ST92141 datasheet(68/179 Pages) STMICROELECTRONICS

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ST92141 - RESET AND CLOCK CONTROL UNIT (RCCU)

Figure 30. Example of Low Power Mode programming

Begin

WFI_CKSEL

← 1

WFI status

User’s Program

LPOWFI

← 1

User’s Program

WFI

End

PROGRAM FLOW

COMMENTS

SYSTEM CLOCK FREQUENCY

Interrupt

Quartz divided by 2

PLL multiply factor

Multiplier divider’s factor set

Wait for the

CK_AF clock selected when WFI

Wait For Interrupt

No code is executed until

Interrupt served

fixed to 10.

to 1, and PLL turned ON

an interrupt is requested

Low Power Mode enabled

Main code execution

continued

2.5 MHz

25 MHz

2.5 MHz

25 MHz

** T2 = Quartz oscillator start-up time

* T1 = PLL lock-in time

T1*

T2**

T1*

FQ=5 MHz, Vcc=5 V and T=25°C

CK_AF/16

WAIT

DX2-0

← 000

Xtal is selected to

restart the PLL quickly

CSU_CKSEL

PLL is

system clock source

CSU_CKSEL<- 1

while the CK_AF is

the system clock

CKAF_SEL <- 0

The system CK

switches to Xtal

The PLL is locked

and becomes the

system clock

XTSTOP

← 0

To restart Xtal and PLL

To stop PLL and Xtal when

a WFI occurs

XTSTOP

← 1

PLL locking

SET UP AFTER RESET PHASE:

DIV2

= 1

XTSTOP = 0

MX(1:0)

= 00

CSU_CKSEL = 0

Interrupt

WAIT

Routine

(LOCK->1)

CK_AF

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