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ST72254G2

8-BIT MCU WITH SINGLE VOLTAGE FLASH MEMORY, ADC, 16-BIT TIMERS, SPI, I2C INTERFACES

STMicroelectronics 

ST72104G, ST72215G, ST72216G, ST72254G

POWER SAVING MODES (Cont’d)

8.4 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 20).

The MCU can exit HALT mode on reception of ei-

ther a specific interrupt (see Table 5, “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 19).

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 15.1 ”OPTION BYTES” on page 129 for

more details).

Figure 19. HALT Mode Timing Overview

RUN

HALT

4096 CPU CYCLE

DELAY

RUN

HALT

IN STRU CTIO N

RESET

OR

INTERRUPT

FETCH

VECTO R

Figure 20. HALT Mode Flow-chart

HALT INSTRUCTION

WDGHALT 1)

1

WATCHDOG

RESET

ENABLE

0

WAT CHDOG

DISABLE

OSCILLATOR OFF

PERIPHERALS 2) OFF

CPU

OFF

I BIT

0

N

RESET

N

Y

INTERRUPT 3)

Y

OSCILLATOR ON

PERIPHERALS OFF

CPU

ON

I BIT

1

4096 CPU CLOCK CYCLE

DELAY

OSCILLATOR ON

PERIPHERALS ON

CPU

ON

I BIT

X 4)

FETCH RESET VECTOR

OR SERVICE INTERRUPT

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 5, “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.

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