The procedure for using an external
oscillator connected to the X1 pin is similar to the procedure for using a crystal
or resonator:
- Clear the XTALCR.OSCOFF bit.
- Set the XTALCR.SE bit to enable
single-ended mode.
- 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).
- Wait for the X1 counter value in
the X1CNT register to reach 2047 (0x7FF).
- Repeat steps 3 and 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 the bit is set, either the external oscillator or the device
has failed.
- If MCLKSTS is clear, the switch
to the external clock is a success. The system clock is now derived from
XTAL.