UM0851 datasheet(224/245 Pages) STMICROELECTRONICS

224 / 245 page

Power management section

UM0851

224/245

Doc ID 16604 Rev 2

Figure 65.

SPEAr clock tree

The advantages of providing the clock framework in software are as follows:

●

Clocks can be managed dynamically at run-time

●

Structured approach

●

Hide complexity of internal clock generation & clock interdependencies

●

Reuse across different SPEAr devices

●

Reuse across different platforms

●

Clock management from multiple cores

●

Using existing OS power management interfaces (when possible)

Clock dependency graph and reference counting

●

The clock framework software generates a clock dependency graph internally.

●

Dependency links are dynamic and can be reconfigured at run time. The clock

dependencies are set up by establishing a Parent-Child Relation between a clock and

its derivatives. This hierarchy provides makes it easy to track all the clocks that are

used in the system.

●

Whenever a device needs a clock, it sends a request to the clock framework. The clock

framework iterates through the total clock dependency tree and sees if the parent has

been enabled for the clock. If not, first the parent clock is enabled followed by the child

clock. Else, if the parent clock is already enabled, the reference counts for each related

clock are incremented.

●

The reference count for the clock usage is incremented or decremented depending

upon the resource utilization. Whenever the clock is enabled, the reference count is

incremented and it is decremented when the clock is disabled. The reference count is

tracked recursively in the total tree to capture all the dependencies.

CLK32KHZ

OSC

30 MHz

32.768 kHz

PLL3

(USB)

PLL1

(Sys)

PLL2

(Aux)

CLKPLL2

CLKPLL1

UCLK12MHZ

UCLK30MHZ

UCLK48MHZ

DDR

CPU

UART

TIMER

SSP

USBH/USBD

RTC

CLK30MHZ

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