Datasheet

138
32133D–11/2011
UC3D
13.4.3 Debug Operation
The SCIF module does not interact with debug operations.
13.4.4 Clocks
The SCIF controls all oscillators on the part. Those oscillators can then be used as sources for
for generic clocks (handled by the SCIF) and for the CPU and peripherals. (In this case, selec-
tion of source is done by the Power Manager.)
13.5 Functional Description
13.5.1 Oscillator (OSC0) Operation
The main oscillator (OSC0) is designed to be used with an external 4 to 20 MHz crystal and two
biasing capacitors, as shown in Figure 13-1. The oscillator can be used for the main clock in the
device, as described in the Power Manager chapter. The oscillator can be used as source for the
generic clocks, as described in ”Generic clocks” on page 140.
The oscillator is disabled by default after reset. When the oscillator is disabled, the XIN and
XOUT pins can be used as general purpose I/Os. When the oscillator is configured to use an
external clock, the clock must be applied to the XIN pin while the XOUT pin can be used as a
general purpose I/O.
The oscillator can be enabled by writing to the OSCEN bits in OSCCTRL0. Operation mode
(external clock or crystal) is chosen by writing to the MODE field in OSCCTRL0. The oscillator is
automatically switched off in certain sleep modes to reduce power consumption, as described in
the Power Manager chapter.
After a hard reset, or when waking up from a sleep mode that disabled the oscillator, the oscilla-
tor may need a certain amount of time to stabilize on the correct frequency. This start-up time
can be set in the OSCCTRL0 register.
The SCIF masks the oscillator outputs during the start-up time, to ensure that no unstable clocks
propagate to the digital logic. The OSC0RDY bit in PCLKSR is automatically set and cleared
according to the status of the oscillator. A zero to one transition on this bit can also be configured
to generate an interrupt, as described in Section 13.6.1.
Figure 13-1. Oscillator connections.
C
LEXT
C
LEXT
C
L
XOUT
XIN
C
i