ST10F272E datasheet(110/181 Pages) STMICROELECTRONICS

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Power reduction modes

ST10F272B/ST10F272E

110/181

Warning:

During power-off phase, it is important that the external

hardware maintains a stable ground level on RSTIN pin,

without any glitch, in order to avoid spurious exiting from

reset status with unstable power supply.

21.3.2

Exiting stand-by mode

After the system has entered the Stand-by Mode, the procedure to exit this mode consists of

a standard Power-on sequence, with the only difference that the RAM is already powered

through V18SB internal reference (derived from VSTBY pin external voltage).

It is recommended to held the device under RESET (RSTIN pin forced low) until external

VDD voltage pin is stable. Even though, at the very beginning of the power-on phase, the

device is maintained under reset by the internal low voltage detector circuit (implemented

inside the main voltage regulator) till the internal V18 becomes higher than about 1.0V, there

is no warranty that the device stays under reset status if RSTIN is at high level during

power ramp up. So, it is important the external hardware is able to guarantee a stable

ground level on RSTIN along the power-on phase, without any temporary glitch.

The external hardware shall be responsible to drive low the RSTIN pin until the VDD is

stable, even though the internal LVD is active.

Once the internal Reset signal goes low, the RAM (still frozen) power supply is switched to

the main V18.

At this time, everything becomes stable, and the execution of the initialization routines can

start: XRAM2EN bit can be set, enabling the RAM.

21.3.3

Real time clock and stand-by mode

When Stand-by mode is entered (turning off the main supply VDD), the Real Time Clock

counting can be maintained running in case the on-chip 32 kHz oscillator is used to provide

the reference to the counter. This is not possible if the main oscillator is used as reference

for the counter: Being the main oscillator powered by VDD, once this is switched off, the

oscillator is stopped.

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