at91sam9g20-cu ATMEL Corporation, at91sam9g20-cu Datasheet - Page 285

• Manufacturer Part Number: at91sam9g20-cu
• Description: At91 Arm Thumb Microcontrollers
• Manufacturer: ATMEL Corporation

26.5
26.6
26.7
6384B–ATARM–15-Dec-08
Peripheral Clock Controller
Programmable Clock Output Controller
Programming Sequence
Figure 26-2. USB Clock Controller
The Power Management Controller controls the clocks of each embedded peripheral by the way
of the Peripheral Clock Controller. The user can individually enable and disable the Master
Clock on the peripherals by writing into the Peripheral Clock Enable (PMC_PCER) and Periph-
eral Clock Disable (PMC_PCDR) registers. The status of the peripheral clock activity can be
read in the Peripheral Clock Status Register (PMC_PCSR).
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 Clock Control registers (PMC_PCER, PMC_PCDR, and
PMC_PCSR) is the Peripheral Identifier defined at the product level. Generally, the bit number
corresponds to the interrupt source number assigned to the peripheral.
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 PLL A output, the PLL B out-
put 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.
1. Enabling the 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.
Source
Clock
USB
USBDIV
Divider
/1,/2,/4
AT91SAM9G20 Preliminary
UDP
UHP
UDP Clock (UDPCK)
UHP Clock (UHPCK)
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