ST72F321 datasheet(43/189 Pages) STMICROELECTRONICS

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ST72321

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POWER SAVING MODES (Cont’d)

8.4 ACTIVE-HALT AND HALT MODES

ACTIVE-HALT and HALT modes are the two low-

est power consumption modes of the MCU. They

are both entered by executing the ‘HALT’ instruc-

tion. The decision to enter either in ACTIVE-HALT

or HALT mode is given by the MCC/RTC interrupt

enable flag (OIE bit in MCCSR register).

8.4.1 ACTIVE-HALT MODE

ACTIVE-HALT mode is the lowest power con-

sumption mode of the MCU with a real time clock

available. It is entered by executing the ‘HALT’ in-

struction when the OIE bit of the Main Clock Con-

troller Status register (MCCSR) is set (see section

10.2 on page 57 for more details on the MCCSR

The MCU can exit ACTIVE-HALT mode on recep-

tion of an MCC/RTC interrupt or a RESET. When

exiting ACTIVE-HALT mode by means of an inter-

rupt, no 256 or 4096 CPU cycle delay occurs. The

CPU resumes operation by servicing the interrupt

or by fetching the reset vector which woke it up

(see Figure 28).

When entering ACTIVE-HALT mode, the I[1:0] bits

in the CC register are forced to ‘10b’ to enable in-

terrupts. Therefore, if an interrupt is pending, the

MCU wakes up immediately.

In ACTIVE-HALT mode, only the main oscillator

and its associated counter (MCC/RTC) are run-

ning to keep a wake-up time base. All other periph-

erals are not clocked except those which get their

clock supply from another clock generator (such

as external or auxiliary oscillator).

The safeguard against staying locked in ACTIVE-

HALT mode is provided by the oscillator interrupt.

Note: As soon as the interrupt capability of one of

the oscillators is selected (MCCSR.OIE bit set),

entering ACTIVE-HALT mode while the Watchdog

is active does not generate a RESET.

This means that the device cannot spend more

than a defined delay in this power saving mode.

CAUTION: When exiting ACTIVE-HALT mode fol-

lowing an MCC/RTC interrupt, OIE bit of MCCSR

the interrupt occurs (tDELAY = 256 or 4096 tCPU de-

lay depending on option byte). Otherwise, the ST7

enters HALT mode for the remaining tDELAY peri-

od.

Figure 27. ACTIVE-HALT Timing Overview

Figure 28. ACTIVE-HALT Mode Flow-chart

Notes:

1. This delay occurs only if the MCU exits ACTIVE-

HALT mode by means of a RESET.

2. Peripheral clocked with an external clock source

can still be active.

3. Before servicing an interrupt, the CC register is

pushed on the stack. The I[1:0] bits of the CC reg-

ister are set to the current software priority level of

the interrupt routine and restored when the CC

4. Only the MCC/RTC interrupt can exit the MCU

from ACTIVE-HALT mode.

MCCSR

OIE bit

Power Saving Mode entered when HALT

instruction is executed

HALT mode

ACTIVE-HALT mode

HALT

RUN

256 OR 4096 CPU

CYCLE DELAY 1)

RESET

INTERRUPT

HALT

INSTRUCTION

FETCH

VECTOR

ACTIVE

[MCCSR.OIE=1]

HALT INSTRUCTION

RESET

CPU

OSCILLATOR

PERIPHERALS 2)

I[1:0] BITS

OFF

FETCH RESET VECTOR

OR SERVICE INTERRUPT

CPU

OSCILLATOR

PERIPHERALS

I[1:0] BITS

OFF

XX 3)

CPU

OSCILLATOR

PERIPHERALS

I[1:0] BITS

XX 3)

256OR 4096CPU CLOCK

CYCLE DELAY

(MCCSR.OIE=1)

INTERRUPT 4)

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