Atmel AT32UC3A3128, Manual

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32072H–AVR32–10/2012
AT32UC3A3
Figure 7-2. Oscillator Connections
7.5.3 32 KHz Oscillator Operation
The 32 KHz oscillator operates as described for Oscillator 0 and 1 above. The 32 KHz oscillator
is used as source clock for the Real-Time Counter.
The oscillator is disabled by default, but can be enabled by writing OSC32EN in OSCCTRL32.
The oscillator is an ultra-low power design and remains enabled in all sleep modes except Static
mode.
While the 32 KHz oscillator is disabled, the XIN32 and XOUT32 pins are available as general
purpose I/Os. When the oscillator is configured to work with an external clock (MODE field in
OSCCTRL32 register), the external clock must be connected to XIN32 while the XOUT32 pin
can be used as a general purpose I/O.
The startup time of the 32 KHz oscillator can be set in the OSCCTRL32, after which OSC32RDY
in POSCSR is set. An interrupt can be generated on a zero to one transition of OSC32RDY.
As a crystal oscillator usually requires a very long startup time (up to 1 second), the 32 KHz
oscillator will keep running across resets, except Power-On-Reset.
7.5.4 PLL Operation
The device contains two PLLs, PLL0 and PLL1. These are disabled by default, but can be
enabled to provide high frequency source clocks for synchronous or generic clocks. The PLLs
can take either Oscillator 0 or 1 as reference clock. The PLL output is divided by a multiplication
factor, and the PLL compares the resulting clock to the reference clock. The PLL will adjust its
output frequency until the two compared clocks are equal, thus locking the output frequency to a
multiple of the reference clock frequency.
When the PLL is switched on, or when changing the clock source or multiplication factor for the
PLL, the PLL is unlocked and the output frequency is undefined. The PLL clock for the digital
logic is automatically masked when the PLL is unlocked, to prevent connected digital logic from
receiving a too high frequency and thus become unstable.
XIN
XOUT
C
2
C
1