Datasheet

SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
522
27.5 Main Clock
Figure 27-3 shows the Main Clock block diagram.
Figure 27-3. Main Clock Block Diagram
The Main Clock has two sources:
 4/8/12 MHz Fast RC Oscillator which starts very quickly and is used at startup.
 3 to 20 MHz Crystal or Ceramic Resonator-based Oscillator which can be bypassed.
27.5.1 4/8/12 MHz Fast RC Oscillator
After reset, the 4/8/12 MHz Fast RC Oscillator is enabled with the 4 MHz frequency selected and it is selected as
the source of MAINCK. MAINCK is the default clock selected to start up the system.
The Fast RC Oscillator 8 and 12 MHz frequencies are calibrated in production. Note that is not the case for the 4
MHz frequency.
Please refer to the “DC Characteristics” section of the product datasheet.
The software can disable or enable the 4/8/12 MHz Fast RC Oscillator with the MOSCRCEN bit in the Clock
Generator Main Oscillator Register (CKGR_MOR).
The user can also select the output frequency of the Fast RC Oscillator, either 4 MHz, 8 MHz or 12 MHz are
available. It can be done through MOSCRCF bits in CKGR_MOR. When changing this frequency selection, the
MOSCRCS bit in the Power Management Controller Status Register (PMC_SR) is automatically cleared and
MAINCK is stopped until the oscillator is stabilized. Once the oscillator is stabilized, MAINCK restarts and
MOSCRCS is set.
XIN
XOUT
MOSCXTEN
MOSCXTCNT
MOSCXTS
Main Clock
Frequency
Counter
MAINF
MAINRDY
SLCK
Slow Clock
3-20 MHz
Crystal
or
Ceramic Resonator
Oscillator
3-20 MHz
Oscillator
Counter
MOSCRCEN
4/8/12 MHz
Fast RC
Oscillator
MOSCRCS
MOSCRCF
MOSCRCEN
MOSCXTEN
MOSCSEL
MOSCSEL
MOSCSELS
1
0
MAINCK
Main Clock
MAINCK
Main Clock
Ref.