Atmel Evaluation Kit AT91SAM9X25-EK AT91SAM9X25-EK Data Sheet

52

SAM9X25 [DATASHEET]

11054E–ATARM–10-Mar-2014

At boot start-up, the processor clock (PCK) and the master clock (MCK) source is the 12 MHz Fast RC Oscillator.

Initialization follows the steps described below:

1.

Stack setup for ARM supervisor mode.

2.

Main Oscillator Detection: the Main Clock is switched to the 32 kHz RC oscillator to allow external clock fre-
quency to be measured. Then the Main Oscillator is enabled and set in bypass mode. If the MOSCSELS bit rises, 
an external clock is connected, and the next step is Main Clock Selection (3). If not, the bypass mode is cleared to 
attempt external quartz detection. This detection is successful when the MOSCXTS and MOSCSELS bits rise, 
else the 12 MHz Fast RC internal oscillator is used as the Main Clock.

3.

Main Clock Selection: the Master Clock source is switched from the 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 Clock. 

4.

C variable initialization: non zero-initialized data is initialized in the RAM (copy from ROM to RAM). Zero-initial-
ized data is set to 0 in the RAM.

5.

PLLA initialization: PLLA is configured to get a PCK at 96 MHz and an MCK at 48 MHz. If an external clock or 
crystal frequency running at 12 MHz is found, then the PLLA is configured to allow communication on the USB link 
for the SAM-BA Monitor; else the Main Clock is switched to the internal 12 MHz Fast RC, but USB will not be 
activated

.

Note that if the clock frequency is provided not at 12 MHz but between 4 and 28 MHz, it is considered by the ROM Code 
as the 12 MHz clock frequency, and the PLL settings are configured accordingly.

The boot sequence on external memory devices can be controlled using the Boot Sequence Configuration Register 
(BSC_CR). The 3 LSBs of the BSC_CR are available to control the sequence. See the “Boot Sequence Controller 
(BSC)” section for more details.

The user can then choose to bypass some steps shown in 

 according 

to the BSC_CR Value.

≤    4

≥ 28

Boot on External Memories

Yes

Yes

Yes

SAM-BA Monitor through DBGU

Yes

Yes

Yes

SAM-BA Monitor through USB

No

Yes

No

0

Y

Y

Y

Y

Y

Y

1

Y

-

Y

Y

Y

Y

2

Y

-

-

Y

Y

Y

3

Y

-

-

Y

Y

Y

4

Y

-

-

-

Y

Y