23.4.4 Using Backup Batteries/Backup Supply

When backup batteries or, more generally, a separate backup supply is used, only VDDIO is present in Backup mode. No other external supply is applied.

Note: Restrictions
With main supply < 3.0V, USB is not usable.
With main supply < 2.7V, MediaLB is not usable.
With main supply < 2.0V, ADC, DAC and Analog comparator are not usable.
With main supply and VDDIN > 3V, all peripherals are usable.

When no separate backup supply for VDDIO is used, since the external voltage applied on VDDIO is kept, all of the I/O configurations (i.e., WKUP pin configuration) are maintained in Backup mode. When not using backup batteries, VDDIORDY is set so the user does not need to program it.

Note: Restrictions
 with main supply < 2.0 V, USB and ADC/DAC and analog comparator are not usable.
With main supply > 2.0V and < 3V, USB is not usable.
With main supply < 2.7V, MediaLB is not usable.
With main supply > 3V, all peripherals are usable.

The following figure illustrates an example of the powering scheme when using a backup battery. Since the PIO state is preserved when in Backup mode, any free PIO line can be used to switch off the external regulator by driving the PIO line at low level (PIO is input, pull-up enabled after backup reset). System wakeup can be performed using a wakeup pin (WKUPx). See the "Wakeup Sources" section for further details.