ST72521 datasheet(42/211 Pages) STMICROELECTRONICS

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ST72521

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8 POWER SAVING MODES

8.1 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 23): 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 divided or multiplied by 2

(fOSC2).

From RUN mode, the different power saving

modes may be selected by setting the relevant

instruction whose action depends on the oscillator

status.

Figure 23. Power Saving Mode Transitions

8.2 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 three bits in the

MCCSR register: the SMS bit which enables or

disables Slow mode and two CPx bits which select

the internal slow frequency (fCPU).

In this mode, the master clock frequency (fOSC2)

can be divided by 2, 4, 8 or 16. The CPU and pe-

ripherals are clocked at this lower frequency

(fCPU).

Note: SLOW-WAIT mode is activated when enter-

ing the WAIT mode while the device is already in

SLOW mode.

Figure 24. SLOW Mode Clock Transitions

POWER CONSUMPTION

WAIT

SLOW

RUN

ACTIVE HALT

High

Low

SLOW WAIT

HALT

SMS

CP1:0

fCPU

NEW SLOW

NORMAL RUN MODE

FREQUENCY

REQUEST

fOSC2

fOSC2/2

fOSC2/4

fOSC2

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