Atmel SAM9X25, Manual

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sion factor. To read the current division factor on a particular peripheral, the user just needs to
write the READ command and the peripheral ID.
Code Example to select divider 8 for peripheral 2 and enable its clock:
write_register(PMC_PCR,0x010031002)
Code Example to read the divider of peripheral 4:
write_register(PMC_PCR,0x00000004)
read_register(PMC_PCR)
When a peripheral clock is disabled, the clock is immediately stopped. The peripheral clocks are
automatically disabled after a reset.
In order to stop a peripheral, it is recommended that the system software wait until the peripheral
has executed its last programmed operation before disabling the clock. This is to avoid data cor-
ruption or erroneous behavior of the system.
The bit number within the Peripheral Control registers is the Peripheral Identifier defined at the
product level. Generally, the bit number corresponds to the interrupt source number assigned to
the peripheral.
22.10 Programmable Clock Output Controller
The PMC controls 2 signals to be output on external pins PCKx. Each signal can be indepen-
dently programmed via the PMC_PCKx registers.
PCKx can be independently selected between the Slow clock, the Master Clock, the
PLLACK/PLLADIV2, the UTMI PLL output and the main clock by writing the CSS field in
PMC_PCKx. Each output signal can also be divided by a power of 2 between 1 and 64 by writing
the PRES (Prescaler) field in PMC_PCKx.
Each output signal can be enabled and disabled by writing 1 in the corresponding bit, PCKx of
PMC_SCER and PMC_SCDR, respectively. Status of the active programmable output clocks
are given in the PCKx bits of PMC_SCSR (System Clock Status Register).
Moreover, like the PCK, a status bit in PMC_SR indicates that the Programmable Clock is actu-
ally what has been programmed in the Programmable Clock registers.
As the Programmable Clock Controller does not manage with glitch prevention when switching
clocks, it is strongly recommended to disable the Programmable Clock before any configuration
change and to re-enable it after the change is actually performed.
22.11 Programming Sequence
1. Enabling the 12MHz Main Oscillator:
The main oscillator is enabled by setting the MOSCEN field in the CKGR_MOR register. In
some cases it may be advantageous to define a start-up time. This can be achieved by writ-
ing a value in the OSCOUNT field in the CKGR_MOR register.
Once this register has been correctly configured, the user must wait for MOSCS field in the
PMC_SR register to be set. This can be done either by polling the status register or by wait-
ing the interrupt line to be raised if the associated interrupt to MOSCS has been enabled in
the PMC_IER register.