ST6380B4 datasheet(21/82 Pages) STMICROELECTRONICS

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ST6380, ST6381, ST6382, ST6383, ST6388, ST6389

3.3 POWER SAVING MODES

STOP and WAIT modes have been implemented

in the ST638x in order to reduce the current con-

sumption of the device during idle periods. These

two modes are described in the following para-

graphs. Since the hardware activated digital

watchdog function is present, the STOP instruc-

tion is de-activated and any attempt to execute it

will cause the automatic execution of a WAIT in-

struction.

3.3.1 WAIT Mode

The configuration of the MCU in the WAIT mode

occurs as soon as the WAIT instruction is execut-

ed. The microcontroller can also be considered as

being in a “software frozen” state where the Core

stops processing the instructions of the routine,

the contents of the RAM locations and peripheral

registers are saved as long as the power supply

voltage is higher than the RAM retention voltage

but where the peripherals are still working. The

WAIT mode is used when the user wants to re-

duce the consumption of the MCU when it is in

idle, while not losing count of time or monitoring of

external events. The oscillator is not stopped in or-

der to provide clock signal to the peripherals. The

timers counting may be enabled (writing the PSI

bit in TSCR1 register) and the timer interrupt may

be also enabled before entering the WAIT mode;

this allows the WAIT mode to be left when timer in-

terrupt occurs. If the exit from the WAIT mode is

performed with a general RESET (either from the

activation of the external pin or by watchdog reset)

the MCU will enter a normal reset procedure as

described in the RESET chapter. If an interrupt is

generated during WAIT mode the MCU behaviour

depends on the state of the MCU Core before the

initialization of the WAIT sequence, but also of the

kind of the interrupt request that is generated. This

case will be described in the following paragraphs.

In any case, the MCU Core does not generate any

delay after the occurrence of the interrupt because

the oscillator clock is still available.

3.3.2 STOP Mode

Since the hardware activated watchdog is present

on the ST638x, the STOP instruction has been de-

activated. Any attempt to execute a STOP instruc-

tion will cause a WAIT instruction to be executed

instead.

3.3.3 Exit from WAIT Mode

The following paragraphs describe the output pro-

cedure of the MCU Core from WAIT mode when

an interrupt occurs. It must be noted that the re-

start sequence depends on the original state of the

MCU (normal, interrupt or non-maskable interrupt

mode) before the start of the WAIT sequence, but

also of the type of the interrupt request that is gen-

erated. In all cases the GEN bit of IOR has to be

set to 1 in order to restart from WAIT mode. Con-

trary to the operation of NMI in the run mode, the

NMI is masked in WAIT mode if GEN=0.

Normal Mode. If the MCU Core was in the main

routine when the WAIT instruction has been exe-

cuted, the Core exits from WAIT mode as soon as

an interrupt occurs; the corresponding interrupt

routine is executed, and at the end of the interrupt

service routine, the instruction that follows the

WAIT instruction is executed if no other interrupts

are pending.

Non-maskable Interrupt Mode. If the WAIT in-

struction has been executed during the execution

of the non-maskable interrupt routine, the MCU

Core outputs from WAIT mode as soon as any in-

terrupt occurs: the instruction that follows the

WAIT instruction is executed and the MCU Core is

still in the non-maskable interrupt mode even if an-

other interrupt has been generated.

Normal Interrupt Mode. If the MCU Core was in

the interrupt mode before the initialization of the

WAIT sequence, it outputs from the wait mode as

soon as any interrupt occurs. Nevertheless, two

cases have to be considered:

– If the interrupt is a normal interrupt, the interrupt

routine in which the WAIT was entered will be

completed with the execution of the instruction

that follows the WAIT and the MCU Core is still in

the interrupt mode. At the end of this routine

pending interrupts will be serviced in accordance

to their priority.

– If the interrupt is a non-maskable interrupt, the

non-maskable routine is processed at first. Then,

the routine in which the WAIT was entered will be

completed with the execution of the instruction

that follows the WAIT and the MCU Core is still in

the normal interrupt mode.

Notes:

If all the interrupt sources are disabled, the restart

of the MCU can only be done by a Reset activa-

tion. The Wait instruction is not executed if an en-

abled interrupt request is pending. In ST638x de-

vices, the hardware activated digital watchdog

function is present. As the watchdog is always ac-

tivated, the STOP instruction is de-activated and

any attempt to execute the STOP instruction will

cause an execution of a WAIT instruction.

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