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Basic Clock Module Operation
4-6
Basic Clock Module
4.2.3 XT2 Oscillator
Some devices have a second crystal oscillator, XT2. XT2 sources XT2CLK
and its characteristics are identical to LFXT1 in HF mode. The XT2OFF bit
disables the XT2 oscillator if XT2CLK is not used for MCLK or SMCLK as
shown in Figure 4−3.
XT2 may be used with external clock signals on the XT2IN pin. When used with
an external signal, the external frequency must meet the datasheet
parameters for XT2.
Figure 4−3. Off Signals for Oscillator XT2
XT2OFF
CPUOFF
SELM1
SELM0
SCG1
SELS
XT2Off (Internal signal)
4.2.4 Digitally-Controlled Oscillator (DCO)
The DCO is an integrated ring oscillator with RC-type characteristics. As with
any RC-type oscillator, frequency varies with temperature, voltage, and from
device to device. The DCO frequency can be adjusted by software using the
DCOx, MODx, and RSELx bits. The digital control of the oscillator allows
frequency stabilization despite its RC-type characteristics.
Disabling the DCO
Software can disable DCOCLK by setting SCG0 when it is not used to source
SMCLK or MCLK in active mode, as shown in Figure 4−4.
Figure 4−4. On/Off Control of DCO
QD
Q
D
CL
CPUOFF
XSELM1
SCG1
SELS
SCG0
SMCLK
POR
DCOCLK
DCOCLK_on
1: on
0: off
DCO_Gen_on
1: on
0: off