st62t25c STMicroelectronics, st62t25c Datasheet - Page 30

• Manufacturer Part Number: st62t25c
• Description: 8-bit Otp/eprom Mcus With A/d Converter, Oscillator Safeguard, Safe Reset And 28 Pins
• Manufacturer: STMicroelectronics

ST62T15C/T25C/E25C
3.5 POWER SAVING MODES
The WAIT and STOP modes have been imple-
mented in the ST62xx family of MCUs in order to
reduce the product’s electrical consumption during
idle periods. These two power saving modes are
described in the following paragraphs.
In addition, the Low Frequency Auxiliary Oscillator
(LFAO) can be used instead of the main oscillator
to reduce power consumption in RUN and WAIT
modes.
3.5.1 WAIT Mode
The MCU goes into WAIT mode as soon as the
WAIT instruction is executed. The microcontroller
can be considered as being in a “software frozen”
state where the core stops processing the pro-
gram instructions, the RAM contents and peripher-
al registers are preserved as long as the power
supply voltage is higher than the RAM retention
voltage. In this mode the peripherals are still ac-
tive.
WAIT mode can be used when the user wants to
reduce the MCU power consumption during idle
periods, while not losing track of time or the capa-
bility of monitoring external events. The active os-
cillator (main oscillator or LFAO) is not stopped in
order to provide a clock signal to the peripherals.
Timer counting may be enabled as well as the
Timer interrupt, before entering the WAIT mode:
this allows the WAIT mode to be exited when a
Timer interrupt occurs. The same applies to other
peripherals which use the clock signal.
If the power consumption has to be further re-
duced, the Low Frequency Auxiliary Oscillator
(LFAO) can be used in place of the main oscillator,
if its operating frequency is lower. If required, the
LFAO must be switched on before entering the
WAIT mode.
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If the WAIT mode is exited due to a Reset (either
by activating the external pin or generated by the
Watchdog), the MCU enters a normal reset proce-
dure. If an interrupt is generated during WAIT
mode, the MCU’s behaviour depends on the state
of the processor core prior to the WAIT instruction,
but also on the kind of interrupt request which is
generated. This is described in the following para-
graphs. The processor core does not generate a
delay following the occurrence of the interrupt, be-
cause the oscillator clock is still available and no
stabilisation period is necessary.
3.5.2 STOP Mode
If the Watchdog is disabled, STOP mode is availa-
ble. When in STOP mode, the MCU is placed in
the lowest power consumption mode. In this oper-
ating mode, the microcontroller can be considered
as being “frozen”, no instruction is executed, the
oscillator is stopped, the RAM contents and pe-
ripheral registers are preserved as long as the
power supply voltage is higher than the RAM re-
tention voltage, and the ST62xx core waits for the
occurrence of an external interrupt request or a
Reset to exit the STOP state.
If the STOP state is exited due to a Reset (by acti-
vating the external pin) the MCU will enter a nor-
mal reset procedure. Behaviour in response to in-
terrupts depends on the state of the processor
core prior to issuing the STOP instruction, and
also on the kind of interrupt request that is gener-
ated.
This case will be described in the following para-
graphs. The processor core generates a delay af-
ter occurrence of the interrupt request, in order to
wait for complete stabilisation of the oscillator, be-
fore executing the first instruction.