ST7260 datasheet(26/117 Pages) STMICROELECTRONICS

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ST7260

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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, 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 divided by 3 (fCPU).

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 HALT Mode

The MCU consumes the least amount of power in

HALT mode. The HALT mode is entered by exe-

cuting the HALT instruction. The internal oscillator

is then turned off, causing all internal processing to

be stopped, including the operation of the on-chip

peripherals.

When entering HALT mode, the I bit in the Condi-

tion Code Register is cleared. Thus, all external in-

terrupts (ITi or USB end suspend mode) are al-

lowed and if an interrupt occurs, the CPU clock be-

comes active.

The MCU can exit HALT mode on reception of ei-

ther an external interrupt on ITi, an end suspend

mode interrupt coming from USB peripheral, or a

reset. The oscillator is then turned on and a stabi-

lization time is provided before releasing CPU op-

eration. The stabilization time is 4096 CPU clock

cycles.

After the start up delay, the CPU continues opera-

tion by servicing the interrupt which wakes it up or

by fetching the reset vector if a reset wakes it up.

Figure 17. HALT Mode Flow Chart

EXTERNAL

INTERRUPT*

RESET

HALT INSTRUCTION

4096 CPU CLOCK

FETCH RESET VECTOR

OR SERVICE INTERRUPT

CYCLES DELAY

CPU CLOCK

OSCILLATOR

PERIPH. CLOCK

I-BIT

SET

CPU CLOCK

OSCILLATOR

PERIPH. CLOCK

I-BIT

OFF

CLEARED

OFF

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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