Atmel SAM9X25, Руководство пользователя

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11054A–ATARM–27-Jul-11
SAM9X25
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11054A–ATARM–27-Jul-11
SAM9X25
21.6.2 Switch from Internal 12 MHz RC Oscillator to the 12 MHz Crystal
For USB operations an external 12 MHz crystal is required for better accuracy.
The programmer controls the main clock switching by software and so must take precautions
during the switching phase.
To switch from internal 12 MHz RC oscillator to the 12 MHz crystal, the programmer must exe-
cute the following sequence:
• Enable the 12 MHz oscillator by setting the bit MOSCXTEN to 1.
• Wait that 12 MHz oscillator status MAINRDY is 1.
• Switch from internal 12 MHz RC oscillator to the 12 MHz oscillator by setting the bit
MOSCSEL to 1.
• If not the bit MOSCSEL is set to 0 by the PMC.
• Disable the 12 MHz RC oscillator by setting the bit MOSCRCEN to 0.
21.6.3 Bypass the 12 MHz Oscillator
Following step must be added to bypass the 12 MHz Oscillator.
• An external clock must be connected on XIN.
• Enable the bypass path MOSCXTBY bit set to 1.
• Disable the 12 MHz oscillator by setting the bit MOSCXTEN to 0.
21.6.4 Switch from the 12 MHz Crystal to Internal 12 MHz RC Oscillator
The same procedure must be followed to switch from a 12 MHz crystal to the internal 12 MHz
RC oscillator.
• Enable the internal 12 MHz RC oscillator for low power by setting the bit MOSCRCEN to 1
• Wait internal 12 MHz RC Startup Time for clock stabilization (software loop).
• Switch from 12 MHz oscillator to internal 12 MHz RC oscillator by setting the bit MOSCSEL
to 0.
• Disable the 12 MHz oscillator by setting the bit MOSCXTEN to 0.
21.6.5 12 MHz Fast RC Oscillator
After reset, the 12 MHz Fast RC Oscillator is enabled and it is selected as the source of MCK.
MCK is the default clock selected to start up the system.
Please refer to the “DC Characteristics” section of the product datasheet.
The software can disable or enable the 12 MHz Fast RC Oscillator with the MOSCRCEN bit in
the Clock Generator Main Oscillator Register (CKGR_MOR).
When disabling the Main Clock by clearing the MOSCRCEN bit in CKGR_MOR, the MOSCRCS
bit in the Power Management Controller Status Register (PMC_SR) is automatically cleared,
indicating the Main Clock is off.
Setting the MOSCRCS bit in the Power Management Controller Interrupt Enable Register
(PMC_IER) can trigger an interrupt to the processor.
21.6.6 3 to 20 MHz Crystal Oscillator
After reset, the 3 to 20 MHz Crystal Oscillator is disabled and it is not selected as the source of
MAINCK.