174
11054A–ATARM–27-Jul-11
SAM9X25
174
11054A–ATARM–27-Jul-11
SAM9X25
21.5
Main Clock
Figure 21-3.
Main Clock Block Diagram
The Main Clock has two sources:
• 12 MHz Fast RC Oscillator which starts very quickly and is used at startup
• 3 to 20 MHz Crystal Oscillator, which can be bypassed
21.6
Main Clock Selection
The main clock can be generated either by an external 12 MHz crystal oscillator or by the on-
chip 12 MHz RC oscillator. This fast RC oscillator allows the processor to start or restart in a few
microseconds when 12 MHz internal RC is selected.
The 12 MHz crystal oscillator can be bypassed by setting the bit MOSCXTBY to accept an exter-
nal main clock on XIN.
XIN
XOUT
MOSCXTEN
MOSCXTCNT
MOSCXTS
Main Clock
Frequency
Counter
MAINF
MAINRDY
SLCK
Slow Clock
3-20 MHz
Crystal
Oscillator
3-20 MHz Crystal
Oscillator
Counter
MOSCRCEN
12 MHz
Fast RC
Oscillator
MOSCRCS
MOSCRCF
MOSCRCEN
MOSCXTEN
MOSCSEL
MOSCSEL
MOSCSELS
1
0
MAINCK
Main Clock
Summary of Contents for SAM9X25
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