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ST9 USER GUIDE

Figure 26

. Clock Control Unit Programming

3.6.1.2 CK_AF Source

When you execute a Wait For Interrupt (WFI) instruction using the Clock Multiplier with output

clock CLOCK2/16, the power is put in Low Power mode. To reduce this power further you

have the possibility of slowing down the INTCLK frequency by using an external clock source

CK_AF. The CK_AF clock will be selected if the WFI_CKSEL bit in the CLKCTL register is set

and if CK_AF is present.

The CK_AF source can also be used in Run mode (no WFI) to reduce power consumption if

CKAF_SEL is set and CK_AF is present.

Quartz

PLL

CK_AF

1/16

1/2

DIV2

CKAF_SEL

1/N

oscillator

MX(1:0)

source

CKAF_ST

CSU_CKSEL

6/8/10/14

XT_DIV16

DX(2:0)

CLOCK2

CLOCK1

(MODER)

(CLK_FLAG)

(CLKCTL)

(PLLCONF)

(CLK_FLAG)

CK_AF

INTCLK

Peripherals

and

CPU Clock Prescaler

XTSTOP

(CLK_FLAG)

Wait for Interrupt and Low Power Modes:

LPOWFI (CLKCTL) selects Low Power operation automatically on entering WFI mode.

WFI_CKSEL (CLKCTL) selects the CK_AF clock automatically, if present, on entering WFI mode.

XTSTOP (CLK_FLAG) automatically stops the Xtal oscillator when the CK_AF clock is present and selected.

CLOCK MULTIPLIER

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