NXP Semiconductors K22F series, Reference Manual, Page: 585 / 1407

26.8.1.2 Oscillator startup
The OSC enters startup state when it is configured to generate clocks (internally the
OSC_EN transitions high) using the internal oscillator circuits by setting the external
reference clock selection bit. In this state, the OSC module is enabled and oscillations are
starting up, but have not yet stabilized. When the oscillation amplitude becomes large
enough to pass through the input buffer, XTL_CLK begins clocking the counter. When
the counter reaches 4096 cycles of XTL_CLK, the oscillator is considered stable and
XTL_CLK is passed to the output clock OSC_CLK_OUT.
26.8.1.3 Oscillator Stable
The OSC enters stable state when it is configured to generate clocks (internally the
OSC_EN transitions high) using the internal oscillator circuits by setting the external
reference clock selection bit and the counter reaches 4096 cycles of XTL_CLK (when
CNT_DONE_4096 is high). In this state, the OSC module is producing a stable output
clock on OSC_CLK_OUT. Its frequency is determined by the external components being
used.
26.8.1.4 External Clock mode
The OSC enters external clock state when it is enabled and external reference clock
selection bit is cleared. For details regarding external reference clock source in this MCU,
see the chip configuration details. In this state, the OSC module is set to buffer (with
hysteresis) a clock from EXTAL onto the OSC_CLK_OUT. Its frequency is determined
by the external clock being supplied.
26.8.2 OSC module modes
The OSC is a pierce-type oscillator that supports external crystals or resonators operating
over the frequency ranges shown in
Table 26-3 . These modes assume the following
conditions: OSC is enabled to generate clocks (OSC_EN=1), configured to generate
clocks internally (MCG_C2[EREFS] = 1), and some or one of the other peripherals
(MCG, Timer, and so on) is configured to use the oscillator output clock
(OSC_CLK_OUT).
Table 26-3. Oscillator modes
Mode Frequency Range
Low-frequency, high-gain f
osc_lo
(32.768 kHz) up to f
osc_lo
(39.0625 kHz)
Table continues on the next page...
Chapter 26 Oscillator (OSC)
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
NXP Semiconductors 585