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ATSAM4S-XPLD
452
SAM4S [DATASHEET]
11100E–ATARM–24-Jul-13
28.1.5 Main Clock
Figure 28-3. Main Clock Block Diagram
The Main Clock has two sources:
12/8/4 MHz Fast RC Oscillator which starts very quickly and is used at start-up.
3 to 20 MHz Crystal or Ceramic Resonator-based Oscillator which can be bypassed.
28.1.5.1 Fast RC Oscillator
After reset, the 12/8/4 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.
selected as the source of MAINCK. MAINCK is the default clock selected to start up the system.
The Fast RC Oscillator frequencies are calibrated in production except the lowest frequency which is
not calibrated.
not calibrated.
Please refer to the “DC Characteristics” section of the product datasheet.
The software can disable or enable the 12/8/4 MHz Fast RC Oscillator with the MOSCRCEN bit in the
Clock Generator Main Oscillator Register (CKGR_MOR).
Clock Generator Main Oscillator Register (CKGR_MOR).
The user can also select the output frequency of the Fast RC Oscillator, either 12/8/4 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.
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
MOSCXTST
MOSCXTS
Main Clock
Frequency
Counter
MAINF
MAINRDY
SLCK
Slow Clock
3-20 MHz
Crystal
or
Ceramic Resonator
Oscillator
3-20 MHz
Oscillator
Counter
MOSCRCEN
Fast RC
Oscillator
MOSCRCS
MOSCRCF
MOSCRCEN
MOSCXTEN
MOSCSEL
MOSCSEL
MOSCSELS
1
0
MAINCK
Main Clock
Main Clock
MAINCK
Main Clock
Ref.
RCMEAS