Atmel SAM4S Series, Preliminary Data Sheet

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485
11100B–ATARM–31-Jul-12
SAM4S Series [Preliminary]
27.2.4 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 pro-
grams 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 27-6. Master Clock Controller
27.2.5 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) processor
instruction.
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.
27.2.6 SysTick Clock
The SysTick calibration value is fixed to 12500 which allows the generation of a time base of 1
ms with SysTick clock to the maximum frequency on MCK divided by 8.
SLCK
Master Clock
Prescaler
MCK
PRES CSS
MAINCK
PLLACK
PLLBCK
To the Processor
Clock Controller (PCK)
PMC_MCKR PMC_MCKR