ST7SCR datasheet(30/101 Pages) STMICROELECTRONICS

30 / 101 page

ST7SCR

30/101

8 POWER SAVING MODES

8.1 INTRODUCTION

To give a large measure of flexibility to the applica-

tion in terms of power consumption, two main pow-

er saving modes are implemented in the ST7.

After a RESET the normal operating mode is se-

lected by default (RUN mode). This mode drives

the device (CPU and embedded peripherals) by

means of a master clock which is based on the

main oscillator frequency.

From Run mode, the different power saving

modes may be selected by setting the relevant

instruction whose action depends on the oscillator

status.

8.2 WAIT MODE

WAIT mode places the MCU in a low power con-

sumption mode by stopping the CPU.

This power saving mode is selected by calling the

“WFI” ST7 software instruction.

All peripherals remain active. During WAIT mode,

the I bit of the CC register is forced to 0, to enable

all interrupts. All other registers and memory re-

main unchanged. The MCU remains in WAIT

mode until an interrupt or Reset occurs, whereup-

on the Program Counter branches to the starting

address of the interrupt or Reset service routine.

The MCU will remain in WAIT mode until a Reset

or an Interrupt occurs, causing it to wake up.

Refer to Figure 21.

Figure 21. WAIT Mode Flow Chart

WFI INSTRUCTION

RESET

INTERRUPT

CPU CLOCK

OSCILLATOR

PERIPH. CLOCK

I-BIT

CLEARED

OFF

CPU CLOCK

OSCILLATOR

PERIPH. CLOCK

I-BIT

SET

FETCH RESET VECTOR

OR SERVICE INTERRUPT

512 CPU CLOCK

CYCLES DELAY

IF RESET

Note: Before servicing an interrupt, the CC

during the interrupt routine and cleared when

the CC register is popped.

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