4.4.1.1 Entering and Exiting Backup Mode

To enter Backup mode, follow the steps in the sequence below:

  1. If VDD3V3 is maintained when in Backup mode, all IOs except SLCD IOs if used in Backup mode, have to be set back to PIO input pull-up or pull-down or output low or high level to avoid power consumption with external on-board devices.
  2. Switch the complete clock tree (Processors and bus clocks) to the 12 MHz RC, and stop all PLLs and fast crystal oscillator if used.
  3. Enable the RTT in 1-Hz mode.
  4. Disable Normal mode of the RTT (RTT runs in 1-Hz mode).
  5. Program one or several wake-up sources(1) and clear any pending wake-up sources.
  6. Disable the VDDCORE Supply Monitor.
  7. Adjust the power switch position in RSTC_CR according to the power supply used for VDDBU (internal switch output)
  8. Select one of the following methods to enter Backup mode:
    1. If the internal core voltage regulator is used, write a 1 to SUPC_CR.VROFF. The pin SHDN remains high in this case.
    2. If an external voltage regulator is used for VDDCORE/VDDPLL, write a 1 to SUPC_CR.SHDW. The pin SHDN is asserted low, allowing an external voltage regulator to be turned OFF.
    3. If the SLCD is used in Backup mode to keep a static message, write a 1 to SUPC_CR.SHDWEOF. At the end of the SLCD frame, the SHDN pin is asserted low
Note:
  1. If a WKUP wake-up pin with tamper functionality is used as wake-up source, the wake-up debouncer (WKUPDBC) value must be set so that its period is greater than or equal to 1/2 x RTCOUT0 period set in the RTC.

After this step, the core voltage regulator is shut down. None of the digital internal logic is powered. Whether VROFF, SHDN or SHDWEOF was used to enter Backup mode, the system exits Backup mode if one of the enabled wake-up events listed in the table Low-Power Mode Summary occurs.

After exiting Backup mode, the device is in Reset state. Only the configuration of the backup area peripherals remains unchanged.

Note that the device does not automatically enter Backup mode if VDD3V3 is disconnected, or if it falls below minimum voltage.

For current consumption in Backup mode, refer to the section Electrical Characteristics.