59
6437E–ATARM–23-Apr-13
SAM9M11
11.3
Device Initialization
11.3.1
Clock at Start Up
At boot start up, the processor clock (PCK) and the master clock (MCK) are found on the slow
clock. The slow clock can be an external 32 kHz crystal oscillator or the internal RC oscillator. By
default the slow clock is the internal RC oscillator. Its frequency is not precise and is between 20
kHz and 40 kHz. Its start up is much faster than an external 32 kHz quartz. If a battery supplies
the backup power and if the external 32 kHz clock was previously started up and selected, the
slow clock at boot is the external 32 kHz quartz oscillator. Refer to the Slow Clock Crystal Oscil-
lator description in the Clock Generator section of the datasheet.
11.3.2
Initialization Sequence
Initialization follows the steps described below:
1.
Stack setup for ARM supervisor mode.
2.
Main Oscillator Detection: (External crystal or external clock on XIN). The Main Oscil-
lator is disabled at startup (MOSCEN = 0). First it is bypassed (OSCBYPASS set at 1).
Then the MAINRDY bit is polled. Since this bit is raised, the Main Clock Frequency field
is analyzed (MAINF). If the value is bigger than 16, an external clock connected on XIN
is detected. If not, an external quartz connected between XIN and XOUT (whose fre-
quency is unknown at this moment) is detected.
3.
Main Oscillator Enabling: if an external clock is connected on XIN, the Main Oscillator
does not need to be started. Otherwise, the OSCBYPASS bit is not set. The Main Oscil-
lator is enabled (MOSCEN = 1) with the maximum start-up time and the MOSC bit is
polled to wait for stabilization.
4.
Main Oscillator Selection: the Master Clock source is switched from Slow Clock to the
Main Oscillator without prescaler. The PMC Status Register is polled to wait for MCK
Ready. PCK and MCK are now the Main Oscillator clock.
5.
C variable initialization: non zero-initialized data are initialized in RAM (copy from
ROM to RAM). Zero-initialized data are set to 0 in RAM.
6.
PLLA initialization: PLLA is configured to allow communication on the USB link for the
SAM-BA Monitor. Its configuration depends on the Main Oscillator source (external
clock or crystal) and on its frequency.
Table 11-1.
External Clock and Crystal Frequencies allowed for Boot Sequence (in Mhz)
Description
4
12
28
Other
Boot on External Memories
Yes
No
SAM-BA Monitor through DBGU
Yes
No
SAM-BA Monitor through USB
No
Yes
No