The X1 and X2 pins double as GPIO19 and GPIO18. At power-up, these pins are in GPIO mode and the
on-chip crystal oscillator is powered off. The following procedure can be used to switch the
pins to X1 and X2 mode and enable the oscillator:
- Clear the XTALCR.OSCOFF bit.
- Wait for the crystal to power up. 1ms is the typical wait time but this depends on the
crystal that is being used.
- Clear the X1 counter by writing a 1 to X1CNT.CLR
and keep clearing until the X1 counter value in the X1CNT register is no longer saturated
2047 (0x7FF).
- Repeat steps 3-4 three additional times.
- Wait for the X1 counter value in the X1CNT
register to reach 2047 (0x7FF). Repeat steps 3-4 three additional times.
- Select XTAL as the OSCCLK source by writing a 1 to CLKSRCCTL1.OSCCLKSRCSEL.
- Check the MCLKSTS bit in the MCDCR register. If it's set, the oscillator has not finished
powering up, and more time is required:
- Clear the missing clock status by
writing a 1 to MCDCR.MCLKCLR.
- Repeat steps 2-7. Do not reset the
device. Doing so powers down the oscillator, which requires the procedure to be
restarted from step 1.
- If the oscillator has not finished
powering up in 10 milliseconds, there is a real clock failure.
- If MCDCR.MCLKSTS is clear, the oscillator startup is a success. The system clock is now derived
from XTAL.