ST72C171 datasheet(29/152 Pages) STMICROELECTRONICS

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ST72C171

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POWER SAVING MODES (Cont’d)

6.4 HALT MODE

The HALT mode is the lowest power consumption

mode of the MCU. It is entered by executing the

ST7 HALT instruction (see Figure 24).

The MCU can exit HALT mode on reception of ei-

ther an specific interrupt (see Table 4, “Interrupt

Mapping,” on page 26) or a RESET. When exiting

HALT mode by means of a RESET or an interrupt,

the oscillator is immediately turned on and the

4096 CPU cycle delay is used to stabilize the os-

cillator. After the start up delay, the CPU resumes

operation by servicing the interrupt or by fetching

the reset vector which woke it up (see Figure 23).

When entering HALT mode, the I bit in the CC reg-

ister is forced to 0 to enable interrupts. Therefore,

if an interrupt is pending, the MCU wakes immedi-

ately.

In the 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 11.1 OPTION BYTES for more details).

Figure 23. HALT Mode Timing Overview

Figure 24. 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 4, “Interrupt Mapping,” on page 26 for

more details.

4. Before servicing an interrupt, the CC register is

pushed on the stack. The I bit of the CC register is

set during the interrupt routine and cleared when

the CC register is popped.

HALT

RUN

4096 CPU CYCLE

DELAY

RESET

INTERRUPT

HALT

INSTRUCTION

FETCH

VECTOR

HALT INSTRUCTION

RESET

INTERRUPT 3)

CPU

OSCILLATOR

PERIPHERALS 2)

I BIT

OFF

FETCH RESET VECTOR

OR SERVICE INTERRUPT

CPU

OSCILLATOR

PERIPHERALS

I BIT

OFF

CPU

OSCILLATOR

PERIPHERALS

I BIT 4)

4096 CPU CLOCK CYCLE

DELAY

WATCHDOG

ENABLE

DISABLE

WDGHALT 1)

WATCHDOG

RESET

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