Atmel Evaluation Kit AT91SAM9X35-EK AT91SAM9X35-EK Data Sheet

171

SAM9X35 [DATASHEET]

11055E–ATARM–10-Mar-2014

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 MOSCXTBY bit to accept an external main clock on XIN.

MOSCRCEN, MOSCXTEN, MOSCSEL and MOSCXTBY bits are located in the PMC Clock Generator Main Oscillator
Register (CKGR_MOR).

After a VDDBU power on reset, the default configuration is MOSCRCEN = 1, MOSCXTEN = 0 and MOSCSEL = 0, the
12 MHz RC oscillator is started as Main clock.

To speed up the wake-up phase, the system boots on 12 MHz RC (Main Clock). This allows the user to perform system
configuration (PLL, DDR2, etc.) at 12 MHz instead of 32 kHz during 12 MHz oscillator start-up. 

On Chip 

12M RC OSC

Main Clock 

Main Clock

Oscillator

MOSCXTEN

MOSCRCEN

MOSCSEL

XIN

XOUT

MOSCXTBY

External Main Cock

Main Supply

POR output

12 MHz RC Startup Time

Wait MOSCRCS = 1
System switches on Main Clock 
to speed-up the boot
System is running at 12 MHz

External oscillator
is started for better accuracy
MOSCXTEN = 1
MOSCSEL = 0

System starts on 32 kHz RC
RCEN = 1
OSC32EN = 0
OSCSEL = 0
MOSCRCEN = 1
MOSCXTEN = 0
MOSCSEL = 0
PMC_MCKR = 1

 Crystal Startup Time

Wait MOSCXTS = 1
User switches on external oscillator 
MOSCSEL=1
Wait while MOSCSELS =1 
System is runnning on 12 MHz Crystal
PLL can be used

12 MHz RC