Power reduction modes
UM0404
DocID13284 Rev 2
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Power reduction modes
Several different power reduction modes with different levels of power reduction have been
implemented in the ST10F276, which may be entered under software and/or hardware
control.
In
Idle mode
the CPU is stopped, while the peripherals continue their operation. Idle mode
can be terminated by any reset or interrupt request.
In
Power Down mode
both the CPU and the peripherals are stopped. Power Down mode
can be configured by software in order to be terminated only by a hardware reset, by a
transition on enabled fast external interrupt pins, by an interrupt generated by the Real Time
Clock, by an interrupt generated by the activity on CAN’s and I
2
C module interfaces.
Note:
All external bus actions are completed before Idle or Power Down mode is entered.
However, Idle or Power Down mode is
not
entered if READY is enabled, but has not been
activated (driven low for negative polarity, or driven high for positive polarity) during the last
bus access.
When Real Time Clock module is used, when the device is in Power Down mode a
reference clock is needed. In this case, two possible configurations may be selected by the
user application according to the desired level of power reduction:
•
A 32 kHz crystal is connected to the low-power oscillator pins (XTAL3 / XTAL4) and
running. In this case the main oscillator is stopped when Power Down mode is entered,
while the Real Time Clock continues counting using 32 kHz clock signal as reference.
The presence of a running low-power oscillator is detected after the Power-On: this
clock is immediately assumed (if present, or as soon as it is detected) as reference for
the Real Time Clock counter and it will be maintained forever (unless specifically
disabled via software, see
Section 22: Real time clock on page 464
).
•
Only the main oscillator is running (XTAL1 / XTAL2 pins). In this case the main
oscillator is not stopped when Power Down is entered, and the Real Time Clock
continues counting using the main oscillator clock signal as reference.
Stand-by mode
is achieved by turning off the main power supply (V
DD
) while V
STBY
remains the only active supply for the device. In this condition V
STBY
pin provides the supply
to a portion of the XRAM (the so called Stand-by RAM, 16 Kbyte in this device) through a
dedicated on-chip low power Voltage Regulator: the content of this RAM can be retained
and will be available at next system start-up.
Note:
V
STBY
should be always powered in the range of 4.5-5.5 volt: 6 volt is acceptable for a
reduced period of time during the life of the device (refer to Electrical Characteristics section
for details); 4 volt is acceptable when no RTC and 32 kHz oscillator are used.
Note:
Exception for V
STBY
value is allowed when RSTIN pin is held low and the main V
DD
is on:
this will allow to properly drive pin EA (mapped together with V
STBY
) and configure the
access to external memory. After RSTIN pin is released, V
STBY
should return high, to be
used as V
STBY
supply voltage.
When Real Time Clock module is counting, it is still possible to enter Stand-by Mode. In
particular, when the on-chip low-power oscillator (XTAL3 / XTAL4 pins) provides a 32 kHz
clock reference for the counter, the main power supply can be turned off since both Real
Time Clock module and the low-power oscillator circuitries are powered (directly or through
the low-power voltage regulator) by the voltage applied on V
STBY
pin.