Datasheet

SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
528
28.4 Master Clock Controller
The Master Clock Controller provides selection and division of the Master Clock (MCK). MCK is the clock provided
to all the peripherals.
The Master Clock is selected from one of the clocks provided by the Clock Generator. Selecting the Slow Clock
provides a Slow Clock signal to the whole device. Selecting the Main Clock saves power consumption of the PLLs.
The Master Clock Controller is made up of a clock selector and a prescaler.
The Master Clock selection is made by writing the CSS field (Clock Source Selection) in PMC_MCKR (Master
Clock Register). The prescaler supports the division by a power of 2 of the selected clock between 1 and 64, and
the division by 3. The PRES field in PMC_MCKR programs the prescaler.
Each time PMC_MCKR is written to define a new Master Clock, the MCKRDY bit is cleared in PMC_SR. It reads 0
until the Master Clock is established. Then, the MCKRDY bit is set and can trigger an interrupt to the processor.
This feature is useful when switching from a high-speed clock to a lower one to inform the software when the
change is actually done.
Figure 28-3. Master Clock Controller
28.5 Processor Clock Controller
The PMC features a Processor Clock Controller (HCLK) that implements the Processor Sleep Mode. The
Processor Clock can be disabled by executing the WFI (WaitForInterrupt) or the WFE (WaitForEvent) processor
instruction while the LPM bit is at 0 in the PMC Fast Startup Mode Register (PMC_FSMR).
The Processor Clock HCLK is enabled after a reset and is automatically re-enabled by any enabled interrupt. The
Processor Sleep Mode is achieved by disabling the Processor Clock, which is automatically re-enabled by any
enabled fast or normal interrupt, or by the reset of the product.
When Processor Sleep Mode is entered, the current instruction is finished before the clock is stopped, but this
does not prevent data transfers from other masters of the system bus.
28.6 SysTick Clock
The SysTick calibration value is fixed to 10500 which allows the generation of a time base of 1 ms with SysTick
clock to the maximum frequency on MCK divided by 8.
28.7 Peripheral Clock Controller
The Power Management Controller controls the clocks of each embedded peripheral by means of the Peripheral
Clock Controller. The user can individually enable and disable the Clock on the peripherals.
The user can also enable and disable these clocks by writing Peripheral Clock Enable 0 (PMC_PCER0),
Peripheral Clock Disable 0 (PMC_PCDR0), Peripheral Clock Enable 1 (PMC_PCER1) and Peripheral Clock
SLCK
Master Clock
Prescaler
MCK
PRESCSS
MAINCK
PLLACK
UPLLCK/2
To the Processor
Clock Controller (PCK)
PMC_MCKR PMC_MCKR