ST72371 datasheet(21/94 Pages) STMICROELECTRONICS

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ST72371/ST72372

3.4 POWER SAVING MODES

3.4.1 Introduction

There are three Power Saving modes. Slow Mode

is selected by setting the relevant bits in the Mis-

cellaneous register. Wait and Halt modes may be

entered using the WFI and HALT instructions.

3.4.2 Slow Mode

In Slow mode, the oscillator frequency can be di-

vided by a value defined in the Miscellaneous

this lower frequency. Slow mode is used to reduce

power consumption, and enables the user to adapt

clock frequency to available supply voltage.

Note: On reset, Slow mode is selected by default

(FOSC/6).

3.4.3 Wait Mode

Wait mode places the MCU in a low power con-

sumption mode by stopping the CPU. All peripher-

als remain active. During Wait mode, the I bit (CC

All other registers and memory remain unchanged.

The MCU will remain in Wait mode until an Inter-

rupt or Reset occurs, whereupon the Program

Counter branches to the starting address of the In-

terrupt 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 13 below.

Figure 13. WAIT 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

4096 CPU CLOCK

CYCLES DELAY

IF RESET

Note: Before servicing an interrupt, the CC register is

pushed on the stack. The I-Bit is set during the inter-

rupt routine and cleared when the CC register is

popped.

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