ST72141K2 datasheet(30/132 Pages) STMICROELECTRONICS

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ST72141K

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POWER SAVING MODES (Cont’d)

5.3 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 are forced to 0, to ena-

ble all interrupts. All other registers and memory

remain 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 22.

5.4 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

MISCR register. This bit enables or disables Slow

mode selecting the internal slow frequency (fCPU).

In this mode, the oscillator frequency can be divid-

ed by 32 instead of 2 in normal operating mode.

The CPU and peripherals are clocked at this lower

frequency except the Motor Control and the SPI

peripherals which have their own clock selection

bit (XT16) in the MISCR register.

Figure 22. WAIT mode flow-chart

WFI INSTRUCTION

RESET

INTERRUPT

CPU

OSCILLATOR

PERIPHERALS

I BIT

OFF

if exit caused by a RESET, a 4096 CPU

clock cycle delay is inserted.

CPU

OSCILLATOR

PERIPHERALS

OFF*

OFF

Note:

* The peripheral clock is stopped only when exit caused by RESET and not by an interrupt.

** Before servicing an interrupt, the CC register is pushed on the stack.

FETCH RESET VECTOR

OR SERVICE INTERRUPT**

CPU

OSCILLATOR

PERIPHERALS

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