaves during standby sleep mode:
0: The oscillator is disabled in standby sleep mode.
1: The oscillator is not stopped in standby sleep mode. If XOSC32K.ONDEMAND is one, the clock source will be
running when a peripheral is requesting the clock. If XOSC32K.ONDEMAND is zero, the clock source will always
be running in standby sleep mode.
z Bit 5 – AAMPEN: Automatic Amplitude Control Enable
0: The automatic amplitude control for the crystal oscillator is disabled.
1: The automatic amplitude control for the crystal oscillator is enabled.
z Bit 4 – Reserved
This bit is unused and reserved for future use. For compatibility with future devices, always write this bit to zero
when this register is written. This bit will always return zero when read.
Table 16-9. Start-Up Time for 32kHz External Crystal Oscillator
STARTUP[2:0]
Number of
OSCULP32K Clock
Cycles
Number of
XOSC32K Clock
Cycles
Approximate Equivalent Time
(OSCULP = 32kHz)
(1)(2)(3)
0x0 1 3 122µs
0x1 32 3 1068µs
0x2 2048 3 62592µs
0x3 4096 3 125092µs
0x4 16384 3 500092µs
0x5 32768 3 1000092µs
0x6 65536 3 2000092µs
0x7 131072 3 4000092µs