ST10F272B_12 STMICROELECTRONICS [STMicroelectronics], ST10F272B_12 Datasheet - Page 112

• Manufacturer Part Number: ST10F272B_12
• Description: 16-bit MCU with 256 Kbyte Flash memory and 12/20 Kbyte RAM
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

Power reduction modes
21.3.1
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active: the portion of XRAM (16Kbytes for ST10F272E), the RTC counters and 32 kHz on-
chip oscillator amplifier.
In normal running mode (that is when main V
during reset to exercise the EA functionality associated with the same pin: the voltage
supply for the circuitries which are usually biased with V
oscillator used in conjunction with Real Time Clock module), is granted by the active main
V
It must be noted that Stand-by Mode can generate problems associated with the usage of
different power supplies in CMOS systems; particular attention must be paid when the
ST10F272 I/O lines are interfaced with other external CMOS integrated circuits: if V
ST10F272 becomes (for example in Stand-by Mode) lower than the output level forced by
the I/O lines of these external integrated circuits, the ST10F272 could be directly powered
through the inherent diode existing on ST10F272 output driver circuitry. The same is valid
for ST10F272 interfaced to active/inactive communication buses during Stand-by mode:
current injection can be generated through the inherent diode.
Furthermore, the sequence of turning on/off of the different voltage could be critical for the
system (not only for the ST10F272 device). The device Stand-by mode current (I
vary while V
and V
phenomenon.
Entering stand-by mode
As already said, to enter Stand-by Mode XRAM2EN bit in the XPERCON Register must be
cleared: this allows to freeze immediately the RAM interface, avoiding any data corruption.
As a consequence of a RESET event, the RAM Power Supply is switched to the internal
low-voltage supply V
The RAM interface will remain frozen until the bit XRAM2EN is set again by software
initialization routine (at next exit from main V
Since V
XRAM2EN bit is no longer able to guarantee its content (logic “0”), being the XPERCON
Register powered by internal V
by Mode switching dedicated circuit continues to confirm the RAM interface freezing,
irrespective the XRAM2EN bit content; XRAM2EN bit status is considered again when
internal V
If internal V
with bit XRAM2EN set, the RAM Supply switching circuit is not active: in case of a
temporary drop on internal V
no spurious Stand-by Mode switching can occur (the RAM is not frozen and can still be
accessed).
The ST10F272 Core module, generating the RAM control signals, is powered by internal
V
switched to V
from ST10F272 Core (active low signal) is low enough to be recognized as a logic “0” by the
RAM interface (due to V
for the RAM and an unwanted data corruption could occur. For this reason, an extra
interface, powered by the switched supply, is used to prevent the RAM from this kind of
potential corruption mechanism.
DD
18
.
supply; during turning off transient these control signals follow the V
STBY
18
18
is falling down (as a consequence of V
pins. System noise on both V
18
DD
comes back over internal stand-by reference V
18SB
becomes lower than internal stand-by reference (V
to V
internal reference. It could happen that a high level of RAM write strobe
STBY
18SB
18
(and vice versa) transition occurs: some current flows between V
(derived from V
lower than V
18
Doc ID 11917 Rev 3
18
voltage versus internal V
. This does not generate any problem, because the Stand-
18SB
DD
STBY
): The bus status could contain a valid address
and V
DD
DD
through the low-power voltage regulator).
power-on reset sequence).
is on) the V
DD
STBY
turning off), it can happen that the
can contribute to increase this
STBY
18SB
18SB
STBY
during normal code execution,
(see in particular the 32 kHz
.
18SB
pin can be tied to V
ST10F272B/ST10F272E
) of about 0.3 to 0.45V
18
, while RAM is
STBY
DD
SS
) may
of
DD