ST72334J datasheet(39/125 Pages) STMICROELECTRONICS

39 / 125 page

ST72334J/N, ST72314J/N, ST72124J

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POWER SAVING MODES (Cont’d)

Standard HALT mode

In this mode the main oscillator is turned off caus-

ing all internal processing to be stopped, including

the operation of the on-chip peripherals. All periph-

erals are not clocked except the ones which get

their clock supply from another clock generator

(such as an external or auxiliary oscillator).

The compatibility of Watchdog operation with Halt

mode is configured by the “WDGHALT” option bit

of the OPTION BYTE. The HALT instruction when

executed while the Watchdog system is enabled,

can generate a Watchdog RESET (see dedicated

section for more details).

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 stabi-

lize the oscillator.

Specific ACTIVE-HALT mode

As soon as the interrupt capability of the main os-

cillator is selected (OIE bit set), the HALT instruc-

tion will make the device enter a specific ACTIVE-

HALT power saving mode instead of the standard

HALT one.

This mode consists of having only the main oscil-

lator and its associated counter running to keep a

wake-up time base. All other peripherals are not

clocked except the ones which get their clock sup-

ply from another clock generator (such as external

or auxiliary oscillator).

The safeguard against staying locked in this AC-

TIVE-HALT mode is insured by the oscillator inter-

rupt.

Note: As soon as the interrupt capability of one of

the oscillators is selected (OIE bit set), entering in

ACTIVE-HALT mode while the Watchdog is active

does not generate a RESET.

This means that the device cannot to spend more

than a defined delay in this power saving mode.

Figure 29. HALT modes flow-chart

HALT INSTR UCTION

OSCILLATOR

CPU

OSCILLATOR

PERIPH ERALS

I BIT

OFF

Notes:

OIE BIT

CPU

OSCILLATOR

PERIPHERAL S

I BIT

OFF

RESET

EXTE RNAL*

CPU

OSCILLATOR

PERIP HERALS

OFF

INTERRUP T

HALT

ACTIV E-HALT

MAIN

FETCH RESET VECTOR

OR SERVICE INTERRUPT **

4096 clock cycles delay

CPU

OSCILLATOR

PERIPH ERALS

External interrupt or internal interrupts with Exit from Halt Mode capability

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

WAT CHDOG

ENABLE

If WDGHA LT

bit reset in

OPTION BYTE

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