ST72321 datasheet(44/185 Pages) STMICROELECTRONICS

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ST72321

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POWER SAVING MODES (Cont’d)

8.4.2 HALT MODE

The HALT mode is the lowest power consumption

mode of the MCU. It is entered by executing the

‘HALT’ instruction when the OIE bit of the Main

Clock Controller Status register (MCCSR) is

cleared (see Section 10.2 on page 57 for more de-

tails on the MCCSR register).

The MCU can exit HALT mode on reception of ei-

ther a specific interrupt (see Table 7, “Interrupt

Mapping,” on page 37) or a RESET. When exiting

HALT mode by means of a RESET or an interrupt,

the oscillator is immediately turned on and the 256

or 4096 CPU cycle delay is used to stabilize the

oscillator. After the start up delay, the CPU

resumes operation by servicing the interrupt or by

fetching the reset vector which woke it up (see Fig-

ure 29).

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

CC register are forced to ‘10b’to enable interrupts.

Therefore, if an interrupt is pending, the MCU

wakes up immediately.

In HALT mode, the main oscillator is turned off

causing all internal processing to be stopped, in-

cluding the operation of the on-chip peripherals.

All peripherals are not clocked except the ones

which get their clock supply from another clock

generator (such as an external or auxiliary oscilla-

tor).

The compatibility of Watchdog operation with

HALT mode is configured by the “WDGHALT” op-

tion bit of the option byte. The HALT instruction

when executed while the Watchdog system is en-

abled, can generate a Watchdog RESET (see

Section 14.1 on page 172 for more details).

Figure 28. HALT Timing Overview

Figure 29. HALT Mode Flow-chart

Notes:

1. WDGHALT is an option bit. See option byte sec-

tion for more details.

2. Peripheral clocked with an external clock source

can still be active.

3. Only some specific interrupts can exit the MCU

from HALT mode (such as external interrupt). Re-

fer to Table 7, “Interrupt Mapping,” on page 37 for

more details.

4. 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 recovered when the CC

HALT

RUN

256 OR 4096 CPU

CYCLE DELAY

RESET

INTERRUPT

HALT

INSTRUCTION

FETCH

VECTOR

[MCCSR.OIE=0]

HALT INSTRUCTION

RESET

INTERRUPT 3)

CPU

OSCILLATOR

PERIPHERALS 2)

I[1:0] BITS

OFF

FETCH RESET VECTOR

OR SERVICE INTERRUPT

CPU

OSCILLATOR

PERIPHERALS

I[1:0] BITS

OFF

XX 4)

CPU

OSCILLATOR

PERIPHERALS

I[1:0] BITS

XX 4)

256 OR 4096 CPU CLOCK

DELAY

WATCHDOG

ENABLE

DISABLE

WDGHALT 1)

WATCHDOG

RESET

(MCCSR.OIE=0)

CYCLE

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