ST7LITE09 datasheet(36/122 Pages) STMICROELECTRONICS

36 / 122 page

ST7LITE0, ST7SUPERLITE

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9 POWER SAVING MODES

9.1 INTRODUCTION

To give a large measure of flexibility to the applica-

tion in terms of power consumption, four main

power saving modes are implemented in the ST7

(see Figure 21): SLOW, WAIT (SLOW WAIT), AC-

TIVE HALT and HALT.

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 (fOSC).

From RUN mode, the different power saving

modes may be selected by setting the relevant

instruction whose action depends on the oscillator

status.

Figure 21. Power Saving Mode Transitions

9.2 SLOW MODE

This mode has two targets:

– To reduce power consumption by decreasing the

internal clock in the device,

– To adapt the internal clock frequency (fCPU) to

the available supply voltage.

SLOW mode is controlled by the SMS bit in the

MCCSR register which enables or disables Slow

mode.

In this mode, the oscillator frequency is divided by

32. The CPU and peripherals are clocked at this

lower frequency.

Notes:

SLOW-WAIT mode is activated when entering

WAIT mode while the device is already in SLOW

mode.

SLOW mode has no effect on the Lite Timer which

is already clocked at FOSC/32.

Figure 22. SLOW Mode Clock Transition

POWER CONSUMPTION

WAIT

SLOW

RUN

ACTIVE HALT

High

Low

SLOW WAIT

HALT

SMS

fCPU

NORMAL RUN MODE

REQUEST

fOSC

fOSC/32

fOSC

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